a study of some complexometric titrations in nonaqueous solvents
TRANSCRIPT
Scholars' Mine Scholars' Mine
Masters Theses Student Theses and Dissertations
1972
A study of some complexometric titrations in nonaqueous A study of some complexometric titrations in nonaqueous
solvents solvents
Ngo The Hung
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A STUDY OF SOME COMPLEXOMETRIC TITRATIONS IN
NONAQUEOUS SOLVENTS
BY
NGO THE HUNG, 1940-
A THESIS
Presented to the Faculty of the Graduate School of the
UNIVERSITY OF MISSOURI-ROLLA
In Partial Fulfillment of the Requirements for the Degree
MASTER OF SCIENCE IN CHEMISTRY
1972
Approved by
T2732 92 pages c.l
To my Parents
ii
ABSTRACT
Complexometric titrations of calcium, zinc and lead with polyamino
carboxylic acids: ethylenediaminetetraacetic acid (EDTA), 1,2-diamino
cyclohexanetetraacetic acid (DCTA), ethyleneglycol-bis(2-aminoethylether)
tetraacetic acid (EGTA) and tetraethylenepentamine (tetren) have been
investigated and compared in the following organic solvents: methanol,
dimethyl formamide, dimethyl sulfoxide and methyl ethyl ketone.
Various end point detection methods have been used: direct visual
titration with metallochromic indicators and instrumental detection by
photometry, potentiometry (mercury electrode and lead ion selective
electrode) and amperometry.
Calcium, zinc and lead can be determined up to trace levels (ppm)
under specific conditions.
A concrete application of this thesis is the determination of zinc
or calcium in a sample of polystyrene within a few minutes while the
classical methods will necessitate several hours.
iii
ACKNOWLEDGMENT
The author wishes to extend his appreciation to his advisor, Dr.
William R. Carroll, Professor of Analytical Chemistry, for suggesting
the topic of this research and for introducing him to the fascinating
field of titrations in nonaqueous media. Without his expert guidance
this thesis could not have been led to completion.
He also wishes to thank Mr. Edward W. Hennecke for diligent help in
some glass blowing situations.
He is particularly grateful to the United States Government (Agency
for International Development) and to the Vietnamese Government for
financing his stay in the United States.
iv
TABLE OF CONTENTS
Page
ABSTRACT . •.•.•...•....•...••.•..•.•.•..•...•..•..•...•...•..••..... ii
ACI<NO\Vl..EDGE:t-fENT • .................................................... iii
TABLE OF CONTENTS • ••.•••••••..•••••••..•••.•••.•.•.••••..•.•..•••• iv
LIST OF FIGURES . .................................................. . vii
LIST OF TABLES ..................................................... viii
ABBREVIATIONS. . • . • • • . • • . • . . • . . . • . . . . . • • . . • . • . . . . . . . . • . . . . . . . . . . . . . ix
I. INTRODUCTION • •••••••••••••••••••••••••••••••.••••••••••• 1
II. REVIEW OF LITERATURE • ..................................... 2
A. Studies with a Mixed Solvent System .•••••.•.• 2
B. Studies with Pure Anhydrous Organic Solvents ••••••.•• 5
c. Conclusion . ......................................... . 6
III. EXPERIMENTAL SECTION ••••••••••••.••••••••••••••••••••.•.• 8
A. Apparatus . ............................................. . 8
B. Reagents and Solutions •••••••••••.••••••••••.••.••... 10
1. Chelan So 1 utions . ................................ . 10
2. Standard Metal Solutions •••.••••••.•••...•••..•.• 13
3. Indicator Solutions •••••••...•.•.••••..•.•.•...•• 14
c. Procedure . ........................................... . 16
1. Direct visual Titrations with Metal1ochromic Indica tors .. ................................. . 16
a} Titrations of zinc with EDTA ••••••••••..•••• 16
b) Titrations of zinc with DCTA ••••••••••••••••• 17
c) Titrations of zinc with EGTA •••••.•••••••.••• 17
d) Titrations of zinc with tetren ••••••••••••••• 17
Table of Contents (continued)
Page
e) Titrations of calcium with EDTA11 ••••• - ••••••••• 18
f) Titrations of calcium with DCTA .. ................ 18
g) Titrations of calcium with EGTA • •.••••••....•.• 18
h) Titrations of calcium + zinc with DCTA •..•..... 19
i) Titrations of calcium + zinc with EGTA • ••••.••. 19
j) Analysis of calcium and zinc in a sample of polystyrene.......................... 19
2. Photometric Titrations............................. 20
3. Potentiometric Titrations.......................... 22
a) Titration of mercury with the mercury elec trade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
b) Titrations of zinc with the mercury electrode...................................... 22
c) Titration of calcium with the mercury elect rode . ..................................... 22
d) Titration of lead with the lead elec trade .•...• 23
e) Titration of zinc with the lead electrode •..... 23
4. Anlperometric Titrations .••••••.•.•••...••••..•.••.•. 24
IV. RESULTS AND DISCUSSION.. . • • • • • . . . . • • • . . . • • . . . • • • . • . . • . • • • . • 25
A. Direct Visual Titrations with Metallochromic In-die a tors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1. General connn.ents .................................... 25
2. Titrations of Zinc ................................... 26
3. Titrations of Calcium . ............................. 39
4. Titrations of Calcium + Zinc . ...................... 45
5. Analysis of Calcium and Zinc in a Sample of Polystyrene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
vi
Table of Contents (continued)
Page 6. Conclusion...................................... 48
B. Photometric Titrations ••••••••••••••••••••••••••••• 51
c. Potentiometric Titrations •••••••••••••••••••••••••• 59
D. Amperometric Titrations •••••••••••••••••••••••••••• 73
v. CONCLUSION • •••••••••••••••••••••••••••••••••••••••••.•. 75
REFERENCES. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • • • . 77
VITA. • • • • • • • • . • • . . • • . • . . . • • . . • . . • . • . • . • . • • • . . . . • • . • • • . • . • • . . • . . . . 82
vii
LIST OF FIGURES
Figure Page
1. Absorbance Curves in Dimethyl Sulfoxide .........•.........•. 52
2. Photometric Titrations of Zinc in Dimethyl Sulfoxide .•...... 54
3. Photometric Titration of Zinc in Dimethyl Formamide ...•..... 56
4. Potentiometric Titrations of Calcium and Zinc with EDTA ..•.. 61
5. Potentiometric Titrations of Lead with EDTA and EGTA .....•.. 66
6. Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA .. 69
viii
LIST OF TABLES
Table Page
I. Results of Titrations of Zinc with EDTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 27
II. Results of Titrations of Zinc with DCTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 31
III. Results of Titrations of Zinc with EGTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 34
IV. Results of Titrations of Zinc with Tetren Using Metallochromic Indicators ••••••••••••••••••••••••••••• 36
V. Results of Titrations of Calcium with EDTA in Methanol Using Methylthymol Blue ••••••••••••••••••••••••• 40
VI. Results of Titrations of Calcium with DCTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 41
VII. Results of Titrations of Calcium with EGTA Using the System Zincon + Zinc-EGTA ••••••••••••••••••••••••• 43
VIII. Results of Titrations of Calcium + Zinc with DCTA Using Methylthymol Blue ••••••••••••••••••••••••••••••••••••• 46
IX. Results of Titrations of Calcium + Zinc with EGTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 47
X. Results of Titration of Zinc and Calcium in Polystyrene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9
XI. Results of Photometric Titrations of Zinc •••••••••••.••••••• 58
XII. Results of Potentiometric Titrations of Calcium and Zinc with EDTA ••••••••••••••••.••••••••••••••••• 60
XIII. Results of Potentiometric Titrations of Lead with EDTA arid EGTA. . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . 65
XIV. Results of Potentiometric Titrations of Zinc With EDTA, DCTA and EGTA. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 68
ix
ABBREVIATIONS
DCTA: 1,2-diamino£Yclohexanetetr~cetic acid
DMSO: Di..!!!ethyl _!Ulfoxide
EDTA: Ethylenediamine.E_etra.!_cetic acid
EGTA: Ethylenealycol-bis(2-aminoethylether)-tetra~cetic acid
S.C.E.: saturated calomel electrode
Tetren: Tetraethylenepentamine
1
I. INTRODUCTION
Complexometric titrations of metal ions with chelating agents like
polyamines or polyaminocarboxylic acids have been extensively studied
for two decades.(l-6) All these studies, though, deal with aqueous
titrations and the field of complexation in non-aqueous media has been
relatively uninvestigated.
When metals in organic products are determined by classical aqueous
complexometry, it is required to destroy the organic matrix by an ashing
procedure followed by a dissolution of the residue. This method is time
comsuming and the ashing procedure can lead to some loss of material.
If the sample can be dissolved in an organic solvent and a direct
complexometric titration be performed in this medium, any loss of
material inherent to the ashing procedure will be prevented and the time
required for the analysis will be tremendously shortened.
It is the purpose of this thesis to investigate some complexometric
titrations in various organic s6lvents and to find a rapid method of
determination of calcium and zinc in polystyrene.
II. REVIEW OF LITERATURE
Studies of complexometric titrations in non-aqueous media using
polyaminocarboxylic acids as chelating agents are met with two main
difficulties: 1) the insolubility of the chelating agent in the
organic solvents and 2) the lack of knowledge concerning the forma
tion of the metal-chelate in these media.
2
These studies can be classified into two divisions: those which
involve a mixed solvent system (organic solvent + water) and those
which deal with pure organic anhydrous solvents.
A. Studies with a Mixed Solvent System
The metals to be titrated are dissolved in a mixed solvent system
and the chelating agent is always dissolved in water. The role of the
organic solvent is to dissolve the sample and/or to prevent the
blocking of the metallochromic indicator.
Schwarzenbach and Ackerman (7) found that the stability constants
of some EDTA-alkaline earth chelates are 100 times greater in a 48%
alcoholic medium than in water.
Gotschalk (8) discovered the same property in titrating aluminum
with EDTA in a 50% ethanolic medium and using dithizone as indicator.
Potentiometric titrations of several metal ions with EDTA, nitrilo
triacetic acid (NTA) and N,N-Di(-hydroxyethyl)glycine in a water +
pyridine mixture have been investigated by Siggia (9) who used various
electrode systems as Pt vs. calomel, Hg on Pt vs. calomel or Ag vs.
calomel.
3
A lead ion-selective membrane electrode was used by Rechnitz and
Kenny (10) to determine lead by a differential measuring technique in
various concentrations of water and methanol, dimethyl sulfoxide, 1,4-
dioxane and acetonitrile.
The determination of metal additives in petroleum products has
been often investigated in various water + organic solvent mixtures.
Gerhardt and Hartmann (11) determined calcium or zinc in lubricating
oils and concentrates by dissolving the samples in acetone, complexing
the metals with an excess of aqueous standard solution of EDTA and then
back titrating the excess EDTA with an a~ueous solution of magnesium
chloride using Eriochrome Black T as indicator.
Fisher (12) used an extraction procedure to separate barium,
calcium and zinc in oil samples but this method could not avoid excessive
emulsion formation and sometimes incomplete extraction.
A toluene + isopropyl alcohol mixture was used by Crump (13) to
dissolve lubricating oils. The additive metals (barium, calcium, lead
and zinc) were complexed with an excess of standard EDTA which was then
back titrated with a standard magnesium iodate solution using Eriochrome
Black T as indicator. The method was rapid but the solution was
cloudy due to the presence of a suspension which tended to obscure some
end-points. Instead of using EDTA Hooks and Noar (14) used
4
diethylenetriaminepentaacetic acid (DTPA) and dithizone as indicator to
determine zinc directly while the other metal additives were complexed
with an excess of DTPA. This excess DTPA was back titrated with a
standard magnesium solution.
Combining their methods Crump and Noar (15) found them having an
accuracy comparable to classical "wet" methods of the ASTI1 and the
Institute of Petroleum. An analysis took 30 minutes.
A double extraction procedure was used by Hypta (16) to determine
barium and calcium in lubricating oils. An analysis took 3 hours and
calcium could be determined down to 0.06% and barium down to 0.02%.
Kiss (17) investigated various water + organic solvent mixtures to
avoid the blocking of Eriochrome Black T and found that ethanol was
the best organic solvent.
Methylthyrnol Blue and Methylthyrnol Blue + Fluorexone have been
used by Kiss, Gaal, Suranyi and Zsigrai (18) to determine consecutively
calcium and magnesium with EDTA in a 60% ethanolic medium.
Micro amounts of magnesium (0.0243 to 12.16 mg) have been titrated
by Kiss and Suranyi (19) with EDTA and Methylthyrnol Blue in a 60%
ethanolic medium.
Magnesium sterate dissolved readily in 96% ethanolic medium and
magnesium could be titrated with EDTA and Methylthyrnol Blue by Kiss
and Knezevic (20).
A high frequency technique to determine bivalent metal ions in
5
water + ethanol and water + ethanol + benzene mixtures was investigated
by Casassas and Fernandex (21) who used 1,2-diaminocyclohexanetetraacetic
acid (DCTA) as chelating agent. They found that the DCTA-metal complexes
are more stable than the EDTA-metal complexes and the high frequency
titration could be used in non-buffered media.
B. Studies with Pure Anhydrous Organic Solvents
Yoshimura and Tamura {22) used a conductometric method· to analyze
antimony (III) and (V) in dimethyl formamide with EDTA. They found
that the end point corresponds to molar ratio EDTA:Sb equal to 4:1.
An amperometric end point detection was used by Arthur and Hunt
(23) to analyze lead, cadmium and zinc in petroleum products. They
dissolved the sample in 2-propanol and the chelating agent was 1,2-
diaminocyclohexanetetraacetic acid (DCTA) dissolved in 95% ethanol made
0.33 M in ethanolamine.
Micro titrations being possible with a high frequency technique
have been successfully investigated by Fernandez, Casassas and Garcia
-5 MOntelongo (24) who studied the complexation between 4.10 M EDTA in
ethanol and cadmium, mercury, zinc, copper and lead solutions in
-4 -5 concentration range from 3.3 x 10 M to 2.5 x 10 M.
A complexation reaction is always followed by a release of protons,
which can be titrated by a high frequency technique. Fernandez and
Casassas (25) applied this method when they studied the reaction be-
tween 1,2-diaminocyclohexanetetraacetic acid (DCTA) and some bivalent
6
Metals in methanol +benzene (1:4 and 4:1) mixtures.
Acetone, acetonitrile and methanol were the media used by Rechnitz
and Kenny (26) when they titrated potentiometrically cupric ions with
EDTA, ethylenediamine, tetraethylenepentamine and 5,6-dimethyl-1,10-
phenanthroline and a solid membrane cupric ion electrode.
Direct visual titrations of zinc, cadmium and magnesium in water
insoluble stearates with EDTA, nitrilotriacetic acid (NTA), 1,2-diamino
cyclohexanetetraacetic acid (DCTA) and N-(2-hydroxyethyl)-ethylene
diamine-NNN'-triacetic acid (HEDTA) have been performed by Kiss,
Zsigrai, Cirin and Krizsan (27) in formamide and dtmethyl sulfoxide with
Eriochrome Black T and PAN as indicators. Magnesium and zinc in micro
amounts from 1 to 12 pg in a 0.5 ml sample could be successfully deter
mined.
A thermometric end point detection was used by Kiss (28) in the
direct titration of EDTA, 1,2-diaminocyclohexanetetraacetic acid (DCTA)
and nitrilotriacetic acid (NTA) with manganese (II) nitrate in dimethyl
sulfoxide and in the indirect titration of some bivalent and tervalent
metals with EDTA or DCTA in dimethyl sulfoxide.
C. Conclusion
From the survey of literature the following conclusions can be
drawn:
1. Complexometric titrations in non-aqueous media are possible.
7
2. The theory of complexometric titrations in aqueous medium is
mainly centered around the notion of pH. In a non-aqueous medium the
pH is not well defined and this explains why all the investigations up
to now have been highly empirical and are still in a stage of trial and
error experiments.
8
III. EXPERIMENTAL SECTION
A. Apparatus
1. Potentiometric studies were performed with a Sargent recording
titrator, model D; a Beckman Zeromatic SS 3 pH meter and a Beckman Ex-
pandomatic pH meter. The following electrodes have been used:
a) Glass electrodes: Beckman No. 39301 on the Expandomatic
pH meter, and Beckman No. 41263 on the SS 3 pH meter.
b) Modified calomel reference electrode made from a Beckman
quartz fiber standard calomel electrode with saturated KCl - methanol
electrolyte, and a Standard calomel electrode, Beckman No. 39402.
c) Silver billet indicator electrode, Bec~n No. 39261. The
electrode was cleaned periodically by light buffing with steel wool.
d) Mercury electrode, J - type, Sargent-Welch No. S-30438.
Before use the mercury electrode cup was cleaned with 5 N nitric acid,
rinsed with distilled water followed by deionized water and dried in
0 an oven at 105 C.
e) Lead specific ion electrode, Orion model 94-82. Before
use the electrode was polished with the abrasive plastic supplied with
the electrode by the manufacturer and soaked in the same lead standar-
dizing solution for five minutes.
2. Photometric instruments included a Beckman DK-2A ratio re-
cording spectrophotometer used for running absorption curves and a
Bausch and Lomb Spectronic 20 spectrophotometer equipped with a 55
milliliter, one inch diameter test tube containing a micro Teflon
coated stirring bar used for measuring absorbance during titrations.
9
The test tube was surrounded by a tube opaque to light and closed by a
rubber stopper with a hole in the middle to let the tip of the buret
dip into the tube. A magnetic stirrer was placed under the instrument,
under the bottom of the test tube. The stirrer was stopped for 15
seconds before any absorbance reading and set into motion for 30 seconds
after each increment of titrant.
3. Amperometric studies were performed with a Sargent Polarograph
model XV. Three different electrolysis vessels have been used:
- polarographic, H form, with stopcock, Sargent No. S-29401
- polarographic, Heyrovsky, 10 ml volume, Sargent No. S-29370.
- 50 ml Pyrex glass beaker.
The reference electrodes were a mercury pool used with the Heyrovsky
polarographic electrolysis vessel, or a standard calomel reference
electrode prepared according to Hume and Harris (29) with a flexible
salt bridge containing a saturated KCl solution and terminated by a
6 mm diameter glass tubing containing an agar KCl gel dipping into a
10 mm diameter glass fritted tube containing a minimum amount of solvent
and an appropriate electrolyte for contact. The glass fritted tube was
fitted in a rubber stopper and plunged into the measuring solution the
level of which was 1 or 2 millimeters higher than the level of the
solution in the tube.(30)
10
The working electrodes used were:
- a capillary dropping mercury electrode, 6 em length, Sar-
gent No. S-29417.
- a silver-silver chloride electrode, Beckman No. 39261.
4. All glassware used was Pyrex glass. 10 milliliter burets with
straight or curved delivery tips were graduated in 0.02 milliliter
divisions.
B. Reagents and Solutions
1. Chelan Solutions.
a) Ethylenediaminetetraacetic acid (EDTA)(Matheson Coleman
& Bell) 99% purity was used as received. Solutions were prepared by
weighting an approximate amount required for 0.01 M solution and after
dissolution in the solvent were standardized against a 0.01 M calcium
chloride standard aqueous solution using Calcon as metallochromic in-
dicator. The pH of the solution was maintained around 13 by the use
of diethyl amine. EDTA dissolved in methanol made 1 M in ethanolamine.
EDTA dissolved in dimethyl formamide upon heating near 148°C. The so-
lution was stable for less than six hours and a voluminous white pre-
cipitate appeared after that period of time. This precipitate could
be redissolved by heating the solution again but the normality had to
be redetermined. EDTA dissolved in dimethyl sulfoxide upon heating
0 near 106 C. The solution was very stable over months.
11
EDTA did not dissolve in the following organic solvents: methanol,
iso propanol, acetonitrile, propylene carbonate, 1,4-dioxane, benzene,
methyl ethyl ketone, methyl isobutyl ketone.
Aqueous 0.01 M standard EDTA solutions used to determine the nor
mality of metal solutions were prepared by dissolving an approximate
amount of the disodium salt (Fisher Scientific Co.) in water and the
titre was checked against the 0.01 M primary standard calcium solution.
b) 1,2-diaminocyclohexanetetranaacetic acid (DCTA) (Ciba
Geigy, Chel CD) was used as received.
A 0.01 M solution of DCTA in methanol, made 0.5 M in ethanolamine
was prepared by weighing an approximate amount of DCTA. The titre
was determined against the 0.01 M primary standard calcium solution
using Calcon as metal indicator and diethyl amine to adjust the pH to
13. A standard zinc nitrate solution can also be used with Eriochrome
Black T as indicator at a pH of 10 obtained with a buffer recommended
by Schwarzenbach (1) (dissolve 70 g ammonium chloride in 570 ml ammonia
of density 0.90, dilute to one liter with demineralized water.)
DCTA solutions in dimethyl formamide or dimethyl sulfoxide were
obtained by dissolving the appropriate amount of reagent in the organic
solvent under heating. Standardization was done with calcium or zinc
standard solutions as with the case of methanolic solutions.
c) Ethylene glycol-bis(2-aminoethylether)-tetraacetic acid
(EGTA)(Ciba-Geigy, Chel DE) was used as received. Solutions were
12
prepared by dissolving an approximate amount of reagent in the organic
solvent and were standardized against a standard zinc aqueous solution
with Zincon as indicator at pH around 9.5 according to Ringbom (31) or
with Eriochrome Black T as indicator at a pH of 10. EGTA dissolved
with heating in dimethyl sulfoxide but dissolved in methanol made 0.25
M in ethanolamine at room temperature.
d) Tetraethylenepentamine (tetren)(Eastman Kodak, technical)
was purified as the pentahydronitrate according to Reilley and Holloway
(32): dissolve 75 g of tetren in 150 ml of ethanol (solution A), cool
0 near 0 C. Prepare a mixture of 150 ml of water and 150 m1 of ethanol
and 2.5 moles (approximately 140 ml of nitric acid, density 1.42) of
nitric acid (solution B) and cool near 0°C. Add slowly with stirring
0 solution B to solution A, maintaining the temperature under 10 C.
Filter the precipitate by suction. Recrystallize five times from a-
queous 5% (by volume) nitric acid solution. The white precipitate is
washed with acetone and dried at 50°C.
To prepare a 0.01 M solution of tetren, weigh approximatly 0.6 g of
the salt and dissolve in 100 ml of organic solvent. Tetren being a
primary amine reacts with ketones to form condensation products
(Schiff bases) (33), therefore it was not prepared in these solvents.
A 0.0129 M tetren solution in methyl ethyl ketone was found to have a
concentration of 0.0041 M one month later. Tetren salt dissolves in
methanol made 1 M in ethanolamine and in dimethyl sulfoxide without
13
ethanolamine with slight heating. Due to the fact that nitrates are
decomposed in dimethyl fomamide (34) no experiments have been attempted
with tetren solutions in this solvent.
Tetren solutions were standardized according to Reilley and Schmid
(35) against a standard zinc nitrate solution with Eriochrome Black T
as indicator and triethanolamine maintaining a pH of 7.8.
2. Standard metal solutions.
a) A one liter 0.01 M calcium standard solution was prepared
by dissolving 1.000 g of calcium carbonate (primary standard, Thorn
Smith Co.) in 20 m1 water and 2 ml concentrated nitric acid, followed
by dilution with demineralized distilled water to volume.
b) A 0.01 M zinc standard solution was prepared by dissolving
0.8137 g of zinc oxide (Mallinckrodt, analytical reagent) in 20 ml water
and 2 ml concentrated nitric acid, followed by dilution to one liter
with demineralized distilled water.
c) Calcium, zinc and lead perchlorates were prepared by
dissolving calcium carbonate (primary standard), zinc oxide and lead
carbonate (Mallinckrodt, analytical reagent) in the minimum amount of
0.2 ~ perchloric acid solution under heating to expel the carbon
dioxide and heating to dryness on the sand bath. To prepare 100 ml
of 0.01 ~ solutions dissolve 0.3, 0.4 and 0.5 g of calcium, zinc or
lead perchlorate respectively in the appropriate solvent. Completely
anhydrous solutions of these metals have been prepared according to
14
Erley (36) and Starke (37) by dissolving the above mentioned amounts of
salts in 20 m1 of organic solvent and 2 m1 of 2,2-dimethoxypropane
followed by 30 minutes of stirring on a combination magnetic stirrer -
hot plate. Toward the end of that period the solution was heated to
expel the last trace of methanol and acetone and diluted to 100 m1 with
the organic solvent. No better results have been obtained with these
completely anhydrous metal solutions.
3. Indicator solutions.
a) Eriochrome Black T (Fisher Scientific Co.) was prepared
in 0.1% solution by dissolving 20 mg in 15 ml triethanolamine and 5 m1
absolute alcohol or 25 mg in 25 ml dimethyl formamide or dimethyl sul
foxide. The solutions were stable for months.
b) Calcon (Fisher Scientific Co.) solutions were prepared in
0.05% solution in methanol (12.5 mg in 25 ml methanol) and 0.1% solution
in dimethyl sulfoxide (25 mg in 25 ml dimethyl sulfoxide). The solutions
were stable for months.
c) Zincon (Aldrich Chemical Co.) was prepared 0.1% in methanol
or dimethyl sulfoxide and 0.065% in dimethyl formamide. The indicator
dissolves in methyl ethyl ketone only when tetrabutylammonium hydroxide
(25% in methanol) (Eastman Kodak) is present: (13 mg of Zincon in 20
ml of methyl ethyl ketone+ 0.5 m1 of tetrabutylammonium hydroxide).
All Zincon solutions were stable for one week only.
d) Methylthymol Blue (pentasodium salt) (Eastman Kodak) is
15
not soluble in alcohol (38), dimethyl sulfoxide or dimethyl formamide.
A 0.1% solution of Methylthymol Blue was prepared by dissolving
20 mg of the indicator in 2 ml of water followed by dilution with 18
m1 of methanol.
e) Aqueous Mercury-EDTA indicator solution (Anderson Labora
tories, Inc.) was used as received. The concentration of the complex
was determined according to Starostin (39) by back titration of an ex
cess of standard bismuth nitrate solution with 0.01 M EDTA using Xylenol
Orange as indicator and found to be 0.020 M.
4. 1,4-Dioxane (City Chemical Co., New York) was purified according
to Huber (40) by filtering through a chromatographic column containing a
strong acid cation exchanger, Amberlite IR-120 (Mallinckrodt). Other re
agents, analytical grade, were used as received. Methanol and methyl
isobutyl ketone were from Matheson Coleman & Bell, dimethyl formamide
dimethyl sulfoxide and methyl ethyl ketone from Fisher Scientific Co.
To adjust the "pH" of the solution during titrations, 0.1 M, 1 M
or concentrated solution of monoethanolamine, diethylamine, triethyl
amine, triethanolamine, tetrabutylammonium hydroxide, tetramethylam
monium hydroxide and perchloric acid were prepared in the solvent used
for titration. Perchloric acid, giving an explosive reaction with di
methyl sulfoxide (40), was first diluted with 1,4-dioxane. MOnoethanol
amine being a primary amine reacts with ketones to form condensation pro
ducts (Schiff bases) (33), therefore it was not used with these solvents.
16
C. Procedure
1. Direct visual titrations with metallochromic indicators.
Titrations were performed in a 50 ml beaker, at room temperature,
without using a dry box. The metal solution, one to five milliliters,
was pipeted into 25 ml of solvent followed by two to five drops of
metallochromic indicator. The acidity of the solution was brought to
the desired value, read on the millivolt scale of the pH meter equiped
with glass electrode and modified S.C.E. A magnetic stirrer was used
continuously during the analysis.
a) Titrations of zinc with EDTA
Zinc was titrated in methanol with Eriochrome Black T, Methylthy
mol Blue or Zincon as indicator. The acidity was maintained around
-300 mV with ethanolamine (around -200 mV for the last indicator). The
color changes were respectively from pink to blue for Eriochrome Black
T, blue to yellow for Methylthymol Blue and blue to pink for Zincon.
In dimethyl formamide the potential was maintained around -250 mV by
use of diethylamine (Eriochrome Black T) or ethanolamine (Zincon). The
color changes were respectively from pink to blue and blue to pink. In
dimethyl sulfoxide the same indicators were used at a potential around
-350 mV.
When methyl ethyl ketone was used as solvent, the titrant was a
solution of EDTA first dissolved in dimethyl sulfoxide then diluted
with methyl ethyl ketone in a 1 to 3 volume ratio. Tetrabutyl ammonium
hydroxide (20% in methanol) diluted 1:10 with the solvent was used to
bring the potential around -400 mV with the occasional addition of
perchloric acid (0.1 Min the solvent). The indicator Eriochrome Black
17
T changed color from pink-violet to blue.
b) Titrations of zinc with DCTA
In methanol as solvent, Eriochrome Black T, Methylthymol Blue and
Zincon required respectively the potential values: -350 mV, -300 mV
and -275 mV, which were obtained with 1 ml of ethanolamine, 1 ml of
diethylamine and 0.25 ml of ethanolamine in each particular case, for a
volume of 25 m1 of solution. They changed color respectively from pink
to blue, violet to canary yellow and blue to pink orange. The reaction
with Eriochrome Black T was very slow at room temperature but increased
noticeably when the solution was heated to 60°C. Micro titrations of
zinc in dimethyl sulfoxide were performed with Zincon as indicator at a
potential around -350 mV, obtained with the addition of ethanolamine.
The color change was from blue to pink orange. When methyl ethyl ketone
0 was used, the solution had to be heated at 60 C, and Methylthymol Blue
changed from blue to yellow at the end of the reaction. Two milliliters
of diethylamine added to 25 ml of solvent gave a potential around -300
mV.
c) Titrations of zinc with EGTA
Zincon used in methanolic medium buffered with ethanolamine around
-275 mV gave a very sharp color change from blue to pink orange. In
dimethyl sulfoxide it reacted slowly with the potential set around
-350 mV with the same buffer. Eriochrome Black T, on the contrary,
gave a very sharp change from violet pink to blue in the same experi-
mental conditions.
d) Titrations of zinc with tetren
The potential of the titrating solution was around -350 mV when
18
dimethyl sulfoxide served as solvent. Zincon changed color as in the
other media. In methyl ethyl ketone the solution was buffered with
triethanolamine, one molar in the same solvent. The ionic strength was
kept at 0.1 by the addition of lithium or sodium perchlorate. Zincon
was used as indicator and gave a very sharp end point.
e) Titrations of calcium with EDTA
In methanol calcium was determined with EDTA using Methylthymol Blue.
The potential was around -275 mV and was attained by the use of 1 ml of
ethanolamine for 25 ml of solvent. The indicator changed rapidly from
blue to yellow with a brief transition of grey before the end point.
f) Titrations of calcium with DCTA
Calcon was used for the titration of calcium in methanol. Two
milliliters of diethylamine added to the solution gave a potential
around -300 mV. The indicator changed from pink to violet.
With the same experimental conditions, Methylthymol Blue gave a
sharper end point from blue to yellow in passing through a grey tran
sition just before the appearance of the yellow color.
In methyl ethyl ketone, with the same experimental procedure as
above, the potential was -330 mV and Methylthymol Blue changed from
blue to canary yellow with a transition of greenish yellow which was
rather persistent.
g) Titrations of calcium with EGTA
In methanol calcium was titrated with the indicator system Zincon
+ Zn-EGTA. To 25 ml of methanol were added 1 ml of ethanolamine, 5
drops of Zincon, 2 drops of 0.005 M Zn-EGTA solution prepared by
mixing equal parts of 0.01 M zinc perchlorate and 0.01 M EGTA in
19
methanol, and the calcium solution. The solution was titrated slowly
and changed color from blue to orange.
In dimethyl sulfoxide the same procedure was used but the Zn-EGTA
solution was prepared in this solvent. The color change was from blue
to pink. The potential was around -400 mV.
If, instead of Zincon, Eriochrome Black Twas used, the color change
was even sharper from pink to blue. Quantitative results were obtained
if the ionic strength of the solution was maintained at 0.1 with potas
sium perchlorate.
h) Titrations of calcium + zinc with DCTA
In methyl ethyl ketone with Methylthymol Blue as indicator and
DCTA in methanol as titrant, the sum of calcium + zinc was determined
with the solution heated to 60° C and buffered around -330 mV with 2
ml of diethylamine. The color change was from blue to yellow.
i) Titrations of calcium + zinc with EGTA
The sum calcium + zinc was determined in dimethyl sulfoxide, using
Eriochrome Black T as indicator and ethanolamine as buffer at -300 mV.
The ionic strength was maintained at 0.1 with potassium perchlorate.
In methanol the indicator used was Zincon and the procedure was iden
tical with the ones concerning the titration of calcium or zinc
separately, but no need for the addition of the zinc-EGTA complex.
j) Analysis of calcium and zinc in a sample of polystyrene
One gram of polystyrene was dissolved in 25 ml of methyl ethyl
ketone and 10 drops of 0.1 M perchloric acid in the same solvent. In
the hot solution were added 2 ml of diethylamine and 5 drops of Methyl
thymol Blue. The solution assumed a blue color and was titrated
20
immediately and slowly with a 0.001 M solution of DCTA in methanol until
a yellow color was reached. The volume of titrant gave the sum of
calcium + zinc.
To titrate zinc, another aliquot was dissolved in the same way with
the addition of 0.25 g of lithium perchlorate. To the resulting solution
cooled to room temperature, were added 1 ml of 1 M triethanolamine in
methyl ethyl ketone, and 5 drops of zincon, (0.1 %) in dimethyl sulfoxide.
The solution assumed a blue color, with the potential around -110 mV.
Zinc was then titrated with a 0.001 M solution of tetren in dimethyl
sulfoxide.until the appearance of the pink color.
2. Photometric titrations.
a) The absorbance curves of the indicators and of the metal-
indicator complexes were recorded on the Beckman DK-2A instrument. The
concentration of the indicator was around 10-6 M, the metal concentration
about one hundred times larger.
Although Eriochrome Black T, Calcon, Methylthymol Blue were also
studied, only Zincon appeared to give the best resolved curves. Ac-
cording to Rush and Yoe (41) a 0.13 % solution of Zincon has a con-
centration of 0.002 M, therefore a 0.065 % or 0.1 % solution of the
indicator has a concentration of 0.001 M or 0.0015 M respectively. The
0.1% soltuion in dimethyl sulfoxide was diluted 50 times with the
solvent and 0.50 ml of the resulting solution was further diluted to
-6 25 m1 with the same solvent, giving a 6 x 10 molar solution. One
tenth milliliter of ethanolamine was added to provide a basic medium
having a concentration of about 0.06 M in ethanolamine. The newly pre-
pared solution was run on the Beckman DK-2A with dimethyl sulfoxide
21
serving as the blank solution. The zinc-Zincon solution was prepared
by the same procedure but one milliliter of 0.01195 M solution of zinc
perchlorate in dimethyl sulfoxide was added before diluting to 25 m1
with the solvent. The concentration of zinc was calculated to be
-4 4.78 x 10 M.
b) Photometric titrations were performed only for the deter-
mination of zinc with Zincon as indicator.
To 25 m1 of dimethyl sulfoxide were added 1 ml of 0.00119 M zinc
perchlorate and 5 drops of 0.1 % Zincon in the same solvent. A paten-
tial of the glass electrode vs. the modified S.C.E. around -350 mV
was obtained by adjusting the acidity with ethanolamine diluted ten
times with the solvent. The absorbance was measured at 650 nm while
titrating with a 0.00103 M solution of DCTA in dimethyl sulfoxide. The
concentration of Zincon was calculated to be 1.44 x 10-5 M in the titrating
solution.
With other titrants the same experimental conditions were followed
except that 20 ml of solvent were used instead of 25, thus giving a
-5 more concentrated Zincon solution (1.78 x 10 M). One milliliter
aliquots of 0.00119 M, 0.00179 M and 0.00171 ~ zinc perchlorate solution
in dimethyl sulfoxide were titrated respectively with 0.00103 M EGTA,
0.00104 M EDTA and 0.00102 M tetren, all dissolved in the same solvent.
For the titration of zinc in dimethyl formamide, 1 m1 of 0.01085 M
zinc perchlorate in dimethyl formamide was added to 25 ml of the solvent
followed by 1 m1 of 0.065 % Zincon solution in the same solvent. The
concentration of Zincon was calculated to be 3.7 x 10-5 M. The titrant
was 0.0070 M EDTA in dimethyl formamide. When the absorbance curves of
22
Zincon and zinc-Zincon in dimethyl formamide were recorded they appeared
to have the same shape as in the case of dimethyl sulfoxide but the
maximum difference in the absorbances was at 625 nm instead of 650 nm.
During the photometric titration the instrument was set at this wave
length of 625 nm.
3. Potentiometric Titrations
Two kindsofmetal indicator electrodes have been successfully used:
the mercury electrode and the lead ion selective electrode.
a) Titration of mercury with the mercury electrode
To 40 ml of dimethyl sulfoxide were added 4 m1 of 1 M potassium
perchlorate, 5 drops of ethanolamine diluted ten times with the same
solvent and 5 drops of 0.01 M mercuric acetate, all dissolved in the
same solvent. The potential indicated by the glass electrode was
around -380 mV. The solution was titrated with 0.01046 M EDTA in di
methyl sulfoxide and the potential of the mercury electrode recorded.
b) Titration of zinc with the mercury electrode
The former titration was stopped after the addition of 0.50 ml of
titrant, during which period the reaction was already completed, 1 m1 of
0.01364 M zinc perchlorate in dimethyl sulfoxide was then added and the
titration started again until a potential break was obtained.
c) Titration of calcium with the mercury electrode
To 40 ml of dimethyl sulfoxide were added 4 m1 of 1 M potassium per
chlorate, one drop of 0.001 M mercuric acetate and 1 m1 of 0.00914 M
calcium perchlorate, all dissolved in the same solvent. Ethanolamine
diluted ten times with dimethyl sulfoxide was used to bring the potential
of the glass electrode around -400 mV. The titration was performed with
23
0.01055 M EDTA in dimethyl sulfoxide.
When methanol was used as solvent, to 25 ml of solution were added
two drops of 0.020 M aqueous mercury-EDTA indicator solution and one
milliliter of 0.01344 M calcium perchlorate in methanol. The titration
was performed with 0.0100 M EDTA in methanol.
d) Titration of lead with the lead electrode
To 25 ml of dimethyl sulfoxide, made 0.1 ~with potassium per
chlorate, were added ethanolamine, diluted ten times with the same
solvent in order to have a potential at the glass electrode around -400
mV, and one milliliter of 0.0125 M lead perchlorate in dimethyl sulfoxide.
The potential at the lead electrode was recorded during the titration
with 0.01055 M EDTA in dimethyl sulfoxide.
e) Titration of zinc with the lead electrode
To 25 ml of dimethyl sulfoxide made 0.1 M with potassium perchlorate
were added ethanolamine, diluted ten times with the same solvent in
order to have a potential at the glass electrode around -400 mV., one
drop of 0.0125 M lead perchlorate and one milliliter of 0.01364 M zinc
perchlorate, both in dimethyl sulfoxide. The titration was performed
with 0.01055 M EDTA in the same solvent.
When 0.00845 M DCTA in dimethyl sulfoxide was used as titrant the
same experimental conditions were followed except for the drop of lead
perchlorate which had a concentration of 0.00125 M.
With 0.0101 M EGTA in dimethyl sulfoxide as titrant the zinc solu
tion had a concentration of 0.0171 M and the same experimental procedure
was used with one drop of 0.001 M lead perchlorate.
24
4. Amperometric Titrations
Before attempting an amperometric titration, different polarograms
for dilute solutions of zinc or calcium (10-4 M) were run in order to
determine the potentials of the plateaus. The amperometric titration
was then performed with the voltage set to the value of the beginning
of the plateau. A wide variety of experimental comditions were tried
but no quantitative results were obtained.
25
IV. RESULTS AND DISCUSSION
A. Direct Visual Titrations with Metallochromic Indicators
1. General comments.
The results of titrations of zinc, calcium and their sum are listed
in Tables I - IX. In performing these titrations the main problem was
the search for suitable "buffer substances" which could be used in the
solvents studied. Since there is no universal pH scale in organic
media the only logical way of recording the acidity of the solution was
to use the millivolt scale on the pH meter, equiped with the electrode
assembly: glass electrode vs. modified S.C.E. This system has been
investigated by Rorabacher et al. (42) in methanol, McClure and Reddy
(43) in dimethyl formamide and by Reddy (44) in di~thyl sulfoxide who
found it to be reliable. A comparison of the relative scales of
acidity of methanol, dimethyl formamide, dimethyl sulfoxide and methyl
ethyl ketone published by Bykova and Petrov (45) and by Kreshkov (46)
show that the pK values for ionization of these solvents are respective
ly 16.7, 24, 31 and 31. These figures prove that the "pH range" in
these media vary widely but according to Petrov and Bykova (47) these
values are valid only for pure anhydrous solvents and if traces of
water are present, which is mostly the case in practical analytical
determinations, then these figures change drastically. Bates (48) and
Reddy (44) have studied some buffer mixtures in methanol and dimethyl
sulfoxide respectively. Some of these mxitures (benzoic acid + sodium
benzoate) have been used ( in dimethyl sulfoxide ) but did not give
any good results.
26
The three metallochromic indicators mostly used, Eriochrome Black T,
Methylthymol Blue and Zincon change color in basic aqueous media. Var
ious amines have been tried to control the basicity of the solution
during the titrations. The potential values listed in each determination
were determined by two factors: 1) they must keep the basicity of the
solution within a reasonable narrow range, 2) the indicator must change
color at these conditions. The first factor was determined experimental
ly by titrating a small amount of amine intended to be used as buffer,
with a standard acid solution dissolved in the same solvent. The
titration curve, titrant added vs. potential at the glass electrode,
was drawn and the potential at the half neutralization point was re
corded. As in the case of aqueous acidimetric titrations the neutral
ization curve shows at the half neutralization point an almost flat
portion in which the potentials do not vary very much. The possibility
of a reaction between the amine buffer and the solvent was another
problem, eg. ethanolamine cannot be used in methyl ethyl ketone (33).
2 Titrations of zinc.
The results are condensed in Tables I - IV.
With EDTA as titrant, Eriochrome Black T seemed to be the best in
dicator in all the media studied. When methyl ethyl ketone was the
solvent, EDTA was first dissolved in dimethyl sulfoxide then diluted
(1:3) with the solvent. This experiment was performed in the hope to
use it for the determination of zinc in polystyrene, since polystyrene
does not dissolve in dimethyl sulfoxide but dissolves in methyl ethyl
ketone or in a mixture of this latter solvent with a small amount of
dimethyl sulfoxide added. Attempts to titrate zinc in dimethyl
Table I. Results of Titrations of Zinc with EDTA Using Metallochromic Indicators
A. Solvent: Methanol
1. Indicator: Eriochrome Black T (Buffer: ethanolamine)
Taken mg.
0.635
1.270
1.905
2.540
3. 085
Found mg.
0. 647
1.273
1.906
2.520
3.085
Number of Determinations
4
4
4
4
4
% Relative Standard Deviation
2.68
0.57
0.43
0.69
0.00
2. Indicator: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard mg. mg. Determinations Deviation
0.635 0.645 4 2.31
1.270 1.262 4 0.86
1905 1.901 4 0.41
2.540 2.518 4 1.02
27
% Error
+1.90
+0.24
+0.05
-0.80
0.00
% Error
+1.60
-0.63
-0.20
-0.86
28
Table I (continued)
3. Indicator: Methylthymol Blue (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.588 0. 540 3 10.00 -8.16
2. 940 2.964 3 0.97 +0.80
3.085 3.087 3 0.16 +0.06
B. Solvent: Dimethyl formamide
1. Indicator: Eriochrome Black T (Buffer: diethylamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.709 0.723 1 +1.97
1.418 1.395 1 -1.62
2.127 2.094 1 -1.55
2.836 2.796 1 -1.41
29
Table I (continued)
2. Indicator: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.709 0.753 1 +6.20
1.418 1.477 1 +4.16
2.127 2.109 1 -0.85
2.836 2.823 1 -0.46
C. Solvent: Dimethyl sulfoxide
1. Indicator: Eriochrome Black T (Buffer: ethanolamine)
Taken mg.
0.781
1.562
2.343
3.124
3.905
Found mg.
o. 779
1.561
2.338
3.127
3.913
Number of Determinations
3
3
3
3
3
% Relative Standard Deviation
0.32
0.32
0.37
0.19
0.28
% Error
-0.26
-0.06
-0.20
+0.96
+0. 20
30
Table I (continued)
2. Indica tor: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.078 0.078 3 1. 74 0.00
0.156 0.157 3 0.86 +0.64
0.234 0.235 3 1. 60 +0.43
0.312 0.311 3 1.38 +0.32
0.391 0.380 3 3.54 -2.81
0. 781 0.784 3 1.12 +0.38
1.562 1.568 3 0.56 +0.38
2.343 2.336 3 0.39 -0.30
3.124 3.102 3 0.92 -0.70
3.905 3.865 3 1. 25 -1.02
D. Solvent: Methyl ethyl ketone
Titrant: EDTA dissolved in DMSO +methyl ethyl ketone (1:3)
Indicator: Eriochrome Black T
(Buffer: tetrabutylammonium hydroxide + perchloric acid)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.891 0.926 1 +3.93
1. 782 1.819 1 +2.07
2.673 2. 729 1 +2.09
Table II. Results of Titrations of Zinc with DCTA Using Metallochromic Indicators
31
A. Solvent: Methanol
1. Indicator: Eriochrome Black T (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.635 o. 634 3 1.03 -0.16
1.270 1.292 3 3.23 +1. 73
1.905 1.920 3 1.53 +0.80
2.540 2.525 3 1.62 -0.60
3.175 3.105 3 2. 74 -2.20
2. Indicator: Methylthymol Blue (Buffer: diethylamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.634 0.641 3 1.35 +1.10
1.268 1. 266 3 0.33 -0.16
1.902 1.902 3 0.61 0.00
2.536 2.512 3 1.16 -0.95
32
Table II (continued)
3. Indicator: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviations
0.634 0.629 3 1.30 -0.79
1.268 1.255 3 1.45 -1.02
1.902 1.873 3 1.77 -1.52
2.538 2.509 3 1.26 -1.14
B. Solvent: Dimethyl sulfoxide
Indicator: Zincon (Buffer: ethanolamine)
.Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.078 0.078 2 o.oo 0.00
0.156 0.158 2 1.43 +1.28
0.234 0.235 2 0.85 +0.49
0.312 0.314 2 1.28 +0.64
0.391 0.391 2 0.25 0.00
Table II (continued)
C. Sol vent: Methyl ethyl ketone
Titrant: DCTA in methanol
Indicator: Methylthymol Blue (Buffer: diethylamine)
Taken mg.
0.065
0.130 .
0.195
0.260
Found mg.
0.064
0.129
0.191
0.257
Number of Determinations
3
3
3
3
% Relative Standard Deviation
1.08
0.54
0.36
0.62
33
% Error
-1.54
-0.77
-2.05
-1.15
Table III. Results of Titrations of Zinc with EGTA Using Metallochromic Indicators
A. Solvent: Methanol
Indicator: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard mg. mg. Determinations Deviation
0.635 0.639 4 1.23
1.270 1.268 4 0.86
1.905 1.914 4 0.92
2.540 2.543 4 0.69
B. Solvent: Dimethyl sulfoxide
34
% Error
+0.63
-0.16
+0.47
+0.12
1. Indicator: Eriochrome Black T (Buffer: ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.892 0.922 2 5.0 +3.36
1.784 1.844 2 5.0 +3.36
2. 676 2.783 2 5.7 +4.00
3.568 3.701 2 5.4 +3.72
4.460 4.646 2 5.9 +4.17
35
Table III (continued)
2. Indicator: Zinc on (Buffer ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.892 0.878 4 2.21 -1.57
1.784 1. 742 4 2.77 -2.35
2.676 2.592 4 3. 71 -3.14
3.568 3.559 4 1.50 -0.25
4.460 4.447 4 1. 72 -0.29
Table IV. Results of Titrations of Zinc with Tetren Using Matallochromic Indicators.
A. Solvent: Dimethyl sulfoxide
1. Indicator: Zincon (Buffer: ethanolamine)
Taken Found Number of % Relative Standard mg. mg. Determinations Deviation
0.112 0.115 1
0.224 0.230 1
0.335 0.345 1
0.447 0.467 1
0.559 0.573 1
1.118 1.127 2 1.44
2.236 2. 241 2 1. 73
3.354 3.368 2 1.52
4.472 4.501 2 1.38
5.590 5.595 2 0.20
36
% Error
+2.68
+2.68
+2.98
+4 .47
+2.50
+0.81
+0.22
+0.41
+0.65
+0.09
37
Table IV (continued)
2. Indicator: Zincon (tetraethylammonium perchlorate +
ethanolamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.112 0.113 2 1. 78 +0.89
0.224 0.233 2 5.35 +4.01
0.335 0.342 2 2.82 +2.09
0.447 0.453 2 1.89 +1.34
0.559 0.568 2 2.15 +1.61
1.118 1.134 1 +1.43
2.236 2. 240 1 +0.18
3.354 3.347 1 -0.21
4.472 4.461 1 -0.25
5.590 5.574 1 -0.29
38
Table IV (continued)
B. Solvent: Methyl ethyl ketone
Titrant: Tetren in DMSO
1. Indicator: Zincon (sodium perchlorate+ triethanolamine)
Taken mg.
0.784
1.568
2.352
3.136
Taken mg.
0.784
1.568
2.352
3.136
Found Number of % Relative Standard % Error mg. Determinations Deviation
0.800 2 2.97 +2.04
1.562 2 1.46 +0.38
2.336 2 0.96 -0.68
3.113 2 1.05 -0.73
2. Indicator: Zincon (lithium perchlorate + triethanolamine)
Found mg.
o. 777
1.555
2.328
3.109
Number of Determinations
2
2
2
2
% Relative Standard Deviation
1.45
1.39
1. 52
1. 22
% Error
-0.89
-0.83
-1.02
-0.86
39
formamide with Eriochrome Black T buffered by ethanolamine or triethanol
amine, with Methylthymol Blue and ethanolamine, with Calcon and diethyl
amine have failed. In methyl ethyl ketone, Zincon buffered with tri
ethanolamine, Calcon with triethylamine, Eriochrome Black T with trieth
anolamine or the mixture tetramethylammonium hydroxide + tetraethyl
ammonium perchlorate did not give any result. In dioxane, which
dissolves polystyrene, Eriochrome Black T buffered by ethanolamine or
triethylamine did not give any quantitative result when EDTA dissolved
in dimethyl sulfoxide was used as titrant.
With DCTA as titrant (Table II), Methylthymol Blue appeared to be
the best indicator, particularly in the solvent methyl ethyl ketone and
diethylamine as buffer the figures suggest a possibility of determination
of zinc in polystyrene. With EGTA {Table III), Zincon proved to be
superior to Eriochrome Black T.
Tetren dissolved in dimethyl sulfoxide could be used to titrate
zinc in methyl: ethyl ketone. It was noticed that lithium perchlorate
used to maintain the ionic strength at 0.1 was much better than sodium
perchlorate since the color change was very rapid and sharp.
3. Titrations of calcium.
Direct titrations of calcium have been less successful than the case
of zinc. To the indicators used for zinc was also added Calcon.
With EDTA (Table V), only Methylthymol Blue gave some results in
methanol. Calcon did not behave quantitatively in all the solvents
and buffers {even tetrabutylammonium hydroxide, one of the most basic
buffers) tried.
DCTA appeared to be more effective than EDTA since results were
Taken mg.
0.397
0. 794
1.191
1.588
Table V. Results of Titrations of Calcium with EDTA in Methanol Using Methylthymol Blue
(Buffer: ethanolamine)
Found Number of % Relative Standard mg. Determinations Deviation
0.404 3 2.36
0.806 3 2.04
1.206 3 1.80
1.608 3 1.68
40
% Error
+1.76
+1.51
+1.26
+1.26
Table VI. Results of Titrations of Calcium with DCTA Using Metallochromic Indicators
A. Solvent: Methanol
Taken mg.
0.842
1.263
1.684
2.105
Taken mg.
0.421
0.842
1.263
1.684
2.105
1. Indicator: Calcon {Buffer: diethylamine)
Found Number of % Relative Standard mg. Determinations Deviation
0.850 4 1.55
1.276 4 1.40
1.695 4 1.12
2.120 4 0.90
2. Indicator: Methylthymol Blue (Buffer: diethylamine)
Found mg.
0.427
0.852
1.280
1. 707
2.128
Number of Determinations
4
4
4
4
4
% Relative Standard Deviation
1.90
1.45
1.58
1.67
1.28
41
% Error
+0.95
+1.02
+0.65
+0.71
% Error
+1.42
+1.18
+1.34
+1.36
+1.09
42
Table VI (continued)
B. Solvent: Methyl ethyl ketone
Titrant: DCTA in methanol
Indicator: Methylthymol Blue (Buffer: diethylamine)
Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation
0.057 0.058 3 2.14 +1.75
0.114 0.115 3 1.52 +0.87
0.171 0.172 3 1.01 +0.58
0.228 0.228 3 0.70 0.00
0.285 0.285 3 0.43 0.00
0.421 0.429 3 2.85 +1.90
0.842 0.851 3 1.39 +1.06
1.263 1. 276 3 1.31 +1.02
1.684 1. 714 3 2.16 +1. 78
2.105 2.135 3 1. 74 +1.42
Table VII. Results of Titrations of Calcium with EGTA Using the System Zincon + Zinc-EGTA
A. Solvent: Methanol
Indicator: Zincon + Zn-EGTA (Buffer: ethanolamine)
Taken Found Number of % Relative Standard mg. mg. Determinations Deviation
0.397 0.401 3 1. 22
0. 794 0.805 3 1. 74
1.191 1. 212 3 2.20
1.588 1.618 3 2.39
B. Solvent: Dimethyl sulfoxide
1. Indicator: Zincon + Zn-EGTA (Buffer: ethanolamine)
Taken Found Number of % Relative Standard mg. mg. Determinations Deviation
0.366 0.367 4 2.24
0.040 0.044 1
0.080 0.089 1
0.120 0.134 1
0.160 0.173 1
43
% Error
+1.00
+1.38
+1. 76
+1.88
% Error
+0.27
+10.00
+11.25
+11.66
+8.13
Table VII. (continued)
2. Indicator: Eriochrome Black T + Zn-EGTA
~uffer: ethanolamine)
Taken Found Number of % Relative Standard mg.
0.040
0.080
0.120
0.160
Taken mg.
0.040
0.080
0.120
mg. Determinations Deviation
0.042 1
0.083 1
0.124 1
0.164 1
3. Indicator: Eriochrome Black T + Zn-EGTA (+KC104)
~uffer: ethanolamine)
Found mg.
0. 040
0.080
0.120
Number of Determinations
1
1
1
% Relative Standard Deviation
44
% Error
+5.00
+3 0 75
+3.34
+2.50
% Error
0.00
0.00
0.00
45
obtained with Calcon and Methylthymol Blue in methanol and with Methyl
thymol Blue in methyl ethyl ketone. This latter case suggested a pos
sibility of determination of calcium in polystyrene with zinc. Calcon
was once more inefficient in all the other media tried.
With EGTA (Table VII.) none of the indicators used gave quantitative
results but if trace quantities of zinc-EGTA complex were added then a
substitution reaction took place and the titration was possible. This
kind of reaction was investigated by Ringbom, Pensar and·Wanninen (31),
who used the system Zincon + zinc-EGTA. In methanol only this latter
reaction was studied, but in dimethyl sulfoxide the system Eriochrome
Black T + zinc-EGTA was also tried and proved to be superior to the
system with Zincon, in particular if the ionic strength was kept at 0.1
with potassium perchlorate, then quantitative results seemed to be
attained.
To sum up, in comparison with EDTA and EGTA, DCTA proved to be the
best titrant for calcium. This result does not contradict the findings
of Holloway and Reilley (49) when they studied the metal stability
constants of various aminopolycarboxylate ligands in water with their
mercury electrode.
4. Titrations of calcium+ zinc.
Calcium + zinc have been determined in methyl ethyl ketone with
DCTA dissolved in methanol and Methylthymol Blue as indicator in an
anticipated try to use the method for the analysis of polystyrene. The
sum of these two metals has also been determined with EGTA, but the
addition of the complex zinc-EGTA was not necessary. EGTA titrant
was much better than DCTA but unfortunately EGTA could not be used
Table VIII. Results of Titrations of Calcium + Zinc with DCTA Using Methylthymol Blue
Solvent: Methyl ethyl ketone
Titrant: DCTA in methanol
(Buffer: diethylamine)
Taken: J.l-mole Ca • ••••••••••••••••• 1.01 1.01
ll-mole Zn . ••.••.••.••.•.... 0.99 1.98
Total JJ-mole Ca+Zn taken . .......... 2.00 2.99
Total ).!-mole Ca+Zn found . .......... 2.00 3.00
Number of determinations •••••••.••• 4 4
% Relative standard deviation ••.... 1.04 1.43
% Error . ........................... 0.00 +0.33
46
1.01
2.97
3.98
4.01
4
1.65
+0.75
Table IX. Results of Titrations of Calcium + Zinc With EGTA Using Metallochromic Indicators
A. Solvent: Dimethyl sulfoxide
Indicator: Eriochrome Black T (Buffer: ethanolamine)
Taken: ~-mole Ca •••••••••••••• 0.91
p-mole Zn .... ......... . 1.71
Total ~-mole Ca + Zn taken ••••• 2.62
Total ~-mole Ca + Zn found ••••• 2.63
Number of determinations •••.••• 5
%Relative Standard Deviation •• 0.02
% Error ....................... . +0.38
B. Solvent: Methanol
Indicator: Zincon (Buffer: ethanolamine)
Taken: ~-mole Ca • ••••••••••••• 9.9 19.8
~-mole Zn • ••••••••••••• 9.7 9.7
Total ~-mole Ca + Zn taken ••••• 19.6 29.5
Total ~-mole Ca + Zn found ••••• 19.9 30.0
Number of determinations ••••.•• 3 3
% Relative Standard deviation •• 0.16 0.21
% Error ......•........ · · · · · · · • · +1.53 +1.69
47
48
with methyl ethyl ketone since an immediate precipitate formed when they
were brought together.
5. Analysis of calcium and zinc in a sample of polystyrene. The
polystyrene sample was dissolved by pouring it into the solution already
stirred on the magnetic stirrer-hot plate; this prevented it from
forming a big coagulate which sticked to the stirring bar and stopped its
motion. The dissolution was complete within ten minutes and the titra
tion performed with the hot solution required less than three minutes
thus the total analysis time was less than 15 minutes. Two samples
were analyzed; one with an assay labled Zn = 100-115 ppm, Ca = 5 ppm,
the other Ca = 100 ppm, Zn ~ 1 ppm,(?), phosphate (?). The sample with
out phosphate gave after analysis of zinc a concentration of calcium
equal to 18 ppm. The sample with phosphate gave a very low result for
calcium and the color of the complex was not well developed. This
interference due to the phosphate has been reported by Garcia Montelongo,
Herrera and Arias (50) who studied the colorimetric determination of
calcium with Methylthymol Blue in aqueous medium. It appeared that this
interference was also valid in methyl ethyl ketone.
6. Conclusion
Direct visual titrations of zinc and calcium with metallochromic
indicators in non aqueous media are possible, although more limited
than in aqeuous solution. Eriochrome Black T and Zincon gave good re
sults for the analysis of zinc, Methylthymol Blue for the analysis of
calcium. This latter indicator has been extensively studied in aqueous
medium by Korbl and Pribil (38), Buben, Korbl and Pribil (51), Korbl
and Kakac (52), Janousek and Studlar (53), Chalmers and Miller (54) and
Table X. Results of Titration of Zinc and Calcium in Polystyrene
Origin of sample: Dow expandable polystyrene (Bendix Co.)
Product: SD - 572
Lot No.: OH - 1701
Assay: Zinc = 100 to 115 ppm
Calcium 5 ppm
A. Titration of Zinc
Solvent methyl ethyl ketone
Titrant 0.001 M tetren in dtmethyl sulfoxide
Indicator Zincon
Buffer Triethanolamine
Weight of sample Weight of zinc found
1.000 g 109 ppm
112
106
98
114
106
Average • •••••..•••• • • • • • • • • • • • • • • 108 ppm
Standard deviation............... 6
%Relative standard deviation.... 6
Confidence ltmit at 90 % Probability level •••••••••••••••• + S
Result: 108 ± 5 ppm Zinc
49
50
Table X. (continued)
B. Microtitration of zinc
Same experimental conditions as in A except titrant is 0.0001 M and
weight of sample is 0.100 g.
Weight of zinc found
100 ppm Average . ..........•............. . 103 ppm
110 Standard deviation. . • • • • • • • • • • • • 10
110 %Relative standard deviation •••• 10
90 Confidence limit at 90 %
100 probability level. • • • • • • • • • • • • •. + 8
110 Result: 103 ± 8 ppm Zinc
C. Titration of calcium+ zinc.
Solvent: methyl ethyl ketone
Titrant: 0.001 M DCTA in methanol
Indicator: Methylthymol Blue
Buffer: diethylamine
Weight of sample
DCTA }l-mole
* Zinc p-mole
*
1.000' g 2.10 2.08 2.16 2.07 2.12 2.12
Value taken from A
1.65
Average for calcium •••• 18 ppm
Standard deviation ••••• 1
Calcium J..l-mole
0.45 0.43 0.51 0.42 0.47 0.47
% Relative standard
Calcium ppm
18 17 20 17 19 19
deviation ................ 7
Confidence limit at 90% probability level •.••••• ± 1
Result: 18 + 1 ppm Calcium
51
Yoshino et al. (55). Since it shows two ranges of pH color changes it
could be used to titrate zinc buffered by urotropin at pH = 6.5 and the
sum zinc + calcium buffered by annnonia. Its behavior in "acidic" medium
in methyl ethyl ketone for the analysis of zinc should be an interesting
investigation.
B. Photometric Titrations
1. The absorbance curves in Figure 1 show a maximum difference be
tween the indicator and the metal-indicator complex at 650 nm. Due to
the fact that the total volume of titrant added represented less than
8 % of the total volume of the solution, a dilution factor was not
necessary in constructing the titration curve.
2. The titration curves in dimethyl sulfoxide as solvent are shown
on Figure 2. DCTA appears to be the best titrant for zinc: the curve
drops sharply and the relative error is +0.84 %. No extrapolation is
needed to locate the end point which lies on the abscissa axis.
For the case of EGTA, EDTA and tetren, the end point has to be extra
polated by the interesection of two straight lines.
Concerning their effectiveness the titrants can be ordered in the
following decreasing order according to Table XI:
DCTA > Tetren > EDTA > EGTA
This compares favorably with the direct visual titration which gives the
following order:
DCTA > EDTA > Tetren > EGTA
52
Figure 1. Absorbance curves in dimethyl sulfoxide
(A) -6 -4 6.0 x 10 M Zincon, 4.78 x 10 M Zinc perchlorate
0.06 M ethanolamine in dimethyl sulfoxide
(B) -6 6.0 x 10 M Zincon, 0.06 M ethanolamine in
dimethyl sulfoxide
0 0 U)
0 I() &
()
0 0 &()
&()
"'" v 0 &()
v &()
~------~------~------~------~----~N
0 v 0
0 0
0 v
ro N
0
0 0
3:>Ntt8~0S8tt
53
E
c: .. :I: ..._ <.!)
z w
...J .
w
r-l
~
Q)
.... ;j
;: 00
..-I ~
54
Figure 2. Photometric titrations of zinc in dimethyl sulfoxide
Wavelength: 650 nm; Indicator: Zincon
(A) Titrations of 1.19 ~-mole of zinc perchlorate in DMSO
Titrant: 0.00103 M DCTA in DMSO
Zincon: 1.44 x 10-5 M in DMSO
(B) Titration of 1.19 ~-mole of zinc perchlorate in DMSO
Titrant: 0.00102 M EGTA in DMSO
Zincon: 1.78 x 10 -S Min DMSO
(C) Titration of 1.79 v-mole of zinc perchlorate in DMSO
Titrant:
Zincon:
0.00104 M EDTA in DMSO
1.78 x 10-5 M in DMSO
(D) Titration of 1.71 ~-mole of zinc perchlorate in DMSO
Titrant: 0.00102 M tetren in DMSO
Zincon: 1.78 x 10-S Min DMSO
55
0.30 r---T1----.,.1----,1---,--1 ---.
0.20 ~ A -
0.10- -
I I I I
lJJ 0.20 u z ~ CD a: 0.10 0 (/) CD <t
0.30
0.20 D
0.10
0 0.5 1.0 1.5 2.0
TITRANT ADDED (MICROMOLES) Figure 2.
56
Figure 3. Photometric titration of zinc in dimethyl formamide
Wavelength: 625 nm; Indicator: Zincon
Titration of 10.85 ~mole of zinc perchlorate in dimethyl formamide
Titrant:
Zincon:
0.0070 ~ EDTA in dimethyl formamide
-5 3.7 x 10 Min dimethyl formamide
57
. 0.20~--------
L&J 0.15 u z <( m a:: 0 0.10 (/) m <(
0.05
0 I 2 3 4 5 6 7 8 9 10 II 12 13
TITRANT ADDED (MICROMOLES)
Figure 3.
Table XI. Results of Photometric Titrations of Zinc
A. Solvent: Dimethyl sulfoxide
1. Indicator: Zincon (Buffer: ethanolamine)
Zinc Taken Zinc Found Number of % Error ll-mole ll-mole Determination
1.19 1.20 1
1.19 1.23 1
1. 79 1.84 1
1. 71 1.68 1
B. Solvent: Dimethyl formamide
1. Indicator: Zincon (Buffer: ethanolamine)
Zinc Taken v-mole
10.85
Zinc Found ll-mole
10.95
Number of Determinations
1
+0.84
+3.36
+2.79
-1.75
% Error
+0.92
58
Titrant
DCTA
EGTA
EDTA
Tetren
Titrant
EDTA
59
In fact the relative errors in the two methods are of the same order
of magnitude and the photometric titration does not give better results
in comparison with the direct visual titration. One further disadvan
tage of the photometric method is the fact that it is time consuming and
the end point must be determined on a graph. A direct visual titration
takes about five minutes while a photometric titration requires twenty
to thirty minutes. The photometric method will only be useful in the
case where impurities in the solution might block the indicator or when
a recording automatic phototitrator is available.
3. In dimethyl formamide the titration curve of zinc at 625 nm in
Figure 3. shows a sharp end point. The relative error of less than 1 %
is much better than the direct visual titration which has a error of 6 %.
Due to the lack of time no investigation have been made in methanol
as solvent.
4. The absorbance curves run for different indicators Calcon, Methyl-
thymol Blue, Eriochrome Black T and Zinconc and their complexes with
calcium or zinc showed that Zincon (Figure 1), gave two best resolved
curves in which the maximum absorbance of one curve occured at a wave
length corresponding to the minimum absorbance of the other, and vice
versa. For this reason, only some photometric titrations of zinc with
Zincon as indicator have been studied. Photometric titrations being an
extension of the direct visual method of titration should of course
give positive results for cases which work in the latter method.
C. Potentiometric Titrations
1. Table XII and Figure 4 show the results of potentiometric
titrations of calcium and zinc with EDTA in dimethyl sulfoxide and in
Metal
Hg
Zn
Ca
Ca
Ca
Ca
Zn
Zn
Taken ll-rnole
2.50
13.64
9.14
18.28
13.44
33.60
13.64
27.28
Table XII. Results of Potentiometric Titrations of Calcium and Zinc with EDTA
Found ll-mole
2.55
13.90
9.35
17.65
13.80
33.50
13.37
27.37
% Error
+2.00
+1.90
+2.30
-2.90
+2.68
-0.30
-1.98
+0.33
Electrode Solvent
Mercury DMSO
Mercury DMSO
Mercury DMSO
Mercury DMSO
Mercury Methanol
Mercury Methanol
Lead DMSO
Lead DMSO
Amplitude of Potential Break
rnV
25
75
25
60
90
96
55
130
Curve in Figure 4.
A
B
c
Not Drawn
D
Not Drawn
E
Not Drawn
0\ 0
61
Figure 4. Potentiometric titrations of calcium and zinc with EDTA
(A) Titration of 2.5 ~-moles of mercuric acetate in DMSO
Titrant: 0.01046 M EDTA in DMSO
Mercury electrode vs. modified S.C.E.
(B) Titration of 13.64 ~-moles of zinc perchlorate in DMSO
Titrant: 0.01046 M EDTA in DMSO
Mercury electrode vs. modified S.C.E.
(C) Titration of 9.14 ~-moles of calcium perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
Mercury electrode vs. modified S.C.E.
(D) Titration of 13.44 ~-moles of calcium perchlorate in DMSO
Titrant: 0.0100 M EDTA in methanol
Mercury electrode vs. modified S.C.E.
(E) Titration of 13.64 ~-moles of zinc perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
Lead ion selective electrode vs. modified S.C.E.
000
. IJJ . 0 . C/)
~ -3.00 IJ.. -0 0 :E
. -400 tn >
> E ..
...J <( -1-
-500
z IJJ -600 1-0 a..
-700
0
62
E
5 10 15 20
TITRANT ADDED (MICROMOLES)
Figure 4.
63
methanol.
a) Volume determinations.
Except for curve A and D where no volume correction is needed since
in A mercury was titrated directly with the mercury electrode and in
D 2 drops of Hg-EDTA indicator solution were used, all the volumes read
on the end point of other curves must be corrected for the indicator
blank. These blank volumes are respectively 2.5 ~-moles for curve B,
0.05 ml x 0.001 M = 0.00005 mM = 0.05 ~-mole for curve C, and 0.05 ml
x 0.00125 M = 0.000625 mM = 0.63 ~-mole for curve E, since one drop of
the indicator has a volume of 0.05 ml.
b) Comments on the results.
The amplitude of the potential break will be defined by the dif
ference in potential between the two points on the curve, one on each
side of the end point, separated by an increment of one micromole.
The titration of zinc in dimethyl sulfoxide with the mercury elec
trode gives a sharper potential break than with the lead electrode (75
mV vs. 55 mV). The volume of indicator ion used was respectively 2.5
~-moles of mercuric ions and 0.63 ~-mole of lead (II) ions. Figure 6
shows that when the lead electrode is used the amplitude of the poten
tial break increases with an increase of the concentration of zinc.
The experiments with the mercury electrode show the opposite.
It has not been possible to determine calcium with the lead elec
trode. When the mercury electrode was used the volume of indicator
was respectively 2 x 0.05 ml x 0.020 M = 0.002 mM = 2 ~-moles of Hg (II)
ions in methanol and 0.05 ~-mole of Hg (II) ions in dimethyl sulfoxide
(see part a) above). In methanol the mercuric ions were introduced
64
in the form of an aqueous solution. The presence of these 2 drops of
water in 45 ml of solution gives a water concentration of 0.22 % and
might explain why the amplitude of the potential break is larger in
methanol than in dimethyl sulfoxide (90 mV vs. 25 mV) (curves C and D).
Other experiments performed at different concentrations of calcium show
that in methanol the amplitude of the potential break increases slightly
with an increase of concentration of calcium while in dimethyl sulfoxide
this trend is more accentuated.
Figure 1 indicates also that when the mercury electrode is used, the
titration curve for zinc appears in a higher potential region than the
curve for calcium. This result in dimethyl sulfoxide is also found in
aqueous medium (56).
A comparison between curves B and C shows that in the titrations of
zinc and calcium with the mercury electrode the potential break is larger
for zinc than for calcium (75 mV vs. 25 mV). This leads to the assumption
that in dimethyl sulfoxide, as in water, the stability constant of the
zinc-EDTA complex is larger than the stability of the calcium-EDTA com
plex.
2. Table XIII and Figure 5 give the results of the potentiometric
titration of lead in dimethyl sulfoxide with EDTA and EGTA using the lead
ion selective electrode.
Since the lead ion is determined with the lead electrode no volume
correction is necessary.
EDTA appears to be a better titrant for lead than EGTA. For almost
the same quantity of lead the potential break with EDTA is 140 mV, with
Table XIII. Results of Potentiometric Titrations of Lead with EDTA and EGTA
Lead ion selective electrode vs. modified S.C.E.
Solvent: Dimethyl sulfoxide
Taken Found % Error Titrant Amplitude of 11-mole 11-mole Potential Break
mV
12.46 12.66 +1.60 EDTA 140
24.92 25.32 +1.60 EDTA 100
12.83 12.73 -0.78 EGTA 44
25.66 25.76 +0.40 EGTA 26
65
Curve in Figure 5
A
B
c
D
66
Figure 5. Potentiometric titrations of lead with EDTA and EGTA
Lead ion selective electrode vs. modified S.C.E.
(A) Titration of 12.5 u-moles of lead perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
(B) Titration of 25.0 umoles of lead perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
(C) Titration of 12.8 u-moles of lead perchlorate in DMSO
Titrant: 0.0100 ~ EGTA in DMSO
(D) Titration of 25.6 u-moles of lead perchlorate in DMSO
Titrant: 0.0100 M EGTA in DMSO
0 It) ,.._ I
·3·~·5 031-:IIO
OW
"S
t\ A
W
'1\1
11
N3
l0d
0 ~
It) (\J
0 (\J
!Q
0
67
- (/) UJ
__. 0 :E 0 a:: u -:E -0
. 1.()
UJ
al
0 ~
&, 0
......
<X: I'Ll
..... z <X: a:: ..... -.....
Table XIV. Results of Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA.
Lead ion selective electrode vs. modified S.C.E.
Solvent: Dimethyl sulfoxide
Taken Found % Error Titrant Amplitude of Jl-mole Jl-mole Potential Break
mV
13.64 13.37 -1.98 EDTA 55
27.28 27.37 +0.33 EDTA 130
13.64 13.35 -2.12 DCTA 120
27.28 26.99 -1.06 DCTA 90
17.10 17.80 +4 .09 EGTA 60
34.20 25.80 +4.67 EGTA 35
68
Curve in Figure 6
A
B
c
not drawn
D
E
69
Figure 6. Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA
Lead ion selective electrode vs. modified S.C.E.
Solvent: Dimethyl sulfoxide
(A) Titration of 13.64 ~-moles of zinc perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
(B) Titration of 27.28 u-moles of zinc perchlorate in DMSO
Titrant: 0.01055 M EDTA in DMSO
(C) Titration of 13.64 ~-moles of zinc perchlorate in DMSO
Titrant: 0.00845 M DCTA in DMSO
(D) Titration of 17.10 ~-moles of zinc perchlorate in DMSO
Titrant: 0.0101 M EGTA in DMSO
(E) Titration of 34.20u-moles of zinc perchlorate in DMSO
Titrant: 0.0101 M EGTA in DMSO
0 0 ~ I
0 0 10
I
0 0 U)
I
0 0 .-... I
0 0 (X)
I
CD
0 0 en I
0 !'()
It) (\J
0 (\J
It)
0 It)
·3·:> ·s
031:JIOO
W
·sA
J\W
'l"llN
3lO
d
70
-(/) LLJ _
J
0 :E 0 a: u -~
-0 . \0
LLJ Q
)
0 ~
;::l
0 bO
•.-l
<X ~
t-z <X a: ~
-.....
71
EGTA it is only 44 mV. This might tell that the stability of the lead
EDTA complex in dimethyl sulfoxide, as in water, is greater than the
stability of the lead-EGTA complex.
Here again the potential break decreases noticeably with an increase
in concentration of lead ions. This phenomenon might be explained by the
fact that at higher lead ion concentrations other reactions, possibly ion
association, compete with the complexation reaction.
3. Figure 6 and Table XIV give the results of potentiometric deter
minations of zinc with the lead ion selective electrode in dimethyl
sulfoxide using different titrants.
All the volumes read on the curves must be corrected for the blank
volume due to the drop of lead perchlorate added before the titration:
the quantity of lead introduced as indicator was respectively 0.63, 0.06
and 0.05 ~-mole in the case of EDTA, DCTA and EGTA used as titrant. EDTA
in contrast with DCTA and EGTA gives a sharper break with an increase in
concentration of zinc. The three titrants can be listed in the following
decreasing effectiveness in their reaction with zinc:
EDTA > DCTA > EGTA
This result differs slightly with the one obtained in the photometric
titration (with zinc solutions ten times more dilute):
DCTA > EDTA > EGTA
One possible explanation of this result may be the fact that the mole
cules of titrant are more complex from EDTA ·to DCTA and EGTA. This
complexity of the molecule is expected to give a slower chelation
72
reaction with zinc ions. In the photometric method the solution was
stirred for 30 seconds after each addition of titrant and let stand for
15 seconds before any absorbance reading, thus the complex formation has
more time to be effective in contrast with the potentiometric method
where the potentials were read continuously with the addition of titrant.
According to the manufacturer (57) the following solvents attack
the epoxy used to seal the sensiting crystal into the body of the elec
trode: acetonitrile, methyl ethyl ketone, dimethyl formamide and methyl
isobutyl ketone. For this reason no attempts have been made to use the
lead electrode in these media.
The following potentiometric titrations have been attempted but did
not give any quantitative results:
Potentiometry with the mer~ury·electrode:
- Titration of zinc with EDTA in m~thanol and ethanolamine as buffer.
The potential at the mercury electrode was not stable.
- Titration of zinc with DCTA in methanol and ethanolamine as buffer:
The potential break was too small.
- Titration of zinc with EDTA in a solvent mixture methanol +methyl
ethyl ketone (1:1), and ethanolamine as buffer: No potential break.
- Titration of mercury with EDTA, both dissolved in dimethyl sulfoxide,
in methyl ethyl ketone with a mixture of tetramethyl ammonium hydroxide
and tetraethylammonium perchlorate as buffer: The potential at the
mercury electrode was not stable.
- Titration of mercury with DCTA in dimethyl formamide: Erratic poten-
tials. Potentiometry with the lead ion selective electrode:
73
- Titration of zinc with EDTA in methanol: No potential break.
- Titration of calcium with EDTA in dimethyl sulfoxide with ethanolamine
as buffer: No potential break.
- Same titration of calcium but use tetrabutylammonium hydroxide as basic
medium: No potential break.
- Titration of calcium with EGTA in dimethyl sulfoxide with ethanolamine
as buffer: Potential not stable.
Potentiometry with the silver electrode.
The silver electrode has been extensively used by Strafelda (58-60)
and by Fritz and Garralda (61) in aqueous medium buffered with borax.
This substance was tried in the titration of calcium with DCTA in di
methyl formamide but did not dissolve in the solvent.
Zinc and calcium could not be determined with EDTA in methanol
buffered with ethanolamine. One explanation might be that the buffer
complexes all the silver ions added to serve as indicator. To avoid
the blocking of these indicator ions it might be interesting to investi
gate the behavior of the ion selective electrodes of the first kind in
organic media, particularly the calcium ion electrode since it has been
successfully: used in many cases in aqueous solution (62-63).
D. Amperometric Titrations
Since polarography in organic solvents, particularly in dimethyl
formamide and dimethyl sulfoxide, has been extensively studied within
the last decade (44, 64-76). It was thought that amperometric titrations
in these two media could be performed as in alcohol (23). All the at
tempts have been unsuccessful despite the fact that almost all the
74
combinations of electrode systems have been tried: mercury pool vs. DME,
S.C.E. vs. DME, Rotating platinum electrode vs. S.C.E.
Polarographic curves for zinc in particular have been recorded but
the amperometric titration curves did not give quantitative results.
75
V. CONCLUSION
Some complexometric titrations in nonaqueous media have been studied.
The principal difficulty encountered was the problem of dissolving the
chelating agents in these solvents. Polyamine carboxylic acids dissolved
in alcohols with the presence of ethanolamine, in dimethyl sulfoxide and
dimethyl formamide upon heating and without any addition of other sub
stances. Their solutions in alcohols and dimethyl sulfoxide were very
stable, in dimethyl formamide they reprecipitated after a period of
several hours.
Direct visual titrations of calcium and zinc have been investigated
in the solvents methanol, dimethyl formamide, dimethyl sulfoxide and
methyl ethyl ketone. The metallochromic indicators which were used
appeared to work well in these media: Eriochrome Black T, Methylthymol
Blue and Zincon proved to be satisfactory for the determination of zinc,
Calcon, Methylthymol Blue, the systems Zincon + zinc-EGTA and Eriochrome
Black T + zinc-EGTA for the determination of calcium.
Photometric titrations of zinc were also investigated with Zincon
as indicator. Microgram quantities of zinc could be determined by this
method.
Potentiometric analysis with the mercury and lead electrodes gave
some results, particularly for the determination of zinc. The use of
the lead ion selective electrode in nonaqueous media was restricted by
the nature of the solvent which must be chosen to be inert to the resin
which composed the body of the electrode.
Amperometric titrations have been investigated without success.
76
A practical application of these experiments was found in the
analysis of trace quantities of zinc and calcium in a sample of poly
styrene dissolved in methyl ethyl ketone. Provided no phosphate is
present in the sample the sum calcium + zinc can be determined with
DCTA dissolved in methanol and Methylthymol Blue as indicator. Zinc is
determined separately with tetren, dissolved in dimethyl sulfoxide, and
Zincon as indicator.
A large number of experiments which did not work do not prove
though that they are impossible under other experimental conditions.
This research has been performed in a one year period, therefore it
does not pretend to be exhaustive. The following topics of investigation
would bring more light to the field of complexometry in nonaqueous media:
research on the problem of dissolving the chelating agents, the determin
ation of metal-chelate stability constants, a comparative study of metal
chelates positively, negatively or not charged, a systematic study on
buffer compositions in nonaqueous media, research on stepwise titrations
by the use of masking agents, a wide investigation to devise ion selec
tive electrodes with materials compatible with organic solvents. If
this last problem could be solved, complexometry in nonaqueous media
would be an extremely powerful, versatile and simple method in the
hands of the analytical chemist.
77
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82
VITA
Ngo The Hung was born on November 18, 1940 in Hanoi, Vietnam. He
received his primary, elementary and almost all his secondary education
in Paris, France. After finishing high school at the lycee Jean-Jacques
Rousseau in Saigon he studied physics and chemistry at the faculty of
science of the university of Saigon. He obtained the licence in Physical
Sciences in 1965. Since 1965 he has been a member of the faculty at the
National Technical Center in Saigon, the only university level engineer
ing college in his country.
In August 1970 his government sent him under the auspices of the
Agency for International Development to the United States for advanced
studies in analytical chemistry at the University of Missouri - Rolla