faculty of biological sciences ugwoke oluchi c

Ugwoke Oluchi C.

FACULTY OF BIOLOGICAL SCIENCES

DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY

SYNTHESIS, CHARACTERIZATION AND SOLVENT EXTRACTION

STUDIES OF 3,5

BENZOIC ACID AND ITS Co(II) AND Ni(II) COMPLEXES

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DN : CN = Webmaster’s name

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Ugwoke Oluchi C.




STUDIES OF 3,5-BIS[(2-HYDROXY-BENZYLIDENE)-AMINO]


UMAR, ABDULLAHI YARO

(PG/M.SC/11/59594)

: Content manager’s Name

Webmaster’s name

a, Nsukka




AMINO]-


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SYNTHESIS, CHARACTERIZATION AND SOLVENT

EXTRACTION STUDIES OF 3,5-BIS[(2-HYDROXY-

BENZYLIDENE)-AMINO]-BENZOIC ACID

AND ITS Co(II) AND Ni(II) COMPLEXES

BY


(PG/M.SC/11/59594)


UNIVERSITY OF NIGERIA, NSUKKA

JULY, 2014.

iii

TITLE PAGE

SYNTHESIS, CHARACTERIZATION AND SOLVENT EXTRACTION STUDIES

OF 3,5-BIS[(2-HYDROXY-BENZYLIDENE)-AMINO]-BENZOIC ACID

AND ITS Co(II) AND Ni(II) COMPLEXES

BY


PG/M.Sc/11/59594

A PROJECT SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUIREMENT FOR THE AWARD OF MASTER OF SCIENCE

DEGREE IN INORGANIC CHEMISTRY


UNIVERSITY OF NIGERIA, NSUKKA

JULY, 2014.

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DECLARATION

I here declare that this project contains the report of my research work and

has not been presented in any previous application for any degree or diploma. All

information from other sources have been acknowledged by means of references.

..................................................


……………………………

DATE

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CERTIFICATION

This thesis entitled “Synthesis, Characterization and Solvent Extraction

Studies on 3,5-[(2-Hydroxy-benzylidene)-Amino]-Benzoic Acid and its Co(II) and

Ni(II) Complexes” by Umar Abdullahi Yaro meets the regulation governing the

award of the degree of Master of Science of the University of Nigeria, and is

approved for its contribution to scientific and literary presentation.

…………………………. ………………………..

Prof. P.O. Ukoha Date

(Supervisor)

………………………… ………………………..

Dr. A. E. Ochonogor Date

(Head, Department of Pure and

Industrial Chemistry)

…………………………… ……………………..

(Dean, Postgraduate School ) Date

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DEDICATION

To almighty God, my father late Alh. Umar-Saje Zakari for his love in quest

for knowledge and the less privileged

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ACKNOWLEDGEMENT

All praise belongs to Allah, the Author and Giver of knowledge and

wisdom.

My sincere gratitude goes to my supervisor, Prof. P. O. Ukoha for his

fatherly role. Who despite his tight schedule finds time to give from his wealth of

experience guidance and counseling without boundaries in academics, social,

moral and spiritual supports.

I like to acknowledge the entire staff of the department of Pure & Industrial

Chemistry, University of Nigeria, Nsukka for their un-alloyed support. Notably:

Dr. Ujam, Dr. Asegbeloyin, Dr. Obasi and Miss Chidinma.

Special thanks to the Management and Staff, Kogi State University,

Anyigba, the out-gone Head of Department of Chemistry, Dr Awodi Y., the

incumbent Head of Department of Chemistry and Tertiary Education Trust Fund

(TET-Fund) for their support.

I wish to acknowledge the entire staff in Chemistry Lab, Biological Science

Lab, Microbiology Lab and Biochemistry Lab, Kogi State University Anyigba.

Notably the Chief Technologist in Biological Science Mr Ogunleye J., Chief

Technologist in Biochemistry Mr. Ekeyi P. and Mr. Usman A. O. in Chemistry.

I say a big thank you to my Mum, Hajiya Sharubutu, my Fiancée Hadiza, my

brothers: Bakatu (Big Bros), Tanimu (TMS), Bello, Yanda, Hamisu, Aptan Nnagi;

and my Confidant & Research Assistant Patricia for your love and understanding

throughout the period of this research.

My warmth appreciations to Dr. Emurotu and family, Mr Agbogun and

family, Mr Samson (My Twin brother), Mr Tobi, Mr Peter (Pete), Mal. Aminu and

family, Alh Jibo and family for your care, support and understanding.

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ABSTRACT

3,5-Bis[(2-hydroxy-benzylidene)-amino]-benzoic acid (H2B) and its

cobalt(II) and nickel(II) complexes were synthesized and characterized via:

electronic, IR, 1H NMR and

13C NMR. Job’s continuous variation method was

used to determine the mole ratio for both metal complexes. Solvent extraction

studies were carried out on H2B in 5% DMF with its cobalt(II) and nickel(II)

complexes using CHCl3 as organic solvent; with variable condition effects of

equilibrium time, buffer pH, mineral acids, salting-out agents and complexing

agents. IR spectral study indicates coordination through (N2O2) azomethine and

protonated hydroxyl groups. Job’s continuous variation method showed a metal to

ligand ratio, 1:1, for both metal complexes of H2B. Cobalt(II) complex of H2B

showed quantitative extraction in pH range 5 – 7, while nickel(II) complex of H2B

showed quantitative extraction in pH range 6 – 8. Nickel was successfully

separated from cobalt by four-cycle extraction at 10-3

M HNO3 aqueous mixture of

Ni(II) and Co(II) {10 µgcm-1

each} in 5% H2B/DMF using 0.05 M cyanide as

masking agent and CHCl3 as organic solvent.

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TABLE OF CONTENTS

Title page. . . . . . . . . . . i

Declaration . . . . . . . . . . ii

Certification . . . . . . . . . iii

Dedication . . . . . . . . . . iv

Acknowledgment . . . . . . . . . v

Abstract . . . . . . . . . . vi

Table of Contents . . . . . . . . . vii

List of Tables . . . . . . . . . xi

List of Figures . . . . . . . . . xii

CHAPTER ONE:

1.0 General Introduction …………………………………………………. 1

1.1 Background of Study…………………………………..…………... 2

1.2 Scope of Study…………………………………………..…………... 3

1.3 Significance of Study………………………………..……………... 4

1.8 Aims and Objectives…………………………………………... 4

CHAPTER TWO: Literature Review

2.0 Brief Chemistry of Metals under Study………,…………….. 6

2.1 Cobalt ……………………………………………………… 6

2.1.1 Aqueous Chemistry of Cobalt …………………………………... 8

2.1.2 Oxidation States.…………………………… …………….. 8

2.2 Nickel………… ……………………………………………. 13

2.2.1 Aqueous Chemistry of Nickel ...……………………………. 15

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2.2.2 Oxidation States………………………………………………... 16

2.2.3 Uses of Nickel and its Compound……………………..………… 17

2.2.4 Nickel and Human Health……………………………………. 18

2.3 Theoretical Fundamentals of Liquid-Liquid Extraction ……..….. 19

2.3.1 Distribution Law …………………………………………………. 20

2.3.2 Limitation of Nernst Distribution Law ………………………….. 22

2.3.3 Thermodynamic Partition Law Constant……………………… 24

2.3.4 Distribution Ratio ……………………………………………… 27

2.4 Efficiency of Extraction ………………………………………… 28

2.4.1 Percentage Extraction ………………………………………….. 29

2.4.2 Separation Factor ……………………………………………… . 31

2.5 Quantitative Treatment of Solvent Extraction Equilibrium …….. 33

2.6 Extraction Methods in Solvent Extraction ……………………… 37

2.6.1 Batch Extraction ………………………………………………… 37

2.6.2 Continuous Extraction …………………………………………… 42

2.6.3 Discontinuous Countercurrent Extraction ……………………….. 43

2.7 Classification of Inorganic Extraction System …………………. 45

2.7.1 Metal Chelate…………………………………………………… . 46

2.7.2 Ion-association Complexes …………………………………….. . 53

2.7.3 Additive Complexes ……………………………………………. 54

2.8 Factors that Influence Stability and Extractability of Metal

Chelate Complexes……………………………………………. 57

2.9 Brief Work on Solvent Extraction of Metals under Study... 62

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2.10 Previous Work on 3,5-Bis[(2-Hydroxy-Benzylidene)-Amino]-Benzoic

Acid………………………………………………………….... 66

2.10.1 Salens………………………………………………………... 68

2.10.1.1 Salen Ligand Synthesis…………………..………………..... 69

CHAPTER THREE:

3.0 Experimental…………………………………………………… 73

3.1 Equipments…...………………………………………………….. 73

3.2 Preparation of Metal Stock Solutions…………………………… 73

3.3 Synthesis of 3,5-Bis[(2-Hydroxy-Benzylidene)-Amino]-Benzoic

Acid …………………………………………………………….... 77

3.4 Synthesis of Co(II) and Ni(II) Complexes of 3,5-Bis[(2-Hydroxy-

Benzylidene)-Amino]-Benzoic Acid……………………….… 77

3.5 Determination of the Composition of the Extracted Species….. 78

3.6 Extraction Procedures ……………………………………………. 78

3.6.1 Extraction from Buffer Solution ………………………………… 79

3.6.2 Extraction from Acid Media……………………………………… 80

3.6.3 Extraction in Salting-out Agents………………………………….. 80

3.6.4 Extraction in Complexing Agents ………………………………… 81

3.7 Measurement of Distribution Ratio……………………………….. 82

3.8 Spectrophotometric Analysis of the Metal Ions…………………… 82

3.9 Calibration Curve………………………………………………….. 83

3.10 Separation Procedures……………………………………………. 84

CHAPTER FOUR:

4.0 Results and Discussion…………………………………………… 85

4.1 Electronic Spectra………………………………………………… 85

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4.2 IR Spectra…………………………………………………………. 86

4.3 1H NMR Spectra …………………………………………………. 96

4.4 13

C NMR Spectra ………………………………………………… 97

4.5 Metal–Ligand Mole Ratio……………………………………….. .. 104

4.6 Molecular Formula of the ligand and the Complexes …………. .. 104

4.7 Solubility Data …………………………………………………... 109

4.8 Dissociation and Protonation Constants of the Ligand ………... … 111

4.9 Equilibration Time……………………………………………….. 115

4.10 Effect of pH Buffer on Extraction of Co(II) and Ni(II) …………… 115

4.11 Effect of Acidity …………………………………………………... 120

4.12 Effect of Salting-out Agent on Extraction …………………………. 122

4.13 Effect of Complexing Agents on Extraction ……………………. 125

4.14 Degree of Metal Separation ………………………………………. 128

4.15 Summary and Conclusion ………………………………………….. 128

4.16 Recommendation………………………..………………………….. 130

4.17 Contribution to Knowledge……….………………………………….. 131

References…………….……………………………………………. 132

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LIST OF TABLES

Page

4.1 Electronic Spectral data of H2B, CoB and NiB 91

4.2 Summary of Infrared spectral data of H2B, CoB and NiB 95

4.3 Summary of Proton resonance data for the H2B, CoB and NiB in

DMSO - D6 (400 MHz) 101

4.4 13

C NMR data for H2B, CoB and NiB in DMSO - D6 (400 MHz) 108

4.5 Solubility Test Data for the Ligand and its CoB and NiB Complexes 110

4.6 The amount of Co(II) extracted into the organic phase at various

time intervals 117

4.7 The amount of Ni(II) extracted into the organic phase at various

time intervals 118

4.8 Degree of Seperation of Ni(II) from Co(II) 129

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LIST OF FIGURES

page

2.1 Graphical representation of Nernst Distribution Law 22

2.2 Deviation from Nernst distribution law 24

2.3 The distribution ratio D for three different substances A, B,

and C, plotted against the variable Z of the aqueous phase. 30

2.4 Same systems showing percentage extraction against Z 31

2.5 Extraction as a function of pH for metals of different formal

Valencies 35

2.6 Separatory funnel of different designs 38

2.7 The graph of ( )

o

orgaqWW n against nth number of extractions 42

2.8 Continuous extraction apparatus 43

2.9 Two interlocking glass units for Craig counter-current distribution 45

2.10 Effect of pH on the extraction of monovalent (Ag+), bivalent (Pb

2+),

tervalent (La3+

), and tetravalent (Th4+

) metal ions by 0.10M

8-hydroxyquinoline in chloroform 58

2.11 Effect of pH on the extraction of cobalt(II) and manganese(II)

by 8-hydroquinoline in chloroform 59

2.12 (R-R) Salen 69

2.13 Metal Complexes of Salen Ligand 1 Utilized in Catalytic

Asymmetric Processes 70

4.1 UV Spectrum of H2B 88

4.2 UV spectrum of CoB 89

4.3 UV Spectrum of NiB

90

4.4 Infra-red spectrum of H2 B 92

4.5 Infra-red spectrum of CoB 93

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4.6 Infra-red spectrum of NiB

94

4.7 HI NMR spectrum of H2B in DMSO-D6 (400 MHz) 98

4.8 HI NMR spectrum of CoB in DMSO-D6 (400 MHz) 99

4.9 HI NMR spectrum of NiB in DMSO-D6 (400 MHz) 100

4.10 1H and

13C nmr assignment for H2B, CoB and NiB complexes 102

4.11 13

C NMR spectrum of H2B in DMSO-D6 (400 MHz) 105

4.12 13


4.13 13


4.14 Job’s plot for Co(II)/H2B mole ratio 112

4.15 Job’s plot for Ni(II)/H2B mole ratio 112

4.16 Structures of ligand, its Co(II) and Ni(II) complexes 113

4.17 Titrimetric determination of pkb for the ligand 114

4.18 Titrimetric method of determining pKa for the ligand 114

4.19 Profile for Co(II) extraction in buffer media 119

4.20 Profile for Ni(II) extraction in buffer media 119

4.21 Profile for extraction of Co(II) in various acid media 121

4.22 Profile for extraction of Ni(II) in various acid media 121

4.23 %E Vs concentration of salting-out agent for Co(II)

extraction with H2B/DMF 124

4.24 %E Vs concentration of salting-out agent for Ni(II)

extraction with H2B/DMF 124

4.25 Effect of complexing agent on the extraction of Co(II) with

H2B/DMF 127

4.26 Effect of complexing agent on the extraction of Ni(II)

with H2B/DMF 127

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CHAPTER ONE

INTRODUCTION

1.0 General Introduction

Extraction is the transfer of a solute from one phase to another. Common

reasons to carry out an extraction in chemistry are to isolate or concentrate the

desired analyte or to separate it from species that would interfere in the analysis.

The most common case is the extraction of an aqueous solution with an organic

solvent that are immiscible with and less dense than water; they form a separate

phase that floats on top of the aqueous phase1.

Solvent or liquid-liquid extraction is based on the principle that a solute can

distribute itself in a certain ratio between two immiscible solvents, one of which is

usually water and the other an organic solvent such as benzene, carbon

tetrachloride or chloroform. In certain cases the solute can be more or less

completely transferred into the organic phase. The technique can be used for

purposes of preparation, purification, enrichment, separation and analysis, on all

scales of working, from microanalysis to production processes. In chemistry,

solvent extraction has come to the forefront in recent years as a popular separation

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technique because of its elegance, simplicity, speed and applicability to both tracer

and macro amounts of metal ions2.

The ability of a solute (inorganic or organic) to distribute itself between an

aqueous solution and an immiscible organic solvent has long been applied to

separation and purification of solutes either by extraction into the organic phase,

leaving undesirable substances in the aqueous phase; or by extraction of the

undesirable substances into the organic phase, leaving the desirable solute in the

aqueous phase.3

1.1 Background of Study

Although solvent extraction as a method of separation has long been known

to the chemists, only in recent years it has achieved recognition among analysts as

a powerful separation technique. Liquid-liquid extraction, mostly used in analysis,

is a technique in which a solution is brought into contact with a second solvent,

essentially immiscible with the first, in order to bring the transfer of one or more

solutes into the second solvent4. The separations that can be achieved by this

method are simple, convenient and rapid to perform; they are clean as much as the

small interfacial area certainly precludes any phenomena analogous to the

undesirable co-precipitation encountered in precipitation separations.

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3

Solvent extraction has one of its most important applications in the

separation of metal cations. In this technique, the metal ion, through appropriate

chemistry, distributes from an aqueous phase into a water-immiscible organic

phase. Solvent extraction of metal ions is useful for removing them from an

interfering matrix, or for selectivity (with the right chemistry) separating one or a

group of metals from others4.

Solvent extraction is one of the most extensively studied and most widely

used techniques for the separation and pre-concentration of elements. The

technique has become more useful in recent years due to the development of

selective chelating agents for trace metal determination5

1.2 Scope of Work

The Scope of this research is limited to synthesis of the Ligand

Bis(salicylidene)3,5-diaminobenzoic acid, its Co(II) and Ni(II) complexes,

spectrophotometric characterization via UV, IR, H and NMR(1H and

13C),

extraction of cobalt and nickel metal ions in water using chloroform as organic

solvent and separation of Ni(II) from aqueous mixture of Ni(II) and Co(II).

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1.3 Significance of Study

The introduction of versatile organic reagent, ‘dithizone’, dimethylglyoxime

about five years later and 8-hydroxylquinoline in the 1940s opened a new in liquid-

liquid extraction studies which suffered a lull from 1900 till then.6

The search for new extractants for metals continues to draw attention with

the quest for reagents that will be discriminatory enough for particular metal ions

and avoid interferences at the conditions of extraction.

Ukoha et al

7 reported the utilization of the compound Bis(4-hydroxypent-2-

ylidene) diaminethane as a good reagent to extract copper(II) and also separated

the element from a mixture of silver(I).

In this research, we are able to synthesize a schiff base Bis(salicylidene)3,5-

diaminobenzoic acid as a ligand to investigate the extraction characteristics of

cobalt(II) and Nickel(II) in various media. The complexes of cobalt(II) and

nickel(II) were characterized spectrophotometrically via UV-visible, IR, and

NMR(1H and

13C)

1.4 Aims and Objectives

This research is aimed at synthesizing a Schiff base ligand: 3,5-Bis-[(2-

hydroxy-benzylidene)-amino]-benzoic acid; its Co(II) and Ni(II) complexes;

characterization of the ligand and its Co(II) and Ni(II) complexes via, Uv-visible,

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IR, 13

C and 1H NMR; using the ligand to extract Co(II) and Ni(II) from aqueous

solutions of varying conditions. Thus, the optimum extraction condition for the

extraction of Co(II) and Ni(II) from aqueous solution with 3,5-[(2-hydroxy-

benzylidene)-amino]-benzoic acid will be achieved and a favourable condition for

the separation of Ni(II) from Co(II) with the ligand will also be ascertained. Hence,

the true nature of the Co(II) and Ni(II) complexes of the ligand will be known.

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CHAPTER TWO

LITERATURE REVIEW

2.0 Brief Chemistry of Metals under Study

2.1 Cobalt

Cobalt is a rare element of the earth’s crust comprising 29 ppm i.e. 0.0029%;

though widely distributed, stands only thirtieth in order of abundance and is less

common than all other elements of the first transition series except scandium (25

ppm)

More than 200 ores are known to contain cobalt but only a few are of

commercial value. The more important are arsenides and sulfides such as smaltite.

CoAs2, cobaltite (or cobalt glance). CoAsS, and linnaeite, Co3S4. These are

invariably associated with nickel, and often also copper and lead, and it is usually

obtained as a byproduct or co-product in the recovery of these metals. The world’s

major sources of cobalt are the African and Canada with smaller reserves in

Australia and the former USSR.

Properties

The metal is lustrous and silvery with a bluish tinge; appreciably hard. By

contrast the atomic weight of cobalt is known with considerable precision since the

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element has one naturally occurring isotope i.e. 59

Co, but bombardment by thermal

neutrons converts this to the radioactive 60

Co. The latter has a half-life of 5.271y

and decays by means of β- and γ emission to non-radioactive

60Ni. It is used in

many fields of research as a concentrated source of γ-radiation, and also medically

in the treatment of malignant growths.

Property Value

Atomic number 27

Number of naturally occurring isotope 1

Atomic weight 58.933200(9)

Electronic configuration [Ar]3d7 4s

2

Electronegativity 1.8

Metal radius (12-coordinate)/pm 125

Effective ionic radius (6-coordinate)/pm IV 53

III 54.5(Is), 61(hs)

II 65(Is), 74.5(hs)

MP/0C 1495

BP/0C 3100

∆Hfus/kJ mol-1

16.3

∆Hvap/kJ mol-1

382

∆Hf (monatomic gas)/kJ mol-1

425(+ 17)

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Density (200C)/gcm

-3 8.90

Electrical resisitivity (200C)/µohm cm 6.24

2.1.1 Aqueous Chemistry of Cobalt

Cobalt shows no important valence higher than 4, its most important being

an electorvalency of 2 in the cobalt(II) ion Co2+

. The simple cobalt(III) ion Co3+

is

very unstable, so as the hydrated into [Co(H2O)6]3+

, but cobalt forms numerous

stable complexes in which it has a covalency (or oxidation state) of +339

The most common oxidation states of cobalt are +2 and +3. [Co(H2O)6]2+

and

[Co(H2O)6]3+

are both known but the latter is a strong oxidizing agent in aqueous

solution, unless it is acidic, it decomposes rapidly as the CoIII

oxidizes the water

with evolution of oxygen. Consequently, in contrast to CoII, Co

III provides few

simple salts, and those which do occur are unstable. However, CoIII

is unsurpassed

in the number of coordination complexes which it forms, especially with N – donor

Ligands. Virtually all of these complexes are low-spin. The �� configuration

producing a particularly high CFSE.40

2.1.2 Oxidation States

Oxidation state III(d6)

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This is the most prolific oxidation state, providing a wide variety of

kinetically inert complexes. The complexes are virtually all low-spin and

octahedral, a major stabilizing influence being the high CFSE associated with the

�� configuration (

��

�∆� , the maximum possible for any d

x configuration). Even

[Co(H2O)6]3+

is low-spin but it is such a powerful oxidizing agent that it is unstable

in aqueous solutions and only a few simple salt hydrates, such as the blue

Co2(SO4)3.18H2O and MCo(SO4)2.12H2O (M=K, Rb, Cs, NH4), which contain the

hexaaquo ion, and CoF3.3½H2O can be isolated. This paucity of simple salts of

cobalt(III) contrasts sharply with the great abundance of its complexes, especially

with N-donor Ligands41

and it is evident that the high CFSE is not the only factor

affecting the stability of this oxidation state.40

Cobalt(III) forms few simple salts, but the green hydrated fluoride

CoF3.3.5H2O and the blue hydrated sulfate Co2(SO4)3.18H2O separate on

electrolytic oxidation of Co2+

in 40% HF and 8 M H2SO4, respectively. Alums,

MCo(SO4)2.12H2O are dark blue; they are reduced by water.

In aqueous solutions containing no complexing agents, oxidation of [Co(H2O)6]2+

to CoIII

is very unfavorable:

VEOHCoeOHCo 84.1])(])([ 02

62

3

62 ==+ ++

…………………..(66)

However, electrolytic or O3 oxidation of cold acidic perchlorate solutions of

Co2+

gives [ +3

62 ])( OHCo , which is in equilibrium with [ +2

52 ])()( OHOHCo . At 00C,

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the half-life of these diamagnetic ions is about a month. In the presence of

complexing agents such as NH3, which form stable complexes with CoIII

, the

stability of CoIII

is greatly improved.

VENHCoeNHCo 1.0])(])([ 02

63

3

63 ==+ ++

………....……….(67)

In basic media we have:

VEOHOHCoOHOHCo ss 17.0)()( 0

)(22)( =+=+ −

………..…(68)

Water rapidly reduces uncomplexed Co3+

at room temperature. This relative

instability is evidenced by the rarity of simple salts and binary compounds, where

CoII forms such compounds in abundance.

42

Oxidation state II (d7)

This is one of the two most stable oxidation states. By contrast, CoII

carboxylates such as the red acetate, Co(O2CMe)2.4H2O, are monomeric and in

some cases the carboxylates ligands are unidentate. The acetate is employed in the

production of catalyst used in certain organic oxidations, and also as a drying agent

in oil-based paints and varnishes. Cobalt(II) gives rise to simple salts with all the

common anions and they are readily obtained as hydrates from aqueous solutions.

The parent hydroxide, Co(OH)2, can be precipitated from the aqueous solutions by

the addition of alkali and is somewhat amphoteric, not only dissolving in acid but

also re-dissolving in excess of conc. alkali, in which case it gives a deep blue

solution containing [Co(OH)4]4-

ions. It is obtainable in both blue and pink

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11

varieties, the former is precipitated by slow addition of alkali at 00C, but it is

unstable and, in the absence of air, becomes pink on warming.

Complexes of cobalt(II), are less numerous than those of cobalt(III) but,

lacking any configuration comparable in stability with the �� of cobalt(III), they

show a greater diversity of types and are more labile. The redox properties and the

possibility of oxidation must always be considered when preparing CoII complexes.

However, providing solutions are not alkaline and the ligands not too high in the

spectrochemical series, a large number of complexes can be isolated without

special precautions. The most common type is high-spin octahedral, though spin-

pairing can be achieved by ligands such as CN-which also favour the higher

oxidation state. Appropriate choice of ligands can however lead to high-spin-low-

spin equilibria as in [Co(terpy)2]X2.nH2O and some 5- and 6-coordinated

complexes of Schiff bases and pyridines.43

Many of the hydrated salts and their

aqueous solutions contain the octahedral, pink [Co(H2O)6]2+

ion, and bidentate N-

donor ligands such as en, bipy and phen form octahedral cationic complexes

[Co(L-L)3]3+

, which are more stable to oxidation than is the hexamine

[Co(NH3)6]2+

. Acac yields the orange [Co(acac)2(H2O)2] which has the trans

octahedral structure and can be dehydrated to form [Co(acac)2] which attains

octahedral coordination by forming the tetrameric specie.

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12

Tetrahedral complexes are also common, being formed more readily with

cobalt(II) than with the cation of any other truly transitional element (i.e. excluding

ZnII). Thus, in aqueous solutions containing [Co(H2O)6]

2+ there are also present in

equilibrium, small amounts of tetrahedral [Co(H2O6)4]2+

, and in acetic acid the

tetrahedral [Co(O2CMe)4]2-

occurs. The most obvious distinction between the

octahedral and tetrahedral compounds is that in general the former are pink to

violet in colour whereas the latter are blue, as exemplified by the well-known

equilibrium:

[Co(H2O)6]2+ + 4Cl- [CoCl4]2- + 6H2O

Pink Blue…………….…..(69)

This is not infallible distinction (as the blue but octahedral CoCl2

demonstrates)

On the other hand, Co(III) complexes in which the complexing agent is other

than water, are much more stable than the corresponding Co(II) complexes, e.g.

potassium pentacyanocobaltate(II), K3[Co(CN)5], is such a powerful reducing

agent, i.e. the Co(II) atom so readily loses an electron to form Co(III), that it

reduces water to hydrogen.

−−−+ +→++ OHHeOHH 221 , ……………………………….…….(70)

and is itself oxidized to potassium hexacyanocobaltate(III), K3[Co(CN)6].

13

13

The hydrated cobalt cobalt(II) ion in solution is pink and in hydrated salts it has a

varying shades of pink or red. Anhydrous salts are blue.39

2.2 Nickel

An alloy of nickel was known in China over 2000 years ago, and Saxon

miners were familiar with the reddish-coloured ore, NiAs, which superficially

resembles Cu2O. These miners attributed their inablility to extract copper from this

source to the work of the devil and named the ore ‘Kupfernickel” (Old Nick’s

copper). In 1751 A. F. Cronstedt isolated an impure metal from some Swedish ore

and, identifying it with the metallic component of Kupfernickel, named the new

metal “nickel”. In 1804 J. B. Richter produced a much purer sample and so was

able to determine its physical properties more accurately.

Nickel is the seventh most abundant transition metal and the twenty-second

most abundant element in the earth’s crust (99 pm). Its commercially important

ores are of two types:

- Laterites, which are oxide/silicate ores such as garnierite,

(Ni,Mg)6Si4O10(OH)8 and nickeliferous limonite, (Fe,Ni)O(OH).nH2O,

which have been concentrated by weathering in tropical rainbelt areas such

as New Caledonia, Cuba and Queensland.

14

14

- Sulfides, such as pentlandite, (Ni,Fe)9S8, associated with copper, cobalt and

precious metals so that the ores typically contain about 1½% Ni. These are

found in more temperate regions such as Canada, the former Soviet Union

and South Africa.

Arsenide ores such as niccolite( Kupfernickel (NiAs), smaltite

(Ni,Co,Fe)As2)are no longer of importance.

The most important single deposit of nickel is at Sudbury Basin, Canada. It was

discovered in 1883 during the building of the Canadian Pacific Railway and

consists of sulfide outcrops situated around the rim of a huge basin 17 miles wide

and 37 miles long (possibly a meteoritic crater). Fifteen elements are currently

extracted from this region (Ni, Cu, Co, Fe, S, Te, Se, Au, Ag and the six platinum

metals).

Properties

The metal is silvery-white and lustrous, malleable and ductile so that they can be

easily worked. Difficulties in attaining high purities have also frequently led to

disparate values for some physical properties, while mechanical history has

considerable effect on such properties as hardness.

Property Value

Atomic number 28

Number of naturally occurring isotopes 5

15

15

Atomic weight 58.6034(2)

Electronic configuration [Ar]3d8 4s

2

Electronegativity 1.8

Metal radius (12-coordinate)/pm 124

Effective ionic radius (6-coordinate)/pm IV 48

III 56(Is), 60(hs)

II 69

MP/0C 1455

BP/0C 2920

∆Hfus/kJ mol-1

17.2(+ 0.3)

∆Hvap/kJ mol-1

375(+ 17)

∆Hf (monatomic gas)/kJ mol-1

429(+ 13)

Density (200C)/gcm

-3 8.908

Electrical resisitivity (200C)/µohm cm 6.84

2.2.1 Aqueous Chemistry of Nickel

All well-known nickel compounds, except oxides and hydroxides, contain

nickel in the bivalent state only.

16

16

Nickel sulphate is NiSO4.7H2O is green, the anhydrous salt yellow. It forms

a green double sulphate with (NH4)2SO4, ammonium nickel(II) sulphate (NH4)2SO-

4.NiSO4.6H2O, used in ammonical solution as the electrolyte in nickel plating39

.

2.2.2 Oxidation States

Oxidation state II(d8)

This is undoubtedly the most prolific oxidation state for nickel.

The absence of any other oxidation state of comparable stability for nickel implies

that compounds of NiII are largely immune to normal redox reactions. Ni

II forms

salts with virtually every anion and has an extensive aqueous chemistry based on

the green [Ni(H2O)6]2+

ion which is always present in the absence of strongly

complexing ligands.

The oxidation number of NiII rarely exceeds 6 and its principal

stereochemistries are octahedral and square planar (4-coordinate) with rather fewer

examples of trigonal bipyramidal (5), square pyramidal (5), and tetrahedral (4).

Octahedral complexes of NiII are obtained (often from aqueous solution by

replacement of coordinated water) especially with neutral N-donor ligands such as

NH3, en, bipy and phen, but also with NCS-, NO2

- and O-donor dimethylsulfoxide,

dmso (MeSO).40

17

17

2.2.3 Uses of Nickel and Its Compounds

The primary use of nickel is in the preparation of alloys such as stainless

steel, which accounts for approximately 67% of all nickel used in manufacture.

The greatest application of stainless steel is in the manufacturing of kitchen sinks

but it has numerous other uses as well.

Other nickel alloys also have important applications. An alloy of nickel and

copper for example is a component of the tubing used in the desalination of sea

water. Nickel steel is used in the manufacture of armour plates and burglar proof

vaults. Nickel alloys are especially valued for their strength, resistance to corrosion

and in the case of stainless steel for example, aesthetic value.

Electroplating is another major use of the metal. Nickel plating is used in

protective coating of other metals. In wire form, nickel is used in pins, staples,

jewellry and surgical wire. Finely divided nickel catalyses the hydrogenation of

vegetable oils. Nickel is also used in the colouring of glass to which it gives a

green hue.

Other applications of nickel include:

- Coinage

- Transportation and construction

- Petroleum industry

- Machinery and household appliances

18

18

- Chemical industry.

Nickel compounds also have useful applications. Ceramics, paints and dyes,

electroplating and preparation of other nickel compounds are all applications of

these compounds. Nickel oxide for example is used in porcelain painting and in

electrodes for fuel cells. Nickel acetate is used as a mordant in the textiles industry.

Nickel carbonate finds use in ceramic colours and glazes.

2.2.4 Nickel and Human Health

The first crystallisation of an enzyme was reported in the 1920's. The

enzyme was urease which converts urea to ammonia and bicarbonate. One source

of the enzyme is the bacterium Helicobacter Pylori. The release of ammonia is

beneficial to the bacterium since it partially neutralizes the very acidic environment

of the stomach (whose function in part helps kill bacteria). In the initial study it

was claimed that there were no metals in the enzyme. Fifty years later this was

corrected when it was discovered that nickel ions were present and an integral part

of the system. The Nobel Prize in Physiology or Medicine for 2005 was awarded to

Barry J. Marshall and J. Robin Warren "for their discovery of the bacterium

Helicobacter pylori and its role in gastritis and peptic ulcer disease".44

19

19

2.3 Theoretical Fundamentals of Liquid-Liquid Extraction

Solvent extraction is a mass transfer process between two phases. Generally,

the solute dissolved in an aqueous solution is extracted into a water-immiscible

organic solvent. When the solute molecule crosses the interface of the two liquids,

its neighbouring solvent molecules changes drastically. If a metal is extracted from

an aqueous solution into an organic non-polar solvent, the metal ion is transferred

across the liquid-liquid boundary as an uncharged particle which can be

electroneutral complex (chelate) formed with the organic reagent or an ion-

association complex. Before the extraction, the metal ion in aqueous solution is in

form of metal-aquo complex while the organic phase consists of the organic

chelating agent. The chelating agent converts the metal-aquo complex into a

neutral metal chelate that distributes itself between the two liquid phases in a

definite ratio8.

Thus, solvent extraction brilliantly highlights the usefulness of phase

distribution as a separation principle. At equilibrium, the distribution between two

liquid phases is governed by Gibbs’ phase rule,9

P + F = C + 2 ……………(1)

Where P is the number of phases in thermodynamic equilibrium with each other, C

is the number of components and F is the number of degrees of freedom, which

means the number of intensive properties such as temperature or pressure.

20

20

When phase rule is applied to a system dealing with a solute distributed

between two immiscible solvents, it gives only one degree of freedom. This

implies that if we choose the concentration of the solute in one phase, the solute

concentration in the other phase is fixed10

. Hence, there is definite relationship

between the solute concentration in each of the solvent phases.

2.3.1 Distribution Law

The distribution of a solute between two immiscible liquids is an equilibrium

process. The distribution equilibrium is the basis for the calculation of the

distribution of a substance between the two phases. The physical basis for

extraction and distribution methods of homogenous mixtures is the Nernst

distribution law. The law states that; a solute will distribute itself between two

essentially immiscible solvents so that at equilibrium the ratio of the concentrations

of the solute in the two phases at a particular temperature will be constant provided

the solute is not involved in chemical interactions10

. Thus, the distribution ratio in

the ideal case depends only on the solvent system and temperature, but not on the

initial concentration of the dissolved substance. Furthermore, in the presence of

several species of molecules, the individual molecule is distributed as if others

were not present. Hence, the law does not consider possible products of side

reactions.

21

21

If a substance, A, is dissolved in water, and this aqueous solution is mixed

with an organic solvent, by bringing it into intimate contact through shaking or

stirring, a portion Aorg is taken up by the organic medium while the remainder, Aaq,

is in the aqueous solution, thus establishing the equilibrium condition below.

Aaq Aorg

In addition, if the volume of the two solvent phases is Vorg and Vaq, the

concentrations of A in both phases are:

[ ]V

AAorg

org

org=

[ ]V

AAaq

aq

aq=

[ ][ ]A

AK

aq

org

D=∴

…………………….(2)

Where KD is called the distribution constant. Graphically, the law may be

expressed as in Fig 2.1

22

22

Figure 2.1: Graphical representation of Nernst Distribution Law.

The value of KD is a reflection of the relative solubility of the solutes in the two

phases.11

2.3.2 Limitation of Nernst Distribution Law

Critical treatment of Nernst distribution law has shown certain limitations as

itemized below:

- It is not thermodynamically rigorous12

. The law does not take cognizance of

the activities of different species. Even for the distribution of a single

monomolecular specie, the law will not hold unless the activity coefficients

in the two phases remain equal regardless of the total concentration of

solute.

Slope = KD,A

[A]org

[A]aq

23

23

- The law did not consider when the solute is present in more than one state.

Polymerization or dissociation of the solute specie, or its association with

other dissolved species may produce a complex set of equilibria such that

the analytically determined ratio of concentrations in the two phases varies

with the total concentration of the solute.

- A given partition coefficient refers to partition equilibrium between two

particular solvents. In ternary diagram, the tie lines are drawn connecting the

composition of the two immiscible phases in equilibrium. Since the

composition of each of these phases is independent of the concentration of

the solute for all points along the tie line, a single constant value for the

partition coefficient of this solute will apply to all mixtures whose

composition is represented by points along this line, irrespective of the

relative volume of the phases. However, for any mixture of composition

represented by points lying along any other tie line, a different (constant)

value of the partition coefficient will apply.

- Strictly, the law is only valid with pure solvents13

.

- The law does not hold when there is solute saturation of a phase.

- It did not consider when there is alteration of conditions like pH during

extraction.

24

24

Consequently, in some cases, instead of getting the perfect graph for the

distribution law, we obtain;

Figure 2.2: Deviation from Nernst distribution law.

Fig.2.2 indicates deviation from Nernst distribution law, which occurs due to

the factors listed above. The first deviation as stated above could be corrected by

considering the partition law from thermodynamic point of view.

2.3.3 Thermodynamic Partition Law Constant

Basically, the thermodynamic condition for a heterogeneous equilibrium

which takes place in partitioning of a substance, A, between an aqueous solution

and an organic solvent is that the chemical potentials, µA, of the solute are equal

for the two phases at constant temperature and pressure.14

[A]org

[A]aq

25

25

[ ] [ ] [ ]αµµ ARTIno

AA aqaqaq+=

[ ] [ ] [ ]αµµ ARTIn

o

AA orgorgorg+=

Since [ ] [ ]µµAA orgaq

=

[ ] [ ] [ ]αµµ ARTIn

o

AA aqaqaq+= = [ ] [ ] [ ]αµµ A

RTIno

AA orgorgorg+= …..(3)

These quantities [ ]µ o

A aq

and [ ]µ o

A org

are the standard chemical potentials and [ ]α A aq

and [ ]α A org

the activities of substance A in aqueous and organic phases respectively.

Such solute will obey Henry’s and Roult’s laws15

. Owing to the fact that the two

phases are liquids, the standard state of the substance refers to unit activity (e.g. in

ideal molar solutions). Equation (3) can be rearranged to give:

( ) ( ) ( )( )

( )ADRTIn

A

ARTInGo

AoA

Ka

aq

orgo

ADorgaq

,, ==∆−=−α

αµµ

The partition equilibrium can be characterized (at constant temperature and

pressure) by the thermodynamic equilibrium constant( )ADKa,. The standard chemical

potentials of the solute A for the two phases as well as the values of o

ADG ,∆ and

( )ADKa, remain constant only for ideal behaviour of the system. This implies that

the mutual solubility of the two liquids is negligible and the solution and the

activity coefficients of the substance are not changed over a wide range of

concentration.

26

26

Consequently, if the solute is strongly solvated, or at high concentration

(mole fraction >0.1), or the ionic strength of the aqueous phase is large (>0.1M) or

changes, equation (2), must be corrected for deviation from ideality according to

this equation21

( )( ) [ ]

( ) [ ]

( )( )

aqA

ADorgA

aqaqA

orgorgAO

AD

KK

A

A

γ

γ

γ

γ ,

,==

……………. (4)

This equation is thermodynamically exact. Where γA is activity coefficient

and KD,A the distribution constant. From equation (4), the distribution constant

remains a constant either for a dilute solution where the activity coefficients

approach unity or for systems where the activity coefficients are controlled by the

use of constant ionic medium method. That is, the ionic strength of the aqueous

phase is kept constant during an experiment by use of a more or less inert medium

like NaClO4. Under such conditions, the activity factor ratio of equation (4) is

assumed to be constant, and KD,A is used as in equation (2) as conditions are varied

at constant ionic strength value. The assumption that the activity factor ratio is

constant has been found to be valid over large solute concentration ranges for some

solute even at high total ionic strengths.

More so, in order to take care of the deviation due to the change of form of

the substance by dissociation, associating, polymerization or formation of complex

27

27

with some other components of the sample or interaction with solvent, another

distribution law is defined.

2.3.4 Distribution Ratio

There are always chemical interactions of the solute with the components in

each phase and these interactions can profoundly affect the concentration of the

solute in both phases. Analytically, the total amount of solute present in each phase

at equilibrium is of prime importance and the extraction process is therefore better

discussed in terms of the distribution ratio, D.

The distribution ratio is defined as the ratio of the total analytical

concentration of a solute in the organic phase (regardless of its chemical form) to

its total analytical concentration in the aqueous phase usually measured at

equilibrium16

. In a two phase system, taking into consideration the existence of a

solute in several chemical forms, A1, A2, A3, …Ax, the distribution ratio is given

by;

D = [ ][ ] totalaq

totalorg

A

A

,

,

= [ ] [ ] [ ][ ] [ ] [ ]

)5....(....................................

.......

21

||

2

|

1

aqxaqaq

orgxorgorg

AAA

AAA

+

+

It is important to briefly distinguish between the distribution constant, KD,

which is valid only for single specified specie and the distribution ratio, D, which

28

28

may involve sum of species of the kind indicated by the index, and thus is not

constant. Therefore, any tendency for the solute to be distributed abnormally in

either phase will show variation from the normal distribution ratio. This variation

occurs due to the fact that the same molecular species is not present in both phases,

because of the tendency of the solute to change its form via association,

dissociation or polymerization. But when the chemical form of the solute remains

the same in both phases, then the value of the distribution constant and the

distribution ratio becomes equal.

2.4 Efficiency of Extraction

Owing to the fact that it is not possible to extract a solute completely from

either an aqueous phase into an organic phase or vice-versa, thus we study the

efficiency of extraction, E.

phaseaqueousinsoluteofionconcentratphaseorganicinsoluteofionconcentrat

phaseorganicinsoluteofionconcentratE

+=

If the concentration of the solute in the organic phase is [A]orgVorg and its

concentration in the aqueous phase is [A]aqVaq,

[ ][ ] [ ] aqaqorgorg

orgorg

VAVA

VAE

+=∴

Dividing through by [A]aqVaq

29

29

[ ] [ ][ ] [ ] 1+

=aqaqorgorg

aqaqorgorg

VAVA

VAVAE

[ ][ ]

[ ][ ]

1+⋅

⋅

=

aq

org

aq

org

aq

org

aq

org

V

V

A

A

V

V

A

A

1+

=

aq

org

aq

org

V

VD

V

VD

E

Multiplying both sides with Vaq/Vorg

( )orgaq VVD

DE

+= ................................. (6)

Obviously, the efficiency of an extraction depends on the magnitude of D

and on the relative volumes of the liquid phases.

For phases of equal volumes, i.e., Vaq = Vorg, the efficiency of extraction

becomes:

1+=

D

DE ……………………..(7)

2.4.1 Percentage Extraction

Practically, the term percentage extraction is more informative than

distribution ratio and it can be correlated with the distribution ratio by the

following equations.

30

30

( )orgaq VVD

DE

+=

100% ………………..(8)

But when Vorg = Vaq, then the equation becomes

1

100%

+=

D

DE ………………………(9)

At very low or high values of D, percentage extraction becomes less

sensitive to change in D. Example, at a phase volume ratio of unity, for any value

of D below 0.001, the solute may be considered to be quantitatively retained in the

aqueous phase. Similarly, at large D values, the change in the value of percentage

extraction is negligible. That is to say that percentage extraction depends on the

value of D up to a limit.

Consider the liquid-liquid distribution plots in Fig 2.3

Figure 2.3: The distribution ratio D for three different substances A, B, and C, plotted against the variable Z of the

aqueous phase. Z may represent pH, concentration of extractant in organic phase and others.

0.1

1

D

10

Z

A

B

C

1 2 3

31

31

Figure 2.4: Same systems showing percentage extraction against Z.

In many practical situations, a plot like Fig 1.3 is less informative than that

of Fig 2.4.

Furthermore, percentage extraction curves are particularly useful for

designing separation schemes.13

2.4.2 Separation Factor

If two solutes A1 and A2 are present in a solution in an initial concentration

ratio [A1]/[A2], then upon extraction their concentration ratio in the organic phase

would be [A1]F1/[A2]F2, where F1 and F2 are the corresponding fractions extracted.

The ratio F1/F2, which is the factor by which the initial concentration ratio is

changed by the separation, is a measure of separation. A corollary measure of

separation which represents the change in the ratio of concentrations remaining in

the aqueous phase is (1-F1)/(1-F2)17

.

100%

E

50

0 1 2 3

Z

A

B

C

32

32

Consequently, the ability to separate two solutes depends on the relative

magnitudes of their distribution ratio. In solvent extraction, separation factor18

, β,

is defined as the ratio of the distribution ratio of the two solutes in a system. If the

corresponding distribution ratio of A1 and A2 are D1 and D2, then the separation

factor becomes;

2

1

DD

=β ……………………..(10)

This factor indicates the tendency for the solute A1 to be separated more

readily from aqueous phase into organic phase than solute A2. Thus, two species

can only be separated with one extraction when the value of D1 and D2 are grossly

different. In addition, two solutes whose distribution ratio differs by a constant

factor would be separated most efficiently if the product, D1D2, is unity. As an

illustration of this principle, solutes A1 and A2 with distribution ratios 103 and 10

1

respectively, if present in equal quantity, then single extraction would remove

99.9% of A1 and 90% of A2. A much more efficient extraction would be obtained

if, using the same factor of 100 between the distribution ratios, the two distribution

ratio were 101 and 10

-1. In this case, respective fraction extracted would be 90%

and 10%.

33

33

2.5 Quantitative Treatment of Solvent Extraction Equilibrium

Chelate complexes of many metals are known19-21

and with a given chelating

agent, the properties of the complexes change from one metal to another. This

chelate is a complex composed of a central metal atom and one or more

multidentate ligand22

. In solvent extraction, the extraction of a metal ion, M+ with

an organic reagent, HL, forming a chelate MLn soluble in an organic solvent is

expressed by the equilibrium;

nHM aqorgnorg

n

aqMLnHL

+++−−−−−+

)(, ………….(11)

The extraction constant expression can be written as;

[ ] [ ][ ] [ ]n

orgaq

n

n

aqorgn

exHLM

HMLK

+

+

= ………………………(12)

For example, the extraction of an aqueous solution of copper ion with a chloroform

solution of 8-hydroxyquinoline (oxime) forms the copper-oxine chelate which is

extracted into chloroform, and the proton released increases the acidity of the

aqueous phase23

.

From equation (12)

If several simplifying assumption are made;

- The concentrations of chelate species other than MLn are negligible

- The concentrations of hydroxyl or other anion coordination complexes are

negligible

34

34

- The reagent HL and the chelate MLn exist as simple undissociated molecules

in the organic phase, then;

Substituting D from equation (5) into equation (12) gives:

[ ][ ]n

org

n

aq

exHL

HDK

+

= …………………………(13)

[ ]

[ ]n

aq

n

org

ex

H

HLKD

+=∴ ………………………(14)

Thus for a given reagent and solvent, the extraction of the metal chelates is

dependent only upon pH and the concentration of reagent in the organic phase but

independent on the initial metal concentration. In practice, a constant and large

excess of reagent is used to ensure that all the metal complexes exist as MLn and D

is then dependent only on pH, i.e.,

[ ] n

aqex HKD−+= ……………..…..(15)

The equation above shows that the distribution ratio varies inversely as the

exponential power of hydrogen ion concentration. The logarithm of equation (15)

gives;

log D = log Kex + npH ……………………….(16)

From equation (7) we have that;

( )E

ED

−=

100 ………………………(17)

The log of equation (17) gives;

35

35

log D = log E – log (100 – E) ……………….(18)

Combining equation (16) and (18) gives;

log E – log (100 – E) = log Kex + npH……….(19)

Equation (19) defines the extraction characteristics for any chelate system, and is

represented graphically in Fig 2.5 for mono, di and trivalent metals, i.e., n = 1, 2

and 3 respectively. Also, it indicates the pH range over which a metal will be

extracted.

Figure 2.5: Extraction as a function of pH for metals of different formal valencies.

The position of the curves relative to the pH scales are determined by the

value of Kex. Therefore, the more acidic the reagent or the stronger the metal

complex, the lower the pH over which the metal will be extracted. Furthermore,

increased reagent concentration has a similar effect.

75

25

50

100

M3+

M2+

M+

pH1/

pH1/

pH1/

2

2 4 6 2 4 6 2 4 6 pH

% e

xtr

act

ion

36

36

The pH at which 50% of the metal is extracted, i.e., pH1/2, can be used to

assess the degree of seperability of two or more metals.

At E = 50, equation (19) reduces to;

log Kex = -npH½ …………………………(20)

Substituting log Kex into equation (16);

log D = n(pH – pH½ ) ……………………….(21)

For two metals with separation factor, β=D/'D

'' where D

' > D

'' and

log β = logD' - logD

'' …………………………..(22)

Therefore, for extraction at a specified pH;

−−

−= ΙΙΙΙΙΙ

21

21log pHpHnpHpHnβ ………(23)

If n' = n

'', i.e. metals have the same formal valency,

Logβ = n∆pH½ ……………………….(24).

Assuming that logβ should be at least 5 for an essentially quantitative separation by

a single extraction, ∆pH½ should be 5, 2.5 and 1.7 for pairs of mono-, di- and tri-

valent metals respectively. Selectivity by pH control is greatest, therefore, for

trivalent metals and least for monovalent. This is reflected in the slope of the

curves, which are determined by n and decrease in the order;

M3+

> M2+

> M+.11

37

37

2.6 Extraction Methods in Solvent Extraction

The two phases may be brought into contact with one another discretely or

continuously, giving rise to three common ways of performing solvent extraction;

batch, continuous and countercurrent. The way selected for any particular

extraction will depend upon the relative values of the distribution ratios of the

components in the original mixture, the equipment available, and convenience.

2.6.1 Batch Extraction

Batch extraction is the simplest and most widely used method employed

where a large distribution ratio for the desired separation is readily obtainable11

.

When the value of the distribution ratio for the desired constituent is large (10 or

greater for equal volume of two phases) and considerably different from those of

other components in the mixture, then batch method is preferred. Under these

conditions, a very large percentage of the solute will pass into the extracting liquid

with only a single equilibrium stage. In this method, a liquid containing the

dissolved solute is shaken or stirred with a second, immiscible liquid in a closed

container until partition equilibrium has been attained. Usually, the apparatus used

for this method is quite simple, a separatory funnel of some convenient design24

.

After shaking, the mixture is transferred into the separatory funnel and

allowed to settle. The denser phase is drained through the stop clock and collected

in another vessel. The phase with the desired constituent is then used for any

subsequent operation.

Figure 2.6: Separatory funnel of different designs.

We employ successive extraction to extract more solutes or when D is small.

Successive Extraction

The extraction is done successively and finally the extracts are combined.

The more the number of extraction, the higher the am

However, it can be shown that for a given volume of extracting solvent, it is more

effective to divide it into several small portions and use each portion successively

rather than to make a single extraction with the whole vo

can be proved mathematically as follows:

Suppose Wo

aq of a solute was originally present in V

phase. Let the solute be extracted with successive portions V

phase. Let Waq and Worg be the weights of the solutes in aqueous and organic

38


funnel of different designs.



The more the number of extraction, the higher the amount extracted, up to a limit.



rather than to make a single extraction with the whole volume. This last statement

can be proved mathematically as follows:

of a solute was originally present in Vaq(mL) of the aqueous

phase. Let the solute be extracted with successive portions Vorg(mL) of the organic

be the weights of the solutes in aqueous and organic

38




ount extracted, up to a limit.



lume. This last statement

(mL) of the aqueous

(mL) of the organic

be the weights of the solutes in aqueous and organic

39

39

phase respectively. The concentration values in aqueous and organic phases are Caq

and Corg respectively.

)()()( orgaq

o

aq WWW += (Wo is the total solute)

)(aqW = )()( aqaq VC

)()()( orgorgorg VCW =

Fraction of Solute Remaining In Aqueous Phase After First Extraction

( ) ( )orgorgaqaq

aqaq

o

aq

aq

VCVC

VC

W

WF

)()(

)()(

)(

)(

1

1

1

+==

1)(aqW = weight of solute remaining in aqueous layer after 1

st extraction.

Dividing through by 1)(aqC

)()(

)(

)(

)(

)(

)(

)(

1

.1

1 orgaq

aq

org

aq

org

aq

aq

DVV

V

VC

CV

VF

+=

+

=

)()(

)(

)(

)( 1

orgaq

aq

o

aq

aq

DVV

V

W

W

+=∴

+=

)()(

)(

)()( 1

orgaq

aqo

aqaq DVV

VWW ………….....(25)

Equation (25) enables the determination of the amount of solute that will

remain unextracted in the aqueous phase in a single equilibrium stage, provided the

volume of the two liquid phases and the distribution ratio for the solute in the

system are known. In a similar fashion, it can be shown that;

40

40

aqorg

orgo

aqorgVDV

DVWW

+=)( ……………..(26)

Equation (26) is employed in the calculation of the amount of solute that will

be extracted into the organic phase in a single equilibrium stage. If there is any

mutual solubility of the two liquids, the equilibrium volumes of the two phases will

not be the initial volume unless each liquid has been pre-saturated with the other

before it is used.

Fraction of Solute After Second Extraction

( ) orgorgaqaq

aqaq

o

aq

aq

VCVC

VC

W

WF

22

2

1

2

)(

)()(

)(

)(

2+

==

Dividing through by 2)(aqC ;

orgaq

aq

org

aq

org

aq

aq

DVV

V

VC

CV

VF

+=

+

=

.2

2

)(

)(

2

( ) ( )

+=∴

orgaq

aq

aqaq DVV

VWW 12 ……………..(27)

If we substitute the value of 1)(aq

W into this expression;

( )

2

2

+=

orgaq

aqo

aqaq DVV

VWW ………………(28)

If we continue for the 3rd

extraction;

41

41

( )

3

3

+=

orgaq

aqo

aqaq DVV

VWW

Then for nth extraction;

( )

n

orgaq

aqo

aqaq DVV

VWW n

+= ……………..(29)

Equation (29) enables us to determine the amount of the solute remaining in

aqueous phase after nth extraction.

The total amount extracted into the organic phase after nth extraction is given by;

( )naq

o

aq WW −

n

orgaq

aqo

aq

o

aqDVV

VWW

+−=

+−=

n

orgaq

aqo

aqDVV

VW 1 ……………….(30)

Consequently, knowing the value of the distribution ratio for a solute and the

volumes of the two phases, the number of extractions required to obtain the desired

degree of extraction can be determined.

Graphical representation;

The fraction remaining in aqueous phase is given by;

( )

n

orgaq

aq

o

aq

aq

DVV

V

W

W n

+= …………………(31)

42

42

Figure 2.7: The graph of ( )

o

orgaqWW n against nth number of extractions

From the graph, it can be observed that it is not possible to extract all the

solute from the aqueous phase, even at infinite number of extractions.

Importantly, since the term containing the volume ratio in equation (31) is

exponential in n, it follows that for a given volume of organic extractant, Vorg,

more solute is extracted when extracting the aqueous solution n times with Vorg/n

portions of the organic liquid, than when extracting once with the entire volume,

Vorg. When the distribution ratio is small, several successive extractions with fresh

portions of extractant can be employed to remove sufficient solute from the

aqueous phase but such procedure is often tedious or involves excessive volume of

the extractant25

.

2.6.2 Continuous Extraction

Continuous extraction method solves the problem of successive batch

extraction. It makes use of a continuous flow of immiscible solvent through a

Waq(n) / Woaq

nth extraction

solution, or a continuous counter

stripped and fresh solvent is added continuously from a reservoir or recycled by

distillation. Most continuous extraction devices operate on the same general

principle which consists of distilling the extracting solvent from a boiler flash,

condensing it and passing it continuously through the solution being extracted. The

figures below illustrate two types of apparatus used for this purpose;

Figure 2.8: Continuous extraction apparatus

solvent heavier than water.

2.6.3 Discontinuous Countercurrent Extraction

This method has repeatedly demonstrated that it stands high in the list of

tools available for this purpose by separating more than one compound from

preparations that were thought to be pure by all other techniques

43

solution, or a continuous counter-current flow of both phases. The spent solvent is



h consists of distilling the extracting solvent from a boiler flash,



tion apparatus26

. (a) Extraction with a solvent lighter than water. (b) Extraction with a

Discontinuous Countercurrent Extraction


le for this purpose by separating more than one compound from

preparations that were thought to be pure by all other techniques

43

current flow of both phases. The spent solvent is



h consists of distilling the extracting solvent from a boiler flash,



. (a) Extraction with a solvent lighter than water. (b) Extraction with a


le for this purpose by separating more than one compound from

preparations that were thought to be pure by all other techniques27

. It has

44

44

fractionated and given clear-cut evidence of purity for the individual fractions from

many substances, for which no clear-cut evidence has been obtained by any other

technique28

. Discontinuous countercurrent extraction was devised by Craig and it

enables substances with similar distribution ratios to be separated. The extraction

of this type is very efficient because fresh extractant is brought in contact with the

solute-depleted aqueous phase and then the solute-enriched extractants contacted

with the fresh aqueous phase till the equilibrium state is attained by the system. It

is used for fractionation purposes, and involves the use of a series of separatory

funnels or more elaborate contacting vessels to achieve many individual

extractions rapidly and in sequence.

The apparatus are fully automatic and comprises many extraction tubes

mounted on a shaking rack whose axis is attached to an automatic control unit.

This control unit automatically affects the operations of shaking, settling and

decantation. After each shaking operation, the rack is tilted through 90o and the

upper phase decanted into the next extraction tube. The cycle is repeated for up to

50 transfers. This method has been applied with great success to the fractionation

of organic compounds particularly where the distribution ratios are of same order

of magnitude. This technique is not common in inorganic compounds because of

different distribution ratios of the materials to be separated. Thus, batch and

continuous extraction techniques are easily employed.

Figure 2.9: Two interlocking glass units for Craig counter

Position during transfer.

2.7 Classification of Inorganic Extraction System

It is convenient to classify metal extraction systems on the basis of the

nature of the extactable species. Extraction can only be possible in an inorganic

extraction system if the charged metal

uncharged species; metal ch

complex is formed in the aqueous phase prior to extraction, but sometimes the

complexing agent is dissolved in the organic phase, in which case complexation

and extraction take place simultaneously when the

45

Two interlocking glass units for Craig counter-current distribution. (a) Position during e

Classification of Inorganic Extraction System



extraction system if the charged metal-aquo complexes are converted into

uncharged species; metal chelate, or ion-association complexes. Usually, the



and extraction take place simultaneously when the two phases are shaken together.

45

current distribution. (a) Position during extraction. (b)



aquo complexes are converted into

association complexes. Usually, the



two phases are shaken together.

46

46

Therefore, inorganic extraction system exists in three forms; metal chelates, ion

association complexes and additive complexes.

2.7.1 Metal Chelate

For a reagent to form an uncharged chelate with a metal, the reagent must

behave as a weak acid whose anion is to participate in the complex formation. In

addition, it must contain hydrophobic groups, which reduce the aqueous solubility

of the metal chelate formed. A metal ion Mn+

that is equilibrated with an organic

and aqueous phase is first solvated in the aqueous phase by water molecule to form

metal-aquo complexes. The organic anion from the dissociated organic reagent,

HL, displaces the water molecule in the aquo-metal complex forming neutral metal

chelate, MLn. The metal chelate distributes itself between the aqueous and organic

phase according to Nernst distribution law. For the fact that charge neutrality

reduces electrostatic interaction between the solute and water and the presence of

hydrophobic groups in the metal chelate reduces its aqueous solubility, the overall

solubility of the metal chelate in the organic phase is enhanced.

The equation of the reaction is given by;

( ) OOM xHMlnLH n

n

x 22+−−−+

−+ ……………..(32)

47

47

Typical examples of metal-chelate solvent extraction systems are the inner

complexes formed by 8-hydroxyquinoline, acetylacetone, dithizone and

dimethylglyoxime.

Organic reagents with one anionic group like –OH, -SH and one neutral

basic group like = N and =O are suitable chelating ligand. They can easily replace

coordinated water molecules from metal ions and provide more than one point of

coordination to the metal ion forming chelate compounds which are essentially

neutral and covalently bonded.

Extraction Equilibrium In Chelate Extraction System

The metal chelate extraction process can be thought to consist of four

equilibrium steps. The equilibrium that develops when an aqueous solution of a

divalent cation, such as Zn(II), is extracted with an organic solution containing

large excess of 8-hydroxyquinoline is as follows. The first step involves

distribution of the 8-hydroxyquinoline, HQ, between the organic and aqueous

layers. The second stage is acid dissociation of HQ to give H+ and Q

- ions in the

aqueous layer. The third equilibrium is the complex-formation reaction giving

MQ2. The fourth stage is distribution of the chelate between the two phases. The

fourth equilibrium ensures that MQ2 is not precipitated out of the aqueous

solution29

. The overall equilibrium is the summation of these four reactions given

as;

48

48

+++−−−−−+ )(2)(2)()(22 222)( aqaqorgaq HOHMQHQOHM ………(33)

The extraction constant for this reaction is

[ ]( )

[ ]

[ ]( )aqorg

aqorg

ex

OHMHQ

HMQK

22

2

)(

2

)(2

)(

=

+ ……………………..(34)

Several useful extractive separations with 8-hydroxyqiunoline have been

developed and similar chelating agents are described in the literature.30

.

On a general note, the extraction of metal complex, ML, that can dissociate

in aqueous phase, using a chelating agent, HL can be formulated based on the four

equilibrium steps below.

(1) The chelating agent HL distributes between the aqueous and the organic

phase31

.

(HL)org (HL)aq …………….(35)

[ ][ ]aq

org

HLD HL

HLK =∴

,

…………………….(36)

Where KD,HL is the distribution constant of the ligand, HL.

(2) The chelating agent dissociates in the aqueous phases;

( ) LHHL aqaqaq

−++−−−−− ……………..(37)

[ ] [ ][ ]aq

aqaq

a HL

LHK

−+

= …………………..(38)

Ka is the dissociation constant of the ligand.

49

49

(3) The metal-aquo complex reacts with the reagent anion to form an uncharged

molecule;

( ) On xHMLLOHM aqnaq

n

aqx 2,)(2+−−−+

−+ ……(39)

[ ][ ] [ ]LM

MLK n

aqaq

aq

Fn

n

−+=

…………………..(40)

KF is the formation constant of the metal chelate.

(4) Then the metal chelate is distributed between the organic and aqueous phases.

( ) ( )MLML nn orgaq−−−

…………………(41)

Thus,

[ ][ ]n

n

ML

MLK

aq

org

MLnD=

,

……………………..(42)

Where KD,MLn is the distribution constant of the metal chelate. The analytical

concentration of this metal in the aqueous phase is the sum of the equilibrium

concentrations of its two forms:

[ ] [ ] [ ]MMLM nn aqaqaqt

++=,

……………...(43)

If we assume that the chelate is essentially undissociated in the non-polar

organic solvent, then the distribution ratio (D) will be

[ ][ ] [ ]MML

MLn

n

nD

aqaq

org

++=

………………..(44)

50

50

Furthermore, assuming that the metal chelate distributes largely into the

organic phase,

[ ] [ ]MLM nn

aqaq⟩⟩+

:. Equation (44) becomes;

[ ][ ]M

MLn

nD

aq

org

+=

………………………..(45)

From equation (42)

[ ] [ ]nn MLKMLaqMLnDorg ,

= ………………..(46)

From equation (40)

[ ] [ ][ ]LK

MLM n

aqF

aq

aq

nn

−=+

…………………(47)

From equation (38)

[ ] [ ][ ]+

=−

H

HLKL

aq

aqa

aq

…………………..(48)

From equation (36)

[ ][ ]K

HLHL

HLLD

org

aq,

= …………………..(49)

Substituting for [ ]MLn org

and [ ]Mn

aq

+ in equation (45);

[ ] [ ][ ]ML

LKMLKD

n

n

aq

n

aqFaq

MLnD

−

=

.

,

…………(50)

i.e.,

51

51

[ ]LKKDn

aqFMLnD

−=.

,

………...………….(51)

Substituting for [ ]L aq

− in equation (51)

[ ][ ]H

HLKKKD n

aq

n

aq

n

aFMLnD

+=

.

.

, .…….……….(52)

Substituting for [ ]HLaq

in equation (52).

[ ][ ]HK

HLKKKD

nHLD

n

aq

n

org

n

aFMLnD

+=

,

, …………………(53)

Since,

…………………….(53i)

( )

( )n

org

n

orgex

H

HLKD

+= ……………………….(53ii)

The logarithm of equation (53ii) gives.

( ) npHHLnLogKLogDLog orgex ++= …….(53iii)

Hence, the distribution ratio depends on the extraction constant together with

the concentrations of the chelating agents, HL, and the pH of the solution. The

distribution ratio is an experimental parameter and its value does not necessarily

imply that distribution equilibrium between the phases has been achieved.

Furthermore, the experimental value of the distribution ratio can be altered

by varying the solution conditions. For example, consider the extraction of benzoic

exn

n

aFMLnDK

HLDK

KKK=

,

,

52

52

acid, HB, from water (acidified with HCl to suppress dissociation of the benzoic

acid) into an organic solvent such as ether. The only equilibrium solution is given

by;

HBHB orgaq−−− ……………(54)

[ ][ ]aq

org

D HB

HBK =∴ …………….(55)

But if the aqueous phase is not acidified, the benzoic acid dissociates in the

aqueous phase;

BHHB aqaqaq

−++−−− ………….(56)

[ ] [ ][ ]aq

aqaq

a HB

BHK

−+

=∴ …….…….(57)

Also, if benzene is used as the organic solvent in place of ether, the benzoic

acid is partially dimerized in benzene.

( ) ( )HBHBHBorgorg

.2 −−− ……...(58)

[ ][ ]HB

HBHBK

org

org

d 2.

.=

………………..(59)

The ratio of the total concentration of benzoic acid in each phase regardless

of its form is given by

[ ] [ ][ ] [ ]BHB

HBHBHBD

aqaq

orgorg

−+

+=

.2 ………(60)

From equation (57)

53

53

[ ] [ ][ ]

aq

aq

a

H

HBKB +

−= …………..…(61)

From equation (59)

[ ] [ ]HBKHBHBorgdorg

2

. = …………(62)

Substituting equation (61) and (62) into equation (60), we obtain,

[ ] [ ]

[ ][ ][ ]

[ ] [ ]( )

[ ] [ ]

++

=

++

+=

+

HKHB

KHB

H

HBKHB

HBKHB

aq

a

aq

dorg

aq

aq

aaq

orgdorgHB

D

1

2122 …………………...……..(63)

[ ]( )[ ]HK

KK

a

dDHB

D+

+=

+1

21 ……………………………………………....…..(64)

Therefore, at low pH, benzoic acid will be found more in organic layer as D

will be large. But at high pH, when D will be small, benzoic acid will be found in

aqueous layer almost entirely as benzoate ion.

2.7.2 Ion-Association Complexes

Ion-association complexes are uncharged species formed by the association

of ions due to electrostatic forces of attraction. Generally, the complexes are

formed when an anionic complex of a metal ion interacts with cationic species

formed by the dissociation of ligand in the aqueous phase. The three equilibrium

steps that are involved for the formation of these complexes include:

54

54

Firstly, the formation of anionic complex by the replacement of the

aquomolecules attached to the metal ion by the major anions in the aqueous phase.

Secondly, the transfer of the organic reagent from the organic phase to the acidic

aqueous phase where it is protonated and it becomes cationic. Thirdly, the

formation of the ion-association complex by the association of the anionic metal

complex with the cationic ligand. The ion-association complexes are easily

attracted into the organic phase because of the weak solvation in the organic phase

and the electrostatic interaction between the cations and the anions. High dielectric

solvents like dichloromethane and nitrobenzene favour their formation.

2.7.3 Additive Complexes

Additive complexes are either ion associated or chelated but in addition may have

other molecules coordinated to them as ligand. The coordinated molecule may be a

reagent molecule, organic solvent molecule or hydroxyl molecules.

Additive Complexes With Organic Reagents

The metal may be incorporated into a large cation or anion or ion containing

bulky organic group. This, in association with an ion of opposite charge,

constitutes an ion pair, which may be readily extracted by an organic solvent. For

example, copper (I) reacts with 2, 9-dimethylphenanthroline to form a large

univalent cation which associates with a nitrate or perchlorate anion to form a

55

55

compound extractable into chloroform39

. Zinc as ZnCl2-

4 associates with two

tribenzylammonium ions [(C6H5CH2)3NH+] to give an uncharged specie soluble in

xylene. Tetraphenylarsonium perrhenate (C6H5)AS+.ReO

-4 and tetraphenyl

arsonium permanganate (C6H5)AS+. MnO

-4 are examples of such additive

complexes.

In order to illustrate the possible equilibrium in additive complexes of this

type, the associations of perrhenate and tetraphenylarsonium chloride are used as

follows32

KD,L KL(R4As+,Cl-)org (R4As+,Cl-)(aq) R4As+

(aq) + Cl-(aq) ……..(65i)

KD,M KM(R4As+,ReO4

-)org (R4As+,ReO4-)(aq) R4As+

(aq) + ReO4-(aq) ....(65ii)

Where R = C6H5

However, not all additive complexes with organic reagent system behaves

fully in accordance with this simple expression because it usually involves many

other complicated equilibria.

Additive Complexes with Organic Solvents

In these additive complexes, the organic solvent plays a major role in the

extraction processes. These complexes are usually formed when hydrophobic

reagents are employed as organic reagent (solvent), and, or the maximum

coordination number of the metal and the geometry of the ligand are favourable.

56

56

For example, uranyl nitrate, UO2(NO3)2, is extracted from nitric acid solutions by

diethyl ether. The hydrated species, UO2(NO3)2. 6H2O, in the aqueous phase

becomes UO2(NO3)2 . 2[(C2H5)2O]. 2H2O in the organic phase. As can be

observed above, the solvent, ether, replaces four water molecules attached to the

metal chelate to form a less hydrophilic additive complex, which is readily

extracted by non-polar solvent. Other solvents beside diethyl ether have been used,

including derivatives of phosphoric acid such as tri-n-butyl phosphate

(C4H9O)3P=O. Some conditions that favours these reactions are;

- Where the maximum coordination number of the metal and the geometry of

the ligands are favourable.

- Where the solvent easily displaces the coordinated water molecule from the

neutral chelate.

- Where the organic reagent is less hydrophilic and less nucleophilic than the

chelating ligand.

The formation of additive complexes of this type is established by measuring

the distribution ratio D1 and D2 of the metal using two different organic solvents at

the same reagent concentration. Variation of the ratio D1/D2 with pH confirms that

these additive complexes are formed, otherwise it will remain constant.

Additive Complexes with Hydroxyl Groups

57

57

The general formula of these complexes is MLn(OH)p. The extraction of

such complexes requires high alkaline media and an organic solvent with high

dielectric constant33

. The conditions above increase the partition constant thus

enhancing the extraction processes.

2.8 Factors that Influence Stability and Extractability of Metal Chelate

Complexes

The quantitative description of extraction given by Kolthoff and Sandell34

and the theoretical treatment of solvent extraction of metal chelates developed by

some workers35,10

are reviewed by briefly discussing some important points below;

Effect of Acidity (pH)

Equation (14) shows the dependence of the dissociation constant on

hydrogen ion concentration. The hydrogen ion concentration of the aqueous phase

governs the degree of dissociation of the chelating agent and hence the amount of

the anion present in the solution to chelate with the metal ion. The value of D

increases with decrease in hydrogen ion concentration, that is, as the pH increases.

This increment depends on the oxidation state of the metal ion being extracted.

Consequently, a high pH will favour dissociation, chelate formation, and

extraction. Also for a change of one pH unit, for example, the concentration of the

organic reagent being constant, the value of D is increased to 100 times if a

58

58

bivalent metal ion is extracted, or 1000 times for the extraction of a tervalent metal

ion36

. The effect of pH on the extraction of metal ions 1+, 2+, 3+, and 4+ charges

with 0.1M 8-hydroxyquinoline in chloroform is shown in Fig 2.10.

Figure 2.10: Effect of pH on the extraction of monovalent (Ag+), bivalent (Pb

2+), tervalent (La

3+), and tetravalent

(Th4+)

metal ions by 0.10 M 8-hydroxyquinoline in chloroform.

At fifty percent extraction, log D in equation (16) is zero. The pH of the

solution at this point is called pH one-half (pH1/2) and is a constant, characteristic

of the system under investigation23

.

Effect of Organic Chelating Agent

As clearly indicated in equation (14), the distribution ratio and the degree of

extraction increases as the concentrations of the chelating agent increase at

constant pH. But the concentration of chelating agent in the organic phase is

limited by its solubility (8-hydroxquinoline is soluble to the limit of 2.6M in

chloroform). Moreso, there is a shift to lower pH value with increasing

concentration of the chelating agent and this enables the extraction to be carried

out at a pH low enough to prevent hydrolysis.

TH4+

100

50

Pb2+

Ag

+

La3+

0 2 4 6 8 10 12

%E

59

59

Fig. 2.11 shows the extraction curves for the extraction of manganese with

8-hydroxyquinoline in chloroform at a concentration range of 0.1 and 0.0lM. As

can be observed, the pH½ value for manganese decreases from 6.66 to 5.66.

Figure 2.11: Effect of pH on the extraction of colbalt (II) and manganese (II) by 8-hydroquinoline in chloroform.

Sometimes, the chelon forms an adduct with metal chelate, MLn (HL)r, and

an increase in the concentration of the chelating agent will have a greater effect

than that predicted by equation (14) since the dependency upon the concentration

of the chelating agent is now (n + r) instead of just n. Cobalt oxinate and oxine

form such an adduct.

Effect of Masking Agent

When a masking agent (sequestering agent) such as ammonia, tartrate,,

nitrilotriacetic acid, or ethylenediaminetetraacetic acid is present in the aqueous

phase it complexes with the metal ion, thereby competing with the chelating agent

and the value of D is lowered. In solvent extraction, a masking agent is a

compound that can form a non-extractible complex with metal ions in the aqueous

0.01M

0.1M

0.01M

Mn

Co2+

0.1M

100

50

%E

2 0 4 6 8 10 12

pH

60

60

phase. Consequently, at constant concentration of extracting agent and pH, the

presence of masking agent decreases the percentage of metal extracted.

Furthermore, there is a shift of the %E-pH curve to higher pH values in the

presence of masking agent. The magnitude of this pH shift depends on the strength

of the metal-masking agent bond and on the concentration of the masking agent.

The application of masking agents in extraction separation of metals is of

extreme importance. By a selective utilization of the masking agents, it is possible

to extract a particular metal of interest from a mixture of metals, which form non-

extractable ionic complexes and remain masked in the aqueous phase.

Effect of Variation of the Oxidation State of Metal

In solvent extraction, a metal in one oxidation state may bond with a

chelating agent but does not in another oxidation state. Thus, such metal must be

converted to the appropriate oxidation state before extraction.

Example, Fe(II) complexes with O-phenanthroline while Fe(III) does not.

Therefore, we have to reduce Fe(III) using hydroxylamine hydrochloride before

treatment with the chelating agent. Also, Pd(II) complexes with dithizone while

Pd(IV) does not.

Effect of Salting-Out Agents

In solvent extraction, salting-out agents are ionic salts which when dissolved

in the aqueous phase increase ionic strength of the medium, hence causing a

61

61

decrease in the solubility of the non-ionic metal chelate in the aqueous phase.

Consequently, salting-out agents increase D of metal during extraction.

The mechanism of salting-out effect is as follows;

- The ionic activity of the extractable metal chelate is decreased.

- The highly hydrated ions of the salt deplete the effective concentration of

free H2O molecules, thereby making the metal chelate formation easy.

- The dielectric constant of the aqueous phase is increased, resulting in

decrease in solubility of the metal chelate in aqueous phase, but increase in

the organic phase.

Effect of the Stability of the Metal Chelate

Equation (39) shows that the stability constant of the metal chelate is

directly proportional to the distribution ratio. Thus, the larger the value of the

stability constant the greater the percent of the metal extracted at a constant pH,

other factors remaining unchanged. Conversely, the more stable the complex the

lower the pH of extraction.

Influence of Organic Solvent

In solvent extraction, choice of organic solvent highly depends on the

solubility of the organic reagent and the degree of immiscibility with the aqueous

phase37

. In addition, the organic solvent should possess the following

properties23,38

;

62

62

- It should have high density difference with the aqueous phase in order not to

form emulsions.

- It should have very low viscosity to permit good contact between the two

phases while shaking.

- The metal chelate should be readily soluble.

- The organic solvent should have low toxicity and inflammability.

- It should have such a property that will make for easy stripping off the

metal.

- It should have high boiling point for easy recovery after extraction.

2.9 Brief Work on Solvent Extraction of Metals under Study

Prior to the introduction of solvent extraction; the separation of cobalt and

nickel was very difficult to achieve, due to the very similar chemical behaviours of

these two metals. One of the early applications of solvent extraction in the

processing of base metals was the separation of cobalt from nickel in chloride

solutions by tertiary amines. The ease and efficiency of separation achievable by

selective extraction of cobalt as the tetrachlorocomplex ion by tertiary amines

(reaction 71), made solvent extraction an attractive route which was soon adopted

industrially.

−− +=+ ClCoClNHRNHClRCoCl orgorg )(423)(3

2

4 )(2 ……………………..(71)

63

63

The main disadvantage of the amine extraction route is the necessity to use

chloride solutions in order to form the extractible cobalt chloro-complexes.

Many efforts have been made to establish a viable cobalt-nickel solvent

extraction separation process for sulphate solutions. Reagents used for this

application have all been cation exchangers comprising various alkyl phosphorous-

based acids. Early on, the familiar di-2-ethylhexyl phospholic acid (DEHPA) was

used. It was found that if extraction was carried out at 50-600C, instead of ambient

temperature, the pH window separating cobalt and nickel expanded to about one

pH unit, enabling a satisfactory separation to be achieved by careful pH control

during extraction. A process based on this principle was operated industrially in

South Africa. Later on it was shown that better separation factors between cobalt

and nickel were achieved by the use of alkyl phosphonic and phosphinic acids45

.

Processes using alkyl phosphonic acids have been operated in Japan and, more

recently, in Finland46

. The commercial availability of alkyl phosphinic acids, in

particular 2,4,4-trimethyl-pentyl phosphinic acid, which is marketed under the

trade name, Cyanex 272, by the American Cyanamid company, resulted in many

papers being published on the application of this reagent to cobalt-nickel

separation47

64

64

Solvent extraction method was widely applied in separation of metal ions

from aqueous phase by contacting with an organic phase which contains a metal

selective organic reagent dissolved in a diluents48

.

Emad and Khalid49

reported the use of N,NI-carbonyl difatty amides

(CDFAs)for the extraction of cobalt(II) and separating it from other elements

(Ni(II), Cd(II), Mn(II) and Fe(II)) owing to its selectivity for cobalt(II) and its fast

rate of extraction from aqueous via: Effect of pH, solvent, shaking time, and

concentration of Co(II).49

Since the first commercial process using di-2-ethyl hexyl phosphoric acid

(D2EHPA) was developed by RITCEY et al50

, the organophosphorus extractants

have been proved to be primary solvents for separation cobalt from nickel in acidic

media solution51,52,53,54

. 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester was

developed and marketed as PC- 88A by the Daihachi Chemical Industry, and as

SME 418 by Shell used by the Nippon Mining55,56,57

. Under the same extraction

condition, this reagent gives a Co/Ni separation factor about 200 times greater than

that of D2EHPA. A similar solvent with PC-88A, named as P507, was produced in

early 1970s and found commercial application of Co/Ni separation in one Chinese

metallurgical enterprises in 198158

. Several phosphinic acids, such as Cyanex272,

Cyanex301 and Cyanex302, were developed by Cytec Chemicals. Cyanex272 is

being used in the Outokumpu Plant in Finland59

.

65

65

A survey of literature has reviewed the cobalt-nickel separation factors from

sulphate media using phosphoric, phosphonic and phosphinic acids60

. The

separation ability of cobalt and nickel increases in the order

phosphinic＞phosphonic＞phosphoric acid due to the increasing stabilization of

tetrahedral coordination compound of cobalt with the extractant in the organic

phase, because the tetrahedral compound is more stable than the octahedral one.

Out of the three types of extractants, Co-Ni separation factor of phosphoric acid,

such as D2EHPA, is usually below 10. Cyanex solvents are usually most expensive

in despite of most selective.

Much basic research has been carried out on the extraction of cobalt and

nickel from sulphate solution. Fujimoto et al61

have patented a process for cobalt-

nickel separation using PC-88A. The date shows 45.8 g/L cobalt extraction at pH

4.5 from a solution containing 30 g/L each of Co and Ni using 20%(volume

fraction) PC-88A. Devi et al62

studied the Co-Ni separation effects of D2EHPA,

PC-88A and Cyanex272 from a sulphate solution containing 0.01 mol/L metal ions

each and 0.1 mol/L Na2SO

4. In the case of extractions with 0.05 mol/L Na-PC88A,

the separation factor can reach over 1 000 at equilibrium pH range of 5.75−6.2, but

was extremely pH sensitive.

66

66

However, studies concerning extraction separation of cobalt and nickel using

PC-88A from chloride media are little. Sarangi et al63

indicated that the Co-Ni

separation factor can reach 37−72 at equilibrium pH range of 5.5−5.7 using

different concentrations of PC-88A from a chloride solution. Separation factors

obtained with binary mixture of extractants gave a value 5.6 times higher in the

case of Na-PC88A as extractant and Na-Cyanex 272 as synergist than that for Na-

Cyanex 272 alone. LIU et al64

reported Co-Ni separation using P507 as extractant

in sulfate and chloride solution.

2.10 Previous Work on 3,5-Bis-[(2-Hydroxy-Benzylidene)-Amino]-Benzoic

Acid

The compound 3,5-Bis[(hydroxyl-benzylidene)-amino]-benzoic acid is a

salen compound belonging to a class of Schiff’s base. Schiff’s bases (also called

azomethines or imines) are named after Hugo Schiff (1834-1915). These

compounds are functional groups with the general formula R1R2C=N-R3. They

contain carbon-nitrogen double bonds in which nitrogen atom are connected to an

aryl or alkyl group. The role of the R3 group is to stabilize the imine Schiff’s

base.65

Strictly speaking Schiff bases are compounds having a formula RR’C=NR”

where R is an aryl group, R’ is a hydrogen atom and R’’ is either an alkyl or aryl

67

67

group. However, usually compounds where R” is an alkyl or aryl group and R’ is

an alkyl or aromatic group are also counted as Schiff bases66

.

These compounds are synthesized from and aromatic imine and a carbonyl

compound (e.g. aldehydes, ketones; scheme1) through a nucleouphilic addition,

which leads to the formation of a hemi-aminal, and the consequent dehydration to

generate an imine. The reaction of 4, 4’-diaminodiphenyl ether with O-vanillin can

be regarded a typical reaction67

.

Application of aldehydes will lead to the formation of imines of R1HC=N-R2

type. This is while the result of the same reaction with ketones are R1R2C=NR3

amines (it should be noted that reaction of ketones occurs more steadily than that

of aldehydes. The overall mechanism of forming Schiff’s bases is according to the

multi-step reaction below:

C O H2 N C N H2O

aldehyde orketone

amineSchiff base orimine ……………………(72)

An imine (Schiff base), such as that formed from O-vanillin, forms

complexes with metal ions via N and O donor atoms. The steric and electronic

effects around the metal core can be finely tuned through appropriate selection of

electron withdrawing or electron donating substituent in the Schiff bases. These N

and O atoms induce two opposite electronic effects: the phenolate oxygen is

+

68

68

regarded as a hard donor, which stabilizes the higher oxidation states while the

imine nitrogen is a softer donor and will hence stabilize the lower oxidation states

of the metal ion68

.

The Schiff base class is very versatile as compounds can have a variety of

different substituent and they can be unbridged or N, N’-bridged. Most commonly

Schiff bases have NO or N2O2-donor atoms but the oxygen atoms can be replaced

by sulphur, nitrogen, or selenium atoms69

. Schiff bases can be classified into:

- Symmetric Schiff base: Salen, Salophen

- Asymmetric Schiff base: Salen, Salophen and Hydrazone

2.10.1 Salens

The term “Salen” is the abbreviation of the name of a popular chelating

ligand used in coordination chemistry and homogenous catalysis. i.e.

salicylaldehyde and ethylenediamine

Salen H2 forms complexes with most transition metals, where in most cases

the metal takes a square pyramidal or octahedral coordination, M(salen)L and

M(salen)L2. Examples include Vo(salen) and Co(salen)Cl(pyridine).

Numerous salen-derivatives are known. E.g. the ligand abbreviated “Salph”

is derived from the condensation of 1,2-phenylenediamine and salicylaldehyde70

.

69

69

2.10.1.1 Salen Ligand Synthesis

Salen Ligands are prepared by the condensation of a salicylaldehyde

derivative with a 1,2-diamine, and the simplest, a chiral version (Fig 2.12) is

prepared from salicylaldehyde and ethylenediamine, abbreviations of which

combine to give this ligand class its name.

N N

H H

t - Bu

t - Bu t - Bu

t - Bu OHHO

Fig 2.12: (R,R)-Salen

Chiral versions of this tetradentate bis(imine) ligand are accessed simply by

using chiral 1,2-diamines, although Ligands derived from other diamines (1,3-,

1,4-, etc.) are often included in this class. Chiral salen Ligands have several

attractive features that constitute the basis for their utility in asymmetric reactions.

The salicyladehyde and diamine components are synthetically accessible and their

condensation to generate the salen ligand generally proceeds in nearly quantitative

yield. Metal complexes of salen Ligands are readily prepared from a variety of first

row and second row transition metal salts as well as main group metals. Once the

appropriate metal for the desired reactivity has been identified, the modularity of

70

70

synthesis of salen Ligands allows for the systematic tuning of catalyst steric and

electronic properties by modification of the metal counterion, the chiral diamine or

the salicylaldehyde components71

. Although a large number of ligand structures

are thus accessible, it is striking that salen 1 has often been found to be the

optimum ligand for a broad range of reactions catalyzed by several different metals

(Fig 2.13)

N

t - Bu

t - Bu t - Bu

t - Bu OO

N

M

M = H.H 2. M = Mn-Cl 3a. M = Co

3b. M = Co-OAc 3c. M = Co-SbF6 4a. M = Cr-Cl

4b. M = Cr-N3 4c. M = Cr-BF4 4d. M = Cr-SbF6

5a. M = Al-Cl 5b. M = Al-N3

Fig 2.13: Metal complexes of salen ligand 1 utilized in catalytic asymmetric processes

Salen ligand 1 was first identified in the context of Mn-catalyzed

asymmetric epoxidation of unfunctionalized olefins72

.

The industrial synthesis of ligand 1 utilizes very inexpensive raw materials

(scheme1)73

and the ligand is now available commercially in both laboratory and

bulk quantities.

71

71

t-Bu

t-Bu

N

N

N

NHOAc. 1200C

OH

t-Bu

OH

CHO

t-Bu

+

NH 2

NH 2

OH

OH

HO 2C

HO 2C

HOAC, M eOH

NH 2

NH 3

OH

OH

HO 2C

O2C

N

OHt - Bu

t - Bu t - Bu

t - BuHO

Nt-Bu

OH

CHO

t-Bu

2+ NH2NH3

OHOH

HO2C O2C

K2CO3

Toluene/EtOH

Mandal et al74

reported the synthesis of N,N,-Bis(salicylidene)-3,4-

diaminobenzoic acid and its nickel(II) complex, by reacting 2 moles of

salicylaldehyde with 1 mole of 3,4-diaminobenzoic acid in 85% yield.

COOH

NH2

NH2

O

OH

Two equiv. in MeOH

(1)

(2) NiCl2/MeOH, 600CNiN

N

O O

CO2H

...(73)

……..(74)

72

72

Laye and Sanudo75

also reported the synthesis (in situ) of 3,5-Bis[{2-

hydroxyphenyl)methylene}amino]benzoic acid by the condensation reaction of 2

moles of salicylaldehyde with 3,5-diaminobenzoic acid:

NH2H2N

HOOH

H

O

HHO

OH

N

HO

N2+

In this research, we are able to synthesize, isolate and re-crystallize the

compound: 3,5-Bis-[(hydroxyl-benzylidene)-amino]-benzoic acid, with its

cobalt(II) and Ni(II) complexes respectively.

...(75)

73

73

CHAPTER THREE

3.0 EXPERIMENTAL

3.1 Equipments

Weighing balance (Metler E-2000), Metler Toledo 300 pH meter, Universal

pH indicator paper, Ground-glass-stoppered extraction bottles, Orbital shaker,

Separatory funnel, Agilent 8453 UV/Visible spectrophotometer, FTIR Shimadzu

spectrophotometer, Agilent-NMR-vnmrs400MHz spectrometer.

3.2 Preparation of Metal Stock Solutions

Cobalt (II):

Standard solution of cobalt(II) containing 100 µg/cm3

was prepared by

dissolving 0.10092 g of cobalt chloride hexahydrate, CoCl2.6H2O, in some quantity

of distilled water and then making it up to mark in 250 cm3 standard volumetric

flask.

Nickel (II):

Standard nickel(II) solution (100 µg/cm3) was prepared by dissolving

0.119697 g of nickel sulphate hexahydrate, NiSO4.6H2O, in some quantity of

distilled water and then making it up to mark in 250 cm3 standard volumetric

flask.

pH Solutions:

74

74

Clark and Lubbs’ procedure8 was used to prepare standard buffer solutions

covering the pH range 1-13 with standard solutions of the following acid/salt

systems:

Hydrochloric acid/potassium chloride,

hydrochloric acid/potassium hydrogen phthalate,

potassium hydrogen phthalate/sodium hydroxide,

and boric acid/sodium hydroxide.

Universal indicator paper and Metler Toledo 300 pH meter were used to

check the pH values of the solutions.

Acid Solutions:

Acid solutions of high concentrations were prepared and lower

concentrations obtained by diluting as required.

5 M HCl: A volume of 42.96 cm3 of 36% HCl (s.g. 1.18) was diluted in some

quantity of distilled water and made up to mark in 100 cm3 standard

volumetric flask.

5 M H2SO4: A volume of 27.78 mL of 98% H2SO4 (s.g. 1.84) was diluted in some


volumetric flask.

75

75

5 M HNO3: A volume of 31.69 cm3 of 70% HNO3 (s.g. 1.42) was diluted in some


volumetric flask.

5M HClO4: A volume of 49.62 cm3 of 61% HClO4 (s.g. 1.66) was diluted in some


volumetric flask.

Salt Solutions:

Sodium chloride, 2 M: A 11.689 g of NaCl was dissolved in some quantity of

distilled water and made up to mark in 100 cm3 standard volumetric flask.

Sodium sulphate, 2 M: A 28.409 g of Na2SO4 was dissolved in some quantity of


Potassium Nitrate, 2 M: A 20.221 g of KNO3 was dissolved in some quantity of


Sodium Perchlorate, 2 M: A 24.488 g of NaClO4 was dissolved in some quantity of


The above high concentrated salt solutions were diluted as required and

fresh solutions were prepared monthly.

Masking Agents Solutions:

Potassium Cyanide, 1 M: A 3.256 g of KCN was dissolved in some quantity of


76

76

Potassium Sodium Tatrate, 1 M: A 14.112 g of potassium sodium tatrate was

dissolved in some quantity of distilled water and made up to mark in 50 cm3

standard volumetric flask.

Ammonium thiocyanate, 1 M: A 76.12 g of NH4SCN was dissolved in some


volumetric flask.

Potassium hydrogen Phthalate, 1 M: A 10.211 g of KH(C8H5O4) was dissolved in

some quantity of distilled water and made up to mark in 50 cm3 standard

volumetric flask.

EDTA, 0.4 M: A 7.445 g of EDTA-disodium salt dehydrate, (C4H6N2O4Na2) was

dissolved in some quantity of distilled water and made up to mark in 50 cm3

standard volumetric flask.

Sodium oxalate, 1 M: A 6.700 g of Na2C2O4 was dissolved in some quantity of


Spectrophotometric Reagents Solutions:

Dimethylglyoxime: Dimethylglyoxime is prepared by dissolving 0.50 g of A. R.

dimethylglyoxime in 250 cm3 ammonium solution and diluting to 500 cm

3

with distilled water.65

77

77

Thiocyanate Ligand: A standard stock solution of ligand (SCN-) 152.588 g/L was

prepared by dissolving (50 g) of NH4SCN in distilled water. The volumetric

flask (250 cm3) was completed to the mark with distilled water.

66

3.3 Synthesis of 3,5-Bis-[(2-Hydroxy-Benzylidene)]-Amino]-Benzoic Acid.

Synthesis of the Schiff base ligand was carried out according to reported

method67

. The procedure was as follows:

To a solution of 400 mg (3.3 mmol) of salicylaldehyde in 10 cm3 of

methanol was added with stirring on magnetic stirrer a methanolic solution of 250

mg (1.6 mmol) of 3,5-diaminobenzoic acid. From this solution after some time, an

orange-colored crystalline solid precipitated out, which was filtered, dried, and

recrystallized from methanol.

3.4 Synthesis of Co(II) and Ni(II) Complexes of 3,5-Bis-[(2-Hydroxy-

Benzylidene)]-Amino]-Benzoic Acid.

Both cobalt(II) and nickel(II) complexes of the N,N/-bis(salicylidene)-3,5-

diaminobenzioc acid were synthesized according to literature.67

The orange Schiff base (100 mg) was dissolved in hot methanol; to this was

added a methanolic solution of equimolar solution of the metals (0.28 mmol), and

the resulting mixture was heated at 600C for 30 minutes. The resulting precipitate

78

78

upon standing at ambient temperature was filtered and recrystallized from a

mixture of methanol and chloroform.

3.5 Determination of the Composition of the Extracted Species

Job’s Continuous variation Method

The experiment was performed as described in literature68

as follows: An

equimolar concentration of Co(II) or Ni(II), and the ligand was mixed. The mixing

was in such a way that a bottle containing X cm3 of the metal solution in 1.00 mL

of aqueous phase also contained (1-X) cm3 of the reagent in the same quantity of

the organic phase. Thus, the sum of the metal ion concentration and ligand

concentration was constant for each solution. The phases were shaken, centrifuged

and separated. The absorbance of the organic phase was measured at wavelength of

maximum absorption of the complex.

3.6 Extraction Procedures

Agitation Speed: The orbital shaker was set at a high constant speed such that

increase in agitation speed did not change the extraction rate.

Equilibration Temperature: Solutions of both organic and aqueous phases were

allowed to equilibrate at room temperature 28±1oC before mixing for extraction. In

addition, all extractions were performed at the temperature stated above.

79

79

Equilibration Time: The time taken by the metal complex to transfer from

aqueous phase into organic phase.

The procedure for the determination of equilibration time is as follows:

0.6mL of 10 µg/cm3 solution of metal ion, Co(II) or Ni(II), was pipetted into

different extraction bottles, 0.2 cm3 of H2B (5%/DMF) was added and each made

up to 6.00 cm3 with distilled water. Equal volume (6.0 cm

3) of chloroform was

added to each bottle. The bottles were shaken and one bottle removed after each of

the desired time interval, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 and 60 minutes.

The two phases were centrifuged and separated. The amount of metal ion left

unextracted in the aqueous phase was determined specrtrophotometrically.

3.6.1 Extraction from Buffer Solution

0.6 cm3 of 10 µg/cm

3 solution of Co(II) or Ni(II), was pipetted into different

extraction bottles representing pH values from 1 to 13. 3.4 cm3 of the buffer

solution of known pH was added into each bottle. The solutions were adjusted to

the appropriate pH using either dilute hydrochloric acid or ammonia solution; 0.2

mL of H2B (5%/DMF) was added and finally made up to 6.0 cm3 with the

corresponding buffer solution.

Equal volume (6.0 cm3) of chloroform was added to each of the extraction

bottle. The phases were equilibrated for the appropriate time determined for each

80

80

metal, centrifuged and separated. The amount of metal ion left extracted and

unextracted in both the organic and aqueous phases were determined

spectrophotometrically after the perchloric acid decomposition.

3.6.2 Extraction from Acid Media

A volume , 0.6 cm3 of 10 µg/cm

3 solution of Co(II) or Ni(II) was pipetted

into different extraction bottles and the appropriate volume of the acid (H2SO4,

HCl, HNO3 or HClO4) was added such that on final dilution to 6.0 cm3 with

distilled water the concentration range 0.001-2.0 M is covered; 0.2 cm3 of H2B

(5%/DMF) was added and finally made up to 6.0 cm3 with distilled water.






3.6.3 Extraction in Salting Out Agents

A volume, 0.6 cm3 of 10 µg/cm


into different extraction bottles. Appropriate volume of salting-out agent (Na2SO4,

NaCl, KNO3 or NaClO4) was added to cover the concentration range 0.001-1.0 M

81

81

in 6.0 cm3 of final solution. A specific volume of the corresponding acid (H2SO4,

HCl, HNO3 or HClO4) was added into the appropriate bottle to give a

concentration at which only partial extraction of the metal ion was obtained; 0.2

cm3 of H2B (5%/DMF) was added and finally made up to 6.0 cm

3 with distilled

water.






3.6.4 Extraction in Complexing Agents

A volume, 0.6 cm3 of 10 µg/cm


into different extraction bottles. Appropriate volume of complexing agent (cyanide,

EDTA, fluoride, phthalate, tartrate or thiocyanate) solution was added to cover the

concentration range 0.001-1.0 M in 6 cm3 of final solution. A specific volume of

acid was added into each bottle such that allows for quantitative extraction of the

metal ion under study; 0.2 cm3 of H2B (5%/DMF) was added and finally made up

to 6.0 cm3 with distilled water.

82

82






3.7 Measurement of Distribution Ratio

After separation of the organic and the aqueous phases, the amount of the

metal ion unextracted in the aqueous phase was analysed spectrophotometrically

after perchloric acid decomposition discussed below. The amount of metal

extracted into the organic phase was also obtained spectrophotometrically.

The distribution ratio of the metal was calculated as the ratio of the

concentration of the metal ion in the organic phase to that in the aqueous phase.

3.8 Spectrophotometric Analysis of the Metal Ions

Cobalt(II) as the Thiocyanate Complex:66

To 3 cm3 of the metal solution 5 cm

3 of acetone was added (reducing the

polarity of water to prevent the dissociation of the complex), followed by 1 cm3 of

concentrated HCl; 1 cm3 of SCN

- solution to produce a blue complex. The

volumetric flask (10 cm3) was completed to the mark with distilled water. The

83

83

absorbance of the cobalt(II)thiocyanate complex was measured at 625 nm against a

reagent blank. The amount of cobalt was determined from the calibration curve.

Nickel(II) as the dimethylglyoxime complex:65

The nickel(II) extraction raffinate (10 cm3) was transferred to a beaker

containing 90 cm3 of water, 5.0 g of A. R. citric acid was added; followed by dilute

ammonia solution until the pH was 7.5. The solution was cooled and transferred to

a separatory funnel. 20 cm3 of dimethylglyoxime solution was added and allowed

to stand for 2 minutes, then 12 cm3 of chloroform. The mixture was shaken for 1

minute; the phases were allowed to settle out. The red chloroform layer was

separated and the absorbance determined at 366 nm against a reagent blank. The

amount of nickel was determined from the calibration curve.

3.9 Calibration Curve

Aliquots of the metal ion solutions were pipetted into separate extraction

bottles such that on final dilution to 10 cm3, the concentration range 0.1-10 µg/cm

3

was covered. The metal ion solution was treated as outlined above and the

absorbance measured at appropriate wavelength. The calibration curve was

constructed by plotting the absorbance against metal concentration. The

84

84

concentrations of each metal ion after extraction were read off from the straight

line graph using the absorbance obtained.

3.10 Separation Procedures

A mixture containing equal amount (10 µg) of Co(II) and Ni(II) in 0.01 M

HNO3, and containing 0.25 cm3 of 1.0 M cyanide was added. Nickel was extracted

with 0.2 cm3 H2B/DMF; the solution was made up to 6.0 cm

3 using distilled water.


bottle. The phases were equilibrated for the appropriate time determined for

Nickel, centrifuged and separated. The extraction was repeated four more times

with fresh 0.2 cm3 of reagent solution and 6.0 cm

3 of chloroform. The aqueous and

the combined organic extract were analyzed spectrophotometrically for both metals

as given above.

85

85

CHAPTER FOUR

4.0 RESULTS AND DISCUSSIONS

4.1 Electronic Spectra

Figures 4.1, 4.2 and 4.3 shows the UV-Visible spectra of H2B, CoB and NiB

complexes respectively. The summary of the results are presented in Table 4.1.

The ligand shows maximum absorption at 346 nm with molar absorptivity of 2917

mol-1

cm-1

which is ascribed to intraligand π-π*

transitions similar to earlier

reported in literature79

.

The CoB complex shows strong absorption peaks of 339 nm (29498.53 cm-

1), 348 nm (28735.63 cm

-1), 370 nm (27027.03 cm

-1) and a weak band at 306 nm

(32679.74 cm-1

) with molar absorptivity of 4770 mol-1

cm-1

, 4201 mol-1

cm-1

, 4009

mol-1

cm-1

and 966 mol-1

cm-1

respectively. Co(II), a d7 in ground state has an

3F

term 25

2 gg et configuration80,81

. The bathochromic shift at 348 nm may be due to

intraligand π-π*

transitions. The peak at 346 nm with molar absorptivity of 2917

mol-1

cm-1

in the ligand shifted to 339 nm (29498.53 cm-1

) in the CoB complex.

This is probably due to ligation of N and O atoms to Co which limited the

availability of their electrons for transition. The NiB complex shows absorption

maxima of 333 nm (30030.03 cm-1

) with a peak at 346 nm (28901.73 cm-1

). Ni(II)

86

86

a d8 in ground state has an

4F term 26

2 gg et configuration the hypsochromic shift of

333 nm (30030.03 cm-1

) is probably due to charge transfer. The intensities in molar

absorptivities of both metal complexes of the ligand (NiB and CoB) shows

symmetry allowed charge transfer (CT) band absorption.82,83

There was no spin

forbidden, and Laporte-forbidden81

d-d transitions.

4.2 IR SPECTRA

Figures 4.4, 4.5 and 4.6 show the IR spectral analysis of the ligand H2B and its

cobalt(II) CoB and nickel(II) NiB complexes. The summary of the result is

tabulated in Table 4.2.

From the spectral result of the ligand, the observed absorbance at 3100-2800

cm-1

centered at 3000 cm-1

was assigned to carboxylic O–H vibration81,82

; the

characteristic absorbance was also seen in the spectrum of both CoB and NiB

complexes indicating that the carboxylic group did not participate in coordination

with the metal in each complexes. Furthermore, the C=O stretch observed at

1687.77 cm-1

, 1688.81 cm-1

and 1687.77 cm-1

, for H2B, CoB and NiB complexes

respectively83,84

confirms the absence of coordination or bond formation with the

carboxylic group. The broad aromatic phenolic O–H85

absorbance 3600 – 3400 cm-

1 centered at 3500 cm

-1 was only observed in the ligand, this broad absorbance

87

87

disappeared in both spectrum of CoB and NiB; these disappearances can be

attributed to the coordination or bond formation between the metal and ligand

through the oxygen group of the ligand. This is further confirmed by the shift of

wavelength of C–O stretch (phenolic) observed at 1289.46 cm-1

of the ligand to

1287.53 cm-1

and 1288.49 cm-1

for CoB and NiB complexes respectively81

. The

aromatic region shows assignment of 2922.25 cm-1

, 2923.22 cm-1

and 2915.50 cm-1

for H2B, CoB and NiB respectively due to vibrations of C–H deformation; 769.94

cm-1

, 759.98 cm-1

and 760.94 cm-1

for H2B, CoB and NiB respectively due to

vibrations of C–H out of plane deformation and 1449.55cm-1

, 1449.55 cm-1

and

1450.52 cm-1

for H2B, CoB and NiB respectively due to vibrations of CC

skeletal ring vibrations82,84

.

The C=N group was assigned 2922.25 cm-1

, 2923.22 cm-1

and 2915.50 cm-1

;

the little changes observed in shift on wavelength is characteristics of soft donation

of unpaired electron on the nitrogen atom to the metal in both complexes of CoB

and NiB.

The additional band observed in only the CoB (415.67 cm-1

; 497.65 cm-1

)

and NiB (471.61 cm-1

; 487.04 cm-1

) complexes were assigned to M–N and M – O

vibrations respectively as reported by El-Ajaily et al86

, and Prakash et al87

.

88

88

Figure 4.1: UV Spectrum of H2B

89

89

Figure 4.2: UV Spectrum of CoB

90

90

Figure 4.3: UV Spectrum of NiB

91

91

Table 4.1: Electronic Spectral data of H2B, CoB and NiB

Compound Absorbance Wavelength(nm) Wavelength(cm-1

) ε (Molar Absorptivity)

in mol-1

cm-1

Assignment

H2B 2.917 346 28901.73 2917 π-π*

CoB 4.770

4.201

4.009

0.966

339

348

370

306

29498.74

28735.63

27027.03

32679.74

4770

4201

4009

966

CT

π-π*

CT

CT

NiB 3.141

3.554

333

346

30030.03

28901.73

3141

3554

CT

π-π*

Figure 4.4: Infra-red Spectrum of H

92

red Spectrum of H2B

92

Figure 4.5: Infra-red Spectrum of CoB.

93

red Spectrum of CoB.

93

Figure 4.6: Infra-red Spectrum of NiB

94

red Spectrum of NiB

94

95

95

Table 4.2: Summary of Infrared Spectral Data of H2B, CoB and NiB.

H2B wavelength

(cm-1

)

CoB wavelength

(cm-1

)

NiB wavelength

(cm-1

)

Assignments

3100-2800

Centered at 3000b

3100-2800

Centered at 3000b

3100-2800

Centered at 3000b

v(OH) Carboxylic

1687.8s 1686.8s 1687.8s v(C=O) Carboxylic

3600-3400

Centered at 3500b

- - v(O–H) Phenolic

1289.5s 1287.5b 1288.5s v(C–O) Phenolic

1585.5s 1584.6b 1585.5b v(C=N)str Azomethine

2922.3s 2923.2w 2915.5w v(C–H)def. Aromatic

760.9s 760.0s 761.0b v(C–H)def. Aromatic out

of plane

1449.6s 1449.6b 1450.5b v(C=C)

skeletal ring vibration

- 415.7s 471.6w v(M–N) metal bonded to

nitrogen

- 497.7w 487.0w v(M–O) metal bonded to

oxygen

Legend:

b = broad,

s = strong

w = weak

96

96

4.3 1H NMR Spectra

Figures 4.7, 4.8, and 4.9 show the 1H nuclear magnetic resonance (NMR)

spectrum for the compound 3,5-[(2-hydroxyl-benzylidene)-amino]-benzoic acid

and its cobalt(II) and nickel(II) complexes in dimethyl sulfoxide (DMSO-

D6,400MHz); and the summary of the shift assignments (δ ppm) are represented in

Table 4.3

The singlet signals (δ ppm) at 10.09, 10.11 and 10.14 are assigned to the

carboxylic protons Ha (COOH)88

in ring A Fig. 4.10 (i) for ligand, its cobalt(II)

and nickel(II) complexes respectiviely; which is in contrast to the 7.7 ppm reported

by Chizoba et al89

. The singlet peaks at 8.78 ppm, 8.81 ppm and 8.84 ppm are

assigned to the 2 symmetrical azomethine protons Hd (HC=N) in the ligand H2B,

CoB and NiB respectively. The four multiplets appearing in the spectrum for the

ligand H2B {δ ppm: 7.61-7.55, (3Jo=8 Hz,

4Jo=4 &5 Hz, 2H); 7.48-7.45, (

3Jo=7

Hz, 5Jo=2 Hz, 2H); 7.39-7.33, (

3Jo=8 Hz, 2H) and 6.94-6.90, (

3Jo=8 Hz,

4Jo=4 Hz,

5Jo=2 Hz, 2H)}; CoB complex {δ ppm: 7.63-7.58, (

3Jo=7 & 8 Hz, 2H); 7.51-7.47,

(3Jo=7 & 8 Hz, 2H); 7.40-7.36, (

3Jo=7 & 8 Hz, 2H) and 6.96-6.90, (

3Jo=8 &10 Hz,

2H)} and NiB complex {δ ppm: 7.66-7.60, (3Jo=7 & 8 Hz, 2H); 7.54-7.50, (

3Jo=6

&8 Hz, 2H); 7.45-7.34, (3Jo=8 &9 Hz, 2H) and 6.99-6.93, (

3Jo=8 & 10 Hz, 2H)}

are due to the unsymmetric di-substituted aromatic ring (AA’BB

’system)

90 protons

He-h of the symmetric ring B Fig. 4.10 (i). The doublets centered at δ ppm: 7.06,

97

97

(J=26 Hz, 2H); δ ppm: 7.07, (J=19 Hz, 2H) and δ ppm: 7.10, (J=24 Hz, 2H) were

assigned to the chemically equivalent protons Ha on the aromatic ring A Fig. 4.10

(i). Holbach et al91

also reported similar J-values. The singlet peak at 7.70 ppm

appearing in only the ligand’s spectrum can be assigned to the 2 symmetrical

aromatic/phenolic OH88

Hi in ring B Fig. 4.10 (i). John et al92

reported values up to

14.70 ppm. The singlet peaks at 6.76 ppm, 6.75 ppm and 6.79 ppm in the spectrum

of the ligand H2B, its complexes CoB and NiB respectively; integrated for 1H each

are assigned to Hc proton para to the carboxylic group in ring A Fig. 4.10 (i).

It can be deduced from the proton magnetic resonance spectrum that the

ligand is symmetrical with HC=N and Ar-OH functional groups appearing on both

sides; the ligand has 16 protons while the two complexes CoB and NiB has 14

protons each; due to the absence of the symmetrical Ar-OH groups Hi Fig. 4.10 (ii)

signal in both spectrum. Hence, no coordination through the carboxylic group.

4.4 13

C NMR Spectra

Figure 4.11, 4.12 and 4.13 shows the 13

C nuclear magnetic resonance of the

ligand H2B, its cobalt(II), CoB, and nickel(II), NiB, complexes respectively in

DMSO-D6 (400MHz). The summary of the results are represented in Table 4.4.

98

98

Figure 4.7: 1H NMR spectrum of H2B in DMSO-D6 (400MHz)

99

99

Figure 4.8: 1H NMR spectrum of CoB in DMSO-D6 (400MHz)

100

100

Figure 4.9: 1H NMR spectrum of NiB in DMSO-D6 (400MHz)

101

101

Table 4.3: Summary of Proton Resonance Data for H2B, CoB and NiB in DMSO-D6 (400MHz).

Assignment Ligand (in ppm) CoB (in ppm) NiB (in ppm)

COOH 10.09 (1H, S) 10.11 (1H, S) 10.14 (1H, S)

Sym. HC=N 8.78 (2H, S) 8.81 (2H, S) 8.84 (2H, S)

Sym Ar-OH 7.70 (2H, S) - -

Sym Ar-H

Salicyldehyde moiety

7.61-7.55 (2H, M); 7.48-7.45 (2H,

M); 7.39-7.33 (2H, M) and 6.94-

6.90 (2H ,M)

7.63-7.58 (2H, M); 7.51-7.47 (2H,

M); 7.40-7.34 (2H, M) and 6.96-

6.90 (2H ,M)

7.66-7.60 (2H, M); 7.54-7.50

(2H, M); 7.45-7.39 (2H, M)

and 6.99-6.93 (2H ,M)

Ar-H

Of the COOH moiety

7.06, (J=26 Hz, 2H) 7.07, (J=19 Hz, 2H) 7.10, (J=24 Hz, 2H)

Ar-H

Of the COOH moiety

6.76 (1H, S) 6.76 (1H, S) 6.76 (1H, S)

102

102

A

B

c

N N CC

OH HO

HO O

A

B

MN N CHdHC

O O

aHO O

H Hb

He

hH

Hg

Hf

H

HH

H

A

B

c

N N CHdHC

OH iHO

aHO O

H Hb

He

hH

Hg

Hf

H

HH

H(i)

(ii)

(iii)

1

2

3

45

6

7

8

9

1011

12

M = Co or Ni

Figure 4.10: 1H and

13C nmr assignment for H2B, CoB and NiB complexes.

B

B

B

e

f

g

h

d

i

g

12

f

e

d

h

b

b

11

10

9

8

7

6

103

103

The signals (δ) at 193.74 ppm, 193.73 ppm and 193.73 ppm for H2B, CoB

and NiB spectrum respectively are due to the vibration of carbon in the C1 (COOH)

of carboxylic acid group. The vibrations (δ): at 133.97 ppm, 134.00 ppm and

133.96 ppm for H2B, CoB and NiB spectrum respectively were assigned for the

quaternary carbon C2 (C-COOH) attached to ring A Fig. 4.10 (iii). The signals at

122.27 ppm(2C), 119.38 ppm; 122.30 ppm(2C), 119.41 ppm and 122.30 ppm(2C),

114.40 ppm for H2B, CoB and NiB spectrum respectively were assigned to the

vibration C3 (C-H) aromatic of ring A Fig. 4.10 (iii); Obasi et al93

reported similar

signals for phenyl carbons. The vibrations at (δ) 160.60 ppm (2C), 160.61 ppm

(2C) and 160.61 ppm (2C) for H2B, CoB and NiB spectrum respectively were

assigned to the symmetric vibration C4 (C-N) aromatic of ring A Fig. 4.10 (iii).

The signals (δ in ppm) at 163.69(2C), 163.63(2C) and 163.67(2C) for H2B, CoB

and NiB spectrum respectively were assigned to the vibration of the symmetric C4

(HC=N) azomethine carbon94

Fig. 4.10 (iii). The vibrations (δ in ppm) at

117.51(2C), 117.54(2C) and 117.53(2C) were assigned to the symmetric

quaternary carbon C7 (C-CHN) aromatic of ring B Fig. 4.10 (iii) for the ligand

H2B, its complexes CoB and NiB respectively. The symmetric C9,10,11 (C-H)

aromatic vibrations of ring B Fig. 4.10 (iii) were observed at (δ in ppm):

133.00(2C), 130.80(2C), 120.23(2C) & 117.00(2C); 133.02(2C), 130.78(2C),

104

104

120.24(2C) & 117.01(2C) for the ligand H2B, its complexes CoB and NiB

respectively. The vibrations (δ in ppm) at 160.87(2C), 160.87(2C) and 160.88(2C)

were assigned to the symmetric C12 (C-OH) aromatic of ring B Fig. 4.10 (iii).

From the 13

C NMR spectrum, the ligand is symmetrical with a total of 21

carbon in 12 chemical environments; thus, the donor atoms are also symmetrical in

each of the complexes giving favourable condition for planar geometries.

4.5 Metal–Ligand Mole Ratio

Figure 4.14 shows the results obtained by using Job’s continuous variation

in investigating Co(II)-ligand mole ratio.

The Co(II) to H2B mole ratio obtained is 1:1. Figure 4.15 shows the mole

ratio of Ni(II) to H2B of Job’s plot, which is also a 1:1 metal-ligand mole ratio.

Lokhande et al95

also reported a 1:1 metal-ligand ratio in Ni(II) complex of Bis[3-

hydroxyimine-5-methyl-N-methyl]-2-imine.

4.6 Molecular Formula of the Ligand and the Complexes

The results of infrared (FTIR) data, nuclear magnetic resonance (1H &

13C NMR)

spectrum and UV-Visible spectrum indicate the structure of the ligand to be as in

Figure 4.16. The results of stoichiometric analysis based on Job’s continuous

105

105

Figure 4.11: 13

C NMR Spectrum of H2B in DMSO-D6 (400MHz)

106

106

Figure 4.12: 13

C NMR Spectrum of CoB in DMSO-D6 (400MHz)

107

107

Figure 4.13: 13

C NMR Spectrum of NiB in DMSO-D6 (400MHz)

108

108

Table 4.4: Summary of 13C Data for H2B, CoB and NiB in DMSO-D6 (400MHz)

Assignment H2B(δ in ppm) CoB(δ in ppm) NiB(δ in ppm)

v (COOH) Carboxylic acid

carbon

193.74 193.73 193.73

v (C-COOH) of aromatic 133.97 134.00 133.96

v (sym. C-N) of aromatic 160.60(2C) 160.61(2C) 160.61(2C)

v (C-H) aromatic of COOH

moiety

122.27 (2C), 119.38 122.30 (2C), 119.41 122.30 (2C), 119.40

v (sym. HC=N) azomethine

carbon

163.69(2C) 163.63(2C) 163.67(2C)

v (sym. C-CHN) of aromatic 117.51(2C) 117.54(2C) 117.53(2C)

v (sym. C-H) aromatic of

salicylaldehyde moiety.

133.00(2C), 130.80(2C),

120.23(2C), 117.00(2C)

133.02(2C), 130.78(2C),

120.27(2C), 117.03(2C)

133.01(2C), 130.78(2C),

120.24(2C), 117.01(2C)

v (sym. C-OH) aromatic of

salicylaldehyde moeity

160.87(2C) 160.87(2C) 160.88(2C)

109

109

variation method further assisted in proposing a geometry for the ligand’s Co(II)

and Ni(II) complexes. The infra-red spectra indicated that the two metal ions,

Co(II) and Ni(II), were coordinated to the azomethine nitrogen atoms and the

oxygen of the deprotonated hydroxyl group. From the proton magnetic resonance

spectra, it was deduced that the ligand has a total of 16 protons and is symmetrical

with some functional groups, HC=N and Ar-OH, appearing in both sides. The 13

C

NMR spectra confirmed that the ligand is symmetrical as reported earlier in

literature89

. Consequently, the proposed structure of the ligand and its Co(II) and

Ni(II) complexes are shown in Figure 4.16.

4.7 Solubility Data

Table 4.5 shows the solubility data for the ligand and its complexes. Both

ligand, CoB and NiB complexes were insoluble in water; this may indicate that, the

hydrophobic component of both the ligand and the complexes are large. Since the

cobalt(II) and nickel(II) complexes were insoluble in water but sparingly soluble in

chloroform and very soluble in dimethyl formamide (DMF); consequently,

chloroform was selected as solvent for the solvent extraction studies of the two

metals.

110

110

Table 4.5: Solubility data for the ligand, its CoB and NiB complexes.

Solvents H2B CoB NiB

Water I I I

Ethanol S S S

Methanol S S S

Chloroform SS SS SS

CCl4 I I I

N-Hexane SS SS SS

di-ethylether I I I

Toluene SS SS SS

Acetone VS VS VS

DMF VS VS VS

DMSO VS VS VS

Legend: I=Insoluble; S=Soluble; SS=Sparingly Soluble; VS=Very Soluble.

111

111

4.8 Dissociation and Protonation Constants of the Ligand

The ligand, 3,5-Bis-[2-hydroxy-benzylidene)-amino]-benzoic acid (0.01 M)

in DMF gave a pH of 9.56. Applying potentiometric titration method95

by titrating

the solution of the ligand with 0.01M HCl, a protonation constant of pKb 4.14 was

obtained as shown in Figure 4.17. This is similar to the pKb 4.2 value earlier

reported in literature89

and show a simultaneous protonation of the two azomethine

nitrogen atoms;

H2B + 2H+ H4B

2+ ………Kb …(76)

The determination of the acid dissociation constant, pKa, using

potentiometric titration method by titrating with 0.01M NaOH gave 10.12 as

shown in Figure 4.18. Similar values of pKa was reported in literature89

. This

means that in a basic medium, there is simultaneous loss of two protons of the

hydroxyl groups.

H2B B2-

+ 2H+ ……….. Ka

……(77)

112

112

Figure 4.14: Job’s plot for Co(II)/H2B mole ratio.

Figure 4.15: Job’s plot for Ni(II)/H2B mole ratio.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10

Ab

sorb

an

ce

Mole ratio

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10

Ab

sorb

an

ce

Mole ratio

1/9 2/8 3/7 4/6 5/5 6/4 7/3 8/2 9/1

1/9 2/8 3/7 4/6 5/5 6/4 7/3 8/2 9/1

113

113

N N CHHC

OH HO

HO O

3,5-Bis[2-Hydroxy-Benzylidene)-Amino]-Benzoic acid {H2B}

3,5-Bis[2-Hydroxy-Benzylidene)-Amino]-Benzoicacidcobalt(II) {CoB}

3,5-Bis[2-Hydroxy-Benzylidene)-Amino]-Benzoicacidnickel(II) {NiB}

CoN N CHHC

O O

HO O

Ni

N N CHHC

O O

HO O

Figure 4.16: Structures of Ligand, its Co(II) and Ni(II) Complexes

114

114

Figure 4.17: Titrimetric determination of pKb for the ligand H2B

Figure 4.18: Titrimetric determination of pKa for the ligand H2B

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30

pH

Volume of titrant in (mL)

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30

pH

Volume of titrant in (mL)

115

115

4.9 Equilibration Time

The equilibration time for the extraction of Co(II) and Ni(II) was carried

out with 5% H2B/DMF in chloroform. The amounts extracted at various time

intervals are tabulated in Table 4.6 and 4.7.

For the extraction of Co(II) and Ni(II) an equilibration time of 15 min.

was established. This time was employed in subsequent extraction of the metal

ions.

4.10 Effect of pH Buffer on Extraction of Co(II) and Ni(II)

The extraction of Co(II) with 5% H2B/DMF was studied as a function of

pH in the pH range 1-13 as shown in Figure 4.19. As the pH decreased from 5

to 1, the percentage extraction also decreased. This may be due to the

competition between H+ and Co2+ for the azomethine ligation sites, which is

unfavourable for the formation of the complex. In addition, it may be due to the

masking effect of the acid component of the buffer. At pH 6, up to 94.50%

extraction was obtained which is in line with extraction at pH 6.2 reported by

Al-Mulla and Al-Janabi96 in the extraction of Co(II) from aqueous solution

using N, N’-carbonyl difatty amides. This may be due to the formation of a

116

116

neutral chelate complex completely extractable into the organic phase. As given

in equation (77), dissociation of the ligand at high pH enhanced the formation of

the chelate with the metal ions. As the pH increased to 13, the percentage

extraction decreased. This is probably due to hydrolysis of the metal ion thereby

decreasing the amount available for complexation and extraction. The masking

effect of the base component of the buffer may contribute to the decrease.

The extraction behaviour of Ni(II) with 5% H2B/DMF was examined by

varying the pH from 1-13 as shown in Figure 4.20. At the pH range 1-6, the

percentage extraction decreased with increase in hydrogen ion concentration.

Also, the reason may be because of high competition between the proton and

Ni(II) for the azomethine ligation sites. The extraction of Ni(II) followed the

same pattern as that of Co(II). At pH 8 up to 97.88% was extracted. As the pH

increased to 13, the percentage extraction decreased appreciably. This may be

because of hydrolysis of Ni(II) at higher pH, thus decreasing the amount

available for bonding.

The extractive behavior of Ni(II) and Co(II) are similar, although

quantitative extraction of Co(II) was seen tending towards lower pH than Ni(II);

hence its application in major separation of the two metal ions in solution97,98.

117

117

Table 4.6: The amount of Co(II) extracted into the organic phase at various time

intervals

(Initial Concentration=10 µg/cm3.)

Time (min) Co(II) in organic phase

(µg/cm3)

Co(II) in aqueous phase

(µg/cm3)

5 4.88 5.00

10 4.33 5.55

15 5.22 4.55

20 5.11 4.88

25 5.11 4.77

30 4.88 4.77

35 4.66 5.11

40 3.88 6.00

45 3.33 6.66

50 3.11 6.55

55 2.77 7.11

60 2.22 7.66

118

118

Table 4.7: The amount of Ni (II) extracted into the organic phase at various time

intervals

(Initial Concentration=10 µg/cm3.)

Time (min) Ni(II) in organic phase

(µg/cm3)

Ni(II) in aqueous phase

(µg/cm3)

5 5.00 4.77

10 5.33 4.44

15 5.88 3.77

20 5.88 3.66

25 4.77 5.22

30 4.55 5.22

35 4.00 4.77

40 3.88 6.00

45 3.55 6.33

50 3.11 6.77

55 3.33 6.44

60 3.33 6.55

119

119

Figure 4.19: Profile for Co(II) extraction in buffer media.

Figure 4.20: Profile for Ni(II) extraction in buffer media.

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

%E

pH

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14

%E

pH

120

120

4.11 Effect of Acidity

The extraction of Co(II) with 5% H2B /DMF was studied as a function of

different mineral acids (HCl, H2SO4, HNO3 and HClO4) within acid concentration

range 10-3

– 2.0 M.

A plot of percentage extraction against different acids

concentrations is shown in Figure 4.21. As the acidity decreased from 10-1

to 10-3

M, the percentage extraction increased. This may be due to the formation of

extractable complexes of cobalt at low [H+]. From equation (77), at high pH

deprotonation of the ligand was favoured to form B2-

, thus enhancing the formation

of the complex. As the acid concentration increased to 2 M the percentage of

Co(II) extracted decreased which may be due to the competition for the

azomethine ligation sites between the protons and the cobalt ions.

The extraction of Ni(II) with 5% H2B/DMF was studied as a function of

different acids (HCl, HNO3, H2SO4 and HClO4) concentrations (10-3

-2.0 M). As

illustrated in Figure 4.22, the extraction pattern of Ni(II) is almost the same as that

of Co(II).

The extraction of both metal ions in HNO3 and HClO4 were higher than the

H2SO4 and HCl, with maximum extraction in the HNO3 media for both metal ions.

Up to 88.51% and 95.39% Co(II) and Ni(II) were extracted respectively in aqueous

HNO3 medium.

121

121

Figure 4.21: Profile for extraction of Co(II) in various acid media.

Figure 4.22: Profile for extraction of Ni(II) in various acid med

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

%E

Acid concentration (M)

HCl

H2SO4

HNO3

HClO4

0

20

40

60

80

100

120

0 1 2 3 4 5 6

%E

Acid concentration (M)

HCl

H2SO4

HNO3

HClO4

10-3

10-2

10-1

1 2

10-3

10-2

10-1

1 2

122

122

4.12 Effect of Salting-Out Agent on Extraction

The extraction of Co(II) with 5% H2B in DMF was studied at varying

concentrations, 10-3

– 1.0 M of salting out agents (NaCl, Na2SO4, NaClO4 and

KNO3) at a constant acid concentration in which partial extraction of the metal ion

was achieved and the results are shown in Figure 4.23. Extraction of Co(II) in 1.0

M HCl gave only 24.98%, but addition of small amount (0.001M) of NaCl

enhanced the percentage extraction up to 86%. This may be attributed to increase

in dielectric constant of the aqueous media that made the complex less ionic, hence

more soluble in the organic phase than in the aqueous phase6. As the concentration

of NaCl increased from 10-2

–1.0 M, the amount extracted decreased appreciably,

owing to the formation of the tetrachloro anionic complex, [CoCl4]2-

. At 0.1 M

H2SO4, the percentage extraction was enhanced from 40.21% to 87.51% by

addition of 0.001 M Na2SO4. The amount extracted decreased with increase in

concentration of Na2SO4. At 1.0 M HClO4, the presence of small quantity of

NaClO4 (0.001 M) increased the percentage extraction from 65.53% to 98.89%.

Also the reason may be due to increased dielectric constant of the aqueous phase

which made the complex less ionic.

The salting out effect of KNO3 is almost the same as that of NaClO4.

Extraction of Ni(II) with 5% H2B/DMF was studied at different concentrations

of salting out agents (NaCl, Na2SO4, NaClO4 and KNO3) and at a constant

123

123

concentration of the acid at which non-quantitative extraction of the metal was

achieved. The results are shown in Figure 4.24. The extraction of Ni(II) in 0.1 M HCl

was enhanced from 43% to 66.01%, however, increase in concentration shows

decrease in extraction which may be attributed to the formation of NiCl42- which

hinders the ligation of the metal. Extraction was enhanced from 43% to 51% in 0.001

M H2SO4 with Na2SO4 concentration of 0.001 M. The amount of Ni(II) extracted did

not show appreciable increase which implies that H2SO4 does not have much effect on

Ni(II)4. At 1.0 M HClO4, extraction of Ni(II) was enhanced from 52.30% to 98.86%

0.005 M NaClO4. Also, this may be due to increase in dielectric constant of the

aqueous phase as the concentration of the salting out agent increases. At a

concentration of 1.0 M HNO3, the salting out effect of KNO3 was observed at a

concentration range of 0.001 – 1.0 M. The amount of Ni(II) extracted increased with

decrease in concentration of the salting out agent. Addition of 0.005 M KNO3

enhanced the extraction up to 97.72%. The reasons for this behaviour may be same as

the ones adduced above.

It was observed that KNO3 and NaClO4 are excellent salting-out agents for

the extraction of both cobalt(II) and nickel(II) complexes of H2B. This may be that

the KNO3 and NaClO4 increases the dielectric constant of the aqueous phase more

readily making the metal ions passive and hence more stable in the organic phase.

Similar effect was reported by Ezugwu et al89

for the extraction of Hg(II) and

Ag(I) using H2B.

124

124

Figure 4.23: %E Vs Concentration of salting-out agent for Co(II)

extraction with H2B/DMF

Figure 4.24: %E Vs Concentration of salting-out agent for Ni(II)

extraction with H2B/DMF

0

20

40

60

80

100

120

0 1 2 3 4 5 6

%E

Concentration of salting-out agent (M)

NaCl/HCl(1.0M)

Na2SO4/H2SO4(0.1M)

NaNO3/HNO3(0.1M)

NaClO4/HClO3(1.0M)

0

20

40

60

80

100

120

0 1 2 3 4 5 6

%E

Concentration of salting-out agent (M)

NaCl/HCl(1.0M)

Na2SO4/H2SO4(0.1M)

NaNO3/HNO3(0.1M)

NaClO4/HClO3(1.0M)

125

125

4.13 Effect of Complexing Agents on Extraction

Figure 4.25 shows the effect of complexing agents in the concentration

range 0.001 – 1.0 M on the extraction of Co(II) from 0.001 M HNO3, which was

the acid concentration at which there was maximum extraction of the metal ion.

The masking effect of EDTA was studied between the concentration range

0.001 – 0.4 M. Cobalt (II) ion was masked 91 – 96% by EDTA in 0.05 to 0.4 M.

At 0.1 to 1.0 M cyanide ion concentration, up to 99% of Co(II) was masked.

Thiocyanate had a maximum masking action (86.64%) at 1.0 M. The masking

effect of phthalate ion was studied between concentration range 0.001 – 0.3 M.

Masking effect was not appreciable, only 21 – 28% Co(II) was masked between

the concentrations 0.1 – 0.3 M. At lower concentration of phthalate ion, the

effect was much less. Up to 70.50% Co(II) was masked at 1.0 M tartrate ion but

the interference was not appreciable in lower concentrations, about 35.21%

Co(II) was masked at 0.001 M. The effect of fluoride ion as a masking agent on

cobalt(II) was also not appreciable. At concentrations 0.001 – 1.0 M only 11.36

– 21.12% Co(II) respectively were masked. This could be that tartrate, phthalate

and fluoride complexes of Co(II) and Ni(II) complexes are not very stable.

The effect of complexing agents on the extraction of Ni(II) from 0.001 M

HNO3 at the concentration range 0.001 – 1.0 M is shown in Figure 4.26. It was

126

126

observed that EDTA and cyanide ions greatly influenced the extraction of

Ni(II). EDTA had the highest interference since up to 80.51% Ni(II) was

masked at 0.1 – 0.4 M. Cyanide at a concentration range 0.1 – 1.0 M masked up

to 77.28% Ni(II). At higher concentration (1.0 M) of thiocyanate ion, up to

70.49% of Ni (II) was masked but as the concentration decreased to 0.001 M,

the interference reduced to 22.35%. Phthalate masked Ni(II) between 24.12 –

14.74% at 0.001 – 1.0 M. The effect did not follow increase or decrease in

concentration. But low masking was observed in 0.001 M and 1.0 M of 19.48

and 14.74% respectively. At lower tartrate ion concentration (0.001 M) up to

34.47% Ni(II) was masked. The interference increased as the concentration

increased; up to 68.17% Ni(II) was masked.

The high masking of Co(II) in the presence of EDTA and cyanide ions

may be due to the formation of very stable non-extractable Co(II) complexes of

EDTA and cyanide as Co-EDTA and [Co(CN)6]4- respectively99. Partial

interference of tartrate, phthalate and flouride ions on both Co(II) and Ni(II)

could be that their complexes are not very stable. Copious comparisons are due

for the reports of Ezugwu et al89 and Ukoha et al

6

127

127

Figure 4.25: Effect of complexing agent on the extraction of Co(II) with

H2B/DMF

Figure 4.26: Effect of complexing agent on the extraction of Ni(II) with

H2B/DMF

-10

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

%E

Concentration of complexing agents (M)

Cyanide

EDTA

Flouride

Phthalate

Tartrate

Thiocyanate

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6

%E

Concentration of complexing agents (M)

Cyanide

EDTA

Flouride

Phthalate

Tartrate

Thiocyanate

128

128

4.14 Degree of Metal Separation

Table 4.8 illustrates the results obtained from the analysis of mixtures of

Co(II) and Ni(II) extracted from 0.001 M HNO3 using 5% H2B/DMF. At this acid

concentration up to 95.39% Ni(II) was quantitatively extracted but the presence of

0.01 M cyanide masked 47.10% of Ni(II).

The separation is based on the fact that at 0.01 M cyanide up to 98.90%

Co(II) was masked while about 33.33% Ni(II) was extracted. Owing to the fact that

Ni(II) is partially masked, a four-cycle extraction process6 was employed for the

separation of the two metals. After extraction very small amount of Ni(II) was

remaining in the aqueous phase but about 99.16% Co(II) was extracted. Although

the procedure is laborious, however, it is clean and efficient; and a five-cycle

extraction will completely separate nickel(II) from cobalt(II).

4.15 Summary and Conclusion

Spectroscopic characterization via electronic, 1H NMR,

13C NMR and IR of

the synthesized ligand, 3,5-Bis[(2-hydroxy-benzylidene)-amino]-benzoic acid

(H2B), its cobalt(II) and nickel(II) complexes were successfully characterized

showing the coordination of the complexes through azomethine and deprotonated

hydroxyl group of the ligand. The Job’s plots shows metal to ligand mole ratio of

1:1 for both metals to H2B.

129

129

Table 4.8: Degree of separation of Ni(II) from Co(II)

Element

Amount taken

(µg)

Amount found (µg)in

organic phase

Amount found

(µg)in aqueous

phase

10 0 9.66

Co(II) 10 0 9.88

10 0 9.88

10 0 9.66

10 9.66 0.11

Ni(II) 10 9.44 0.22

10 9.77 0

10 9.66 0.11

Standard Deviation = 0.12

130

130

Cobalt(II) was quantitatively extracted as [CoB] chelate in 5% DMF using

CHCl3 in the pH range 5 – 7, as well as in 10-3

– 10-1

M HCl, HNO3, HClO4 and

H2SO4. The presence of cyanide and EDTA masked Co(II). Nickel(II) was

quantitatively extracted as [NiB] chelate in 5% DMF using CHCl3 in the pH range

6 – 8, as well as in 10-3

– 10-1

M HCl, HNO3, HClO4 and H2SO4. KNO3 and

NaClO4 proved to be a good salting out agents for the extraction of both Co(II) and

Ni(II).

Conclusively, 3,5-Bis[(2-hydroxy-benzylidene)-amino]-benzoic acid (H2B)

proved to be a good extractant for Co(II) and Ni(II) in aqueous solution. Nickel(II)

has been successfully separated from cobalt(II) by four-cycle extraction at 10-3

M

HNO3 aqueous mixture of Ni(II) and Co(II) in 5%H2B/DMF using 0.05 M cyanide

as masking agent and CHCl3 as organic solvent.

4.16 Recommendation

Further work is required to fully characterize the extracted species.

Preparation of the complexes in various reaction media (low acidity, high acidity,

low pH, and high pH) is required. Characterization which includes elemental

analysis and mass spectroscopy will also be necessary.

To prove on the extractability of the metals, new solvents or solvent

mixtures will be required.

131

131

4.17 Contributions to Knowledge

The project synthesis, characterization and solvent extraction studies of 3,5-

Bis[(2-hydroxy-benzilidene)-amino]-benzoic acid and its Co(II) and Ni(II)

complexes were successfully synthesized. Characterization of the ligand and

complexes based on IR, NMR showed that the ligand is tetradentate and

coordinates through ONNO atoms. The complexes characterized are neutral square

planar compounds.

132

132

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