Silica chemically modi®ed with N-benzoyl-N-phenylhydroxylaminein chemisorption of hydrogen and metal ions

V.N. Zaitsev1,a, Yu. V. Kholinb,*, E. Yu Gorlovaa, I.V. Khristenkob

aChemistry Department, Taras Shevchenko University, 60 Vladimirskaya Street, Kiev 252033, UkrainebChemistry Department, Kharkov University, 4 Svobody Square, Kharkov 310077, Ukraine

Received 14 January 1998; received in revised form 11 August 1998; accepted 15 August 1998

Abstract

New chemically modi®ed silicas ± macroporous and pyrogenic ± with covalently bonded chelating groups of N-benzoyl-N-

phenylhydroxylamine (SiO2±BPHA) were obtained in three-step surface reaction. From elemental analysis, metal

chemisorption and pH titration the concentration of bonded groups was determined as 0.076 and 0.21 mmol gÿ1 for

Silochrome and Aerosil, correspondingly. In both cases about 50% transformation of initial aminopropyl groups to BPHA was

achieved. A chemisorption of hydrogen and some metal ions (Fe(III), V(V), Nb(V), Mn(II), Co(II), Ni(II), Cu(II), Zn(II),

Cd(II), Pb(II)) from aqueous and acetonitrile media was investigated. The adequate approximation of interfacial equilibria was

only possible if strong lateral interactions between bonded ligands were presumed. From the results of a quantitative

physicochemical analysis it was found that dissociation constant of bonded BPHA considerably rose in comparison with the

dissociation constants of analogs in solution. Bonded complexes with all studied metals were less stable. Linear correlation

between stability of complexes in dioxane±water solution and on the silica surface was found as log �(surf.)�log �(sol.)ÿ2.9.

Strong ®xation of counter-ions near the surface was concluded from experiments. In aqueous media SiO2±BPHA demonstrates

high selectivity towards Fe(III) and V(V). Linear correlation that was found between concentration of sorbed Fe3� and the

Kubelka±Munk function for SiO2±BPHA can be used for ion optical sensoring. Vanadium(V) forms several bonded complexes

with different composition instead. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Chemically modi®ed complexing silicas; N-Benzoyl-N-phenylhydroxylamine; Chemisorption; Bonded metal-ion complex

compounds; Quantitative physicochemical analysis; Stability constants

1. Introduction

The concept and development of chemical sensors

are often based on the utilization of inorganic oxides

with immobilized chelating ligands, due to highly

selective interaction of such materials with analytes

and also due to important electrochemical [1±3] and

optical properties of complex compounds ®xed on a

carrier surface [4±7]. Additionally, oxides with cova-

lently bonded ligands were proved to be important as

adsorbants and catalysts [8,9]. Because of their unique

physical and chemical properties silicas are the most

commonly used rigid matrixes for ligand immobiliz-

ation [8±11]. Some silicas chemically modi®ed with

Analytica Chimica Acta 379 (1999) 11±21

*Corresponding author. Fax: +38444835405; e-mail:

kholin@fuzzy.univer.kharkov.ua1E-mail: zaitsev@chemuniv.kiev.ua

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 0 0 3 - 2 6 7 0 ( 9 8 ) 0 0 5 9 2 - 3

chelating ligands (chemically modi®ed complexing

silicas, CMCS) were used to extract and separate

metal ions from liquid media [8,9,12] and to measure

their content employing a hyphenated procedure of a

sorptional-solid phase spectrophotometric (or ¯uori-

metric) determination [4±7].

This paper presents a characterization of new com-

plexing silicas prospective for analytical usage,

namely silicas chemically modi®ed with N-benzoyl-

N-phenylhydroxylamine (BPHA). The preparation

and investigation of optochemical and binding proper-

ties of these materials towards Fe(III), V(V), Nb(V),

Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II)

from water and acetonitrile solutions are described

and discussed. The interest to immobilization of

BPHA arises from the fact that this analytical reagent

forms stable complex compounds with transition

metal ions, and in case of Fe(III), V(V), Nb(V) these

complexes are intensively colored [13,14]. Hence,

SiO2±BPHA could possess important adsorptive prop-

erties with potential to be used as a selective material

for the development of optical sensors.

To ®nd the optimal conditions for the utilization of

silicas with grafted BPHA and to evaluate their prop-

erties compared to characteristics of analogs in solu-

tions it is necessary to determine stoichiometric

compositions and thermodynamical stabilities of com-

plex compounds ®xed on a silica surface. Hence, in

addition to spectroscopic methods commonly applied

to explore the features of grafted complexes, a quan-

titative physicochemical analysis (QPCA) [15±17]

should be used to investigate chemisorptional equili-

bria on the SiO2±BPHA surfaces.

2. Experimental

2.1. Reagents and materials

Macroporous silica ± Silochrome (Luiminophore

Plant, Stavropol, Russia) with speci®c surface area

S�120 m2 gÿ1, particle size 0.1 mm and average pore

diameter about 200 nm as well as non-porous pyro-

genic silica ± Aerosil (Degussa, S�175 m2 gÿ1, par-

ticle size 16±40 nm) were used in this work. Silicas

were activated by heating at 5008C for 8 h before

modi®cation. Metal salts were used as analytical grade

reagents. The salt concentrations in initial solutions

were determined with EDTA as described elsewhere

[14]. Double distilled water was used throughout. The

water-free acetonitrile was obtained with the distill-

ation of pure grade solvent over P2O5 and then over

CaH2. N-phenylhydroxylamine was synthesized as

described [18]. Other reagents were distilled or recrys-

tallized.

2.2. Spectra

IR spectra of adsorbents were recorded on Specord

M-80 spectrophotometer, UV±Vis spectra on Specord

M-40 and photocolorimeter Specol-1 (all Carl Zeiss,

Jena).

2.3. Procedures

All chemisorptional experiments were carried out at

(20�1)8C.

2.4. Studying protolytic properties of SiO2±BPHA

Slurries of adsorbents in water were titrated with

NaOH solution. Both the batch technique and the

titration of separate samples were used. Ionic strengths

(I) of water phase were maintained in the range 0.1±

1 mol lÿ1 by KCl. After attainment of the equilibrium

states, pH were measured with the circuit included

H�-selective glass electrode ESL 43-07 (Analytpry-

bor, Gomel, Belarus), silver chloride electrode EVL

1M3 (Analytprybor, Gomel, Belarus) and salt bridge

®lled with saturated KCl solution in agar. The error of

pH determination did not exceed 0.02.

2.5. Chemisorption of Cu(II), Cd(II) and Pb(II) ions

from aqueous solutions

Chemisorption of Cu(II), Cd(II) and Pb(II) ions

from aqueous solutions was studied similarly to the

procedure described above but pH titration of the

adsorbent slurries was performed in the presence of

metal salts. Additionally pCd, pPb or pCu were mea-

sured potentiometrically with ion-selective electrodes:

Orion 94-48 for Cd, Orion 94-82 for Pb and Crytur 29-

17 and Isary Company (Tbilisi, Georgia) for Cu. The

ionic strengths of aqueous phases were maintained

constant by KCl. Separate experiments were made to

ensure that Cu2�-selective electrodes are suitable for

12 V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21

copper determination in the concentration range

10ÿ5±10ÿ3 mol lÿ1 in the presence of KCl. It was

found that the error of pM determination did not

exceed 0.05 throughout. To avoid PbCl2 precipitation,

the total Pb(II) concentration was limited to

5�10ÿ3 mol lÿ1.

2.6. Chemisorption of Mn(II), Co(II), Ni(II), Cu(II),

Zn(II), Fe(III) and V(V) from aqueous solutions

In these experiments the batch technique has been

used. The Silochrome±BPHA samples (usually 0.1

and 0.3 g in case of FeCl3) were stirred with 20 ml

of water solution containing a certain amount of a

metal salt MnCl2, CoCl2, NiCl2, Cu(NO3)2, ZnCl2 or

KVO3), NaOH (or HCl) and KCl. After stirring the

adsorbants were allowed to contact with solutions for

two days. This term exceeds substantially the time

necessary for the systems to achieve equilibrium states

[9]. Then, pH of water phases were measured and

equilibrium concentrations of the metal ions in solu-

tion ([M]) were determined spectrophotometrically

with 5-sulfosalicylic acid for Fe or 4-(2-pyridylazo)

resorcinol for other metals [14]. The relative error of

the determinations did not exceed 5%.

The pH-metric circuits were calibrated with the use

of standard pH-buffer solutions [19]. To calibrate pM-

metric circuits the Cu(NO3)2, Cd(NO3)2 and Pb(NO3)2

solutions with concentrations in the range 10ÿ5±

10ÿ3 mol lÿ1 were used. The concentrations of back-

ground electrolyte were equal to their concentrations

in the aqueous phases of chemisorptional systems

under study. Thus, the pH measurements gave esti-

mates of H� activities in solutions and the pM mea-

surements established the determination of

equilibrium concentrations.

2.7. Chemisorption of metal salts from acetonitrile

solutions

The adsorption isotherms of CuCl2, KVO3, NbCl3or FeCl3 on Silochrome±BPHA and Aerosil±BPHA

were determined by the batch technique. Samples of

the sorbents (0.1 g) were shaken with 25 ml of the salt

solutions for 2 h. Then the sorbents were allowed to

contact with the solutions for 12 h, and they were

separated by centrifugation, and solutions being

examined for metal content with a spectrophotometric

method [14]. Throughout the initial salt concentration

in chemisorptional systems were varied from 3�10ÿ5

to 2�10ÿ3 mol lÿ1.

2.8. Preparation of SiO2±BPHA

A scheme for SiO2±BPHA preparation is presented

in Fig. 1. Initially silicas were modi®ed with amino-

propyl groups by treatment with g-aminopropyl-

triethoxysilane in dry toluene [11].

To run step (2) aminosilica (50 g) was suspended in

250 ml of dry acetonitrile and solution of terephthal-

oyl chloride (5.5 g) with N-ethyldiisopropylamine

(5 ml) in the same solvent was added. The mixture

was stirred for 1 h at 50±808C. After formation of

white ¯akes of ammonium salt, the modi®ed silica was

decanted. To run step (3) fresh portion of acetonitrile

(200 ml) was added to the silica together with N-

ethyldiisopropylamine (1 ml) and N-phenylhydroxyl-

amine (3 g). Then the mixture was stirred at 608C for

2 h. Resulting SiO2±BPHA was washed out in a

Soxhlet apparatus with acetonitrile and then toluene.

Ultimately, SiO2±BPHA was dried in vacuum to

remove residual solvent.

3. Simulation of chemisorptional equilibria bymeans of a quantitative physicochemicalanalysis

The quantitative physicochemical analysis is a sub-

section of physicochemical analysis [15]. The concept

of the QPCA as applied to investigating the processes

on modi®ed silica surfaces was recently reviewed and

generalized [16,17]. In this paper we only brie¯y

discuss the QPCA.

Let us consider the chemisorption of species M

from solution on silica modi®ed with reagents Q. The

results of chemisorptional measurements can be pre-

sented as a composition±property dependence:

gk � f �tk�M�; tk�Q�; tk�X�; . . . ; ak; Vk�; (1)

where g is a measured property of the equilibrium

system (pH, adsorption of M, amount of M remained

in solution after sorption, light absorbance of solution,

etc.); X the reagents present in solution (except entity

M to be sorbed); t the total (initial) concentrations of

reagents known from the conditions of preparations

V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21 13

(mol lÿ1 for species in solution, mol gÿ1 for grafted

species); a the weighed sample of a sorbent, g; V the

initial volume of a liquid phase, l; k the number of

experimental point; and f is a certain (a priori

unknown) function. The aim of a QPCA is to deter-

mine, on the basis of dependence (1), the number of

species, stoichiometric compositions and thermody-

namic stabilities of complex compounds MmQq (m and

q are the stoichiometric indices) grafted on complex-

ing silica surface. To solve this problem, meaningful

models are applied. These models are constructed in

two stages. First, the structural identi®cation is carried

out, i.e. such a form of function f is speci®ed at which

parameters having a physical sense (for instance,

stability constants) are considered as unknown para-

meters of the model. The next stage, the parametric

identi®cation, is the determination of such values of

these parameters which allow one to ®t dependence

(1) within the limits of experimental errors. In this

work, from a set of models proposed to describe

equilibria on surfaces of complexing silicas (for a

review see [16,17]), we have chosen a model of

chemical reactions [16]. According to this model,

the chemisorptional system is considered as composed

of three phases: an inner volume of the adsorbant, a

homogeneous liquid solution and an inhomogeneous

surface adsorption layer (AL) [20]. The method of

drawing boundary-lines between the phases affects the

value of the AL volume. For this reason the AL

volume seems to be an indeterminable quantity. To

overcome this dif®culty the concentrations of the

grafted ligands and complexes are to be referred to

weighed portions of complexing silica (a heteroge-

neous approach) or to the volume of a liquid phase

(a homogeneous approach) rather than to the AL

volume. In this work we have used a homogeneous

approach. The model of chemical reactions does not

take into account an inhomogeneity of AL in an

explicit form. The speci®city of interactions in AL

(including electrostatic ones) and the features of ®xed

species compared to non-immobilized analogs in

solutions may manifest themselves in an anomalous

stoichiometry and/or stability constants of grafted

complexes.

The parametric identi®cation of a model is per-

formed in several steps. First, the discrete parameters

are ®xed (namely, the number of ®xed complexes and

stoichiometric indices in their formulae). For the

prescribed hypothesis about surface reactions a criter-

ial function is minimized with respect to stability

constants of grafted complexes. To calculate unknown

stability constants a non-linear least squares method

Fig. 1. Scheme of N-benzoyl-N-phenylhydroxylamine immobilization on a silica surface.

14 V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21

was used to minimize the residual variance:

s20 �

1

N ÿ Z

XN

k�1

wk�2k ; (2)

where N is the number of points of the composition±

property dependence, Z the number of parameters to

be calculated, �k the difference between calculated

and measured g values: �k � gmodelk ÿ g

expk , and wk is a

statistical weight of the kth measurement. The statis-

tical weights are equal to wk � 1=�2k , where �2

k is the

variance of discrepancy �k evaluated from the model

of errors in initial experimental data [21]. The phy-

sicochemical model under testing is accepted as an

adequate one if the corresponding value of s20 is of the

order of unity [22]. If s20 is too large, one should

introduce new complexes into the model and the

calculations should be repeated. To calculate stability

constants by minimizing criterion (2) the numerically

stable algorithm based on a Gauss±Newton iterative

procedure [21] and a specially created software pro-

gram CLINP 1.0 [23] have been applied. The least

squares problem (2) is of ill-posed nature [24,25].

From the chemical point of view, it means that the

model can include redundant complexes. To detect

and reject redundant complexes we have used a pro-

cedure described elsewhere [21]. To estimate errors of

determined stability constants from the covariation

matrix calculated after convergence of iterations,

Bonferoni's approach [22] was used.

In a case when the adsorptional layer is energeti-

cally homogeneous and only single complex MQq is

formed with no lateral interactions between grafted

species, determination of composition (namely, the

value of index q) and stability constant of bonded

complex may be simpli®ed. The formation of grafted

compound MQq can be described as a reaction of M

with one q-dentate ligand (Qq) [26]:

M� �Qq� � MQq; �q � �MQq�=�M��Qq�; (3)

and ®t initial experimental data by the transformed

equation of the Langmuir isotherm [27]:

1

D� �M��MQq�

� 1

��q

� �M��; (4)

where D is the distribution ratio (l gÿ1), [MQq] the

concentration of adsorbed M (mol gÿ1), and � is the

total concentration of fragments (Qq) (mol gÿ1). The

value of q is determined as t(Q)/� .

4. Results and discussion

4.1. Covalently bonded BPHA

Covalently bonded BPHA was obtained in a three-

step procedure as illustrated in Fig. 1.

1. Aminosilicas obtained by a common routine [11]

have cluster (island-like) distribution of bonded

groups [11,28]. In these circumstances changing

the aminosilane concentration in the ®rst step of

surface synthesis does not affect the density of

bonded groups. Therefore aminosilica was ob-

tained in excess to aminosilane to cover all the

available surface with aminopropyl moieties.

Concentration of surface amine groups determined

from elemental analysis and pH titration was

1.6�10ÿ4 and 4.2�10ÿ4 mol gÿ1 for modi®ed

Silochrome and Aerosil, respectively.

2. The second step of the surface modification was

accomplished with excess of acylated reagent and

strained amine to promote maximum transform-

ation of bonded amino-groups and to reduce cross-

linking (Fig. 1, scheme 2(a)). Trans-location of

carboxylic groups in terephthaloyl chloride and

rigid structure of the linkage molecule are addi-

tional factors that can increase the yield of the main

product.

3. The final step in SiO2±BPHA preparation has

apparently had no difficulties. Functional analysis

on active chlorine with copper wire showed no

residual acetyl chloride groups on SiO2±BPHA.

Concentration of bonded BPHA groups (CHQ) was

calculated from the results of elemental analysis

with taking into account an incomplete transform-

ation of bonded amino groups to N-benzoyl-N-

phenylhydroxylamine as it was proposed in [11]

from the equation

CHQ �!BPHA

C ÿ !NH2

C

ÿ �ArCn� 100ÿ !BPHA

C Mr

�mol=g�; (5)

where !C is the carbon content on SiO2±BPHA and

SiO2±NH2 (%); ArC the atomic mass of carbon; n

the number of carbon atoms in analyzed moiety (in

V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21 15

our case it is ±C(O)±Ph±C(O)±N(OH)±Ph with 14

carbon atoms), Mr the molecular mass of the

additional moiety. From Table 1 it can be seen

that for both SiO2±BPHA samples about 50%

transformation of amino-groups into hydroxamic

acid can be achieved. A good agreement between

non-selective method of elemental analysis and

highly selective method based on determination

of SiO2±BPHA capacity towards iron ions supports

the conclusion. So SiO2±BPHA is a striking exam-

ple of a material with chemically inhomogeneous

surface containing mixture of desirable bonded

hydroxamic acids and residual alkylamino-groups

in approximately equal concentrations.

The IR spectrum of SiO2±BPHA has absorbance

bands at 1710, 1660, 1545, 1500 and 1450 cmÿ1. The

most intensive ones are at 1660 and 1550 cmÿ1.

Regarding their intensity and position they can be

attributed to Amid-I and Amid-II absorbance, re-

spectively. In contrast to the band at 1660 cmÿ1

that overlaps with non-speci®c adsorption of silica

gel matrix (1645 cmÿ1), the band at 1550 cmÿ1 evi-

dences the hydroxamic acid formation on the silica

surface.

4.2. Protolytic properties of bonded BPHA

To ®t the measured equilibrium pH values within

the limits of their errors by the model of chemical

reactions it is insuf®cient to take into account the

dissociation of bonded BPHA only. The results of pH-

metric titration were described adequately if reactions

of two types were included in the model: (a) dissocia-

tion of bonded BPHA molecules:

K� � HQ�KaH� � KQ (6)

and (b) their association:

KQ� HQ�KhKHQ2; (7)

where K� is the cation of the background electrolyte

KCl, the line above the formulae marks grafted spe-

cies. The model of errors of initial experimental data

included two sources of random errors: in pH with

standard deviation 0.02 and in titrant volumes with

standard deviation 0.005 ml. The calculated mixed

constants Ka, concentration constants Kh and the

residual variances s20 are given in Table 2. The errors

of pKa and log Kh determination did not exceed 0.15

throughout. Reaction (7) does not occur in solutions of

native hydroxamic acids [13,29,30]. It is a form of

accounting for the AL inhomogeneity and strong

lateral interactions between bonded groups. Under

the conditions when the average concentration of

bonded functional groups is small, Kh can be used

to estimate the character of ligand distribution on the

surface. Judging from Kh one can conclude that up to a

half of the total quantity of BPHA groups form

homoconjugates (KHQ2). This conclusion is in good

agreement with clustertype distribution of bonded

groups on SiO2±BPHA precursors ± aminosilicas.

Table 1

Concentrations of bonded groups on modified silicas (mmol gÿ1)

Method of determination Silochrome±BPHA Aerosil±BPHA

Fe3� chemisorption 0.078 0.22

pH±potentiometric analysis 0.077 0.20

Elemental analysis (C) (1.85)a 0.076 ±

Accepted value 0.076 0.21

aCarbon content (%).

Table 2

Logarithms of protolytic equilibrium constants for bonded BPHA

I (mol lÿ1) Silochrome±BPHA Aerosil±BPHA

ÿlog Ka log Kha s2

0 ÿlog Ka log Kha s2

0

0.10 5.64 4.17 3.0 5.19 3.52 4.8

0.5 6.15 4.19 3.5 6.83 2.80 2.0

0.75 7.125 4.46 2.2 6.965 2.75 2.0

1.00 7.30 4.94 3.1 7.20 2.84 3.1

aThe concentrations of bonded groups are referred to the volume of the liquid phase and measured in mol lÿ1.

16 V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21

Immobilization of BPHA affects their dissociation

constants: pKa values found for bonded BPHA at the

ionic strength 1 mol lÿ1 (7.2±7.3) differ signi®cantly

from those for native BPHA (8.0±8.4) [13,29,30]. The

phenomenon observed can be explained by consider-

ing a mixed-ligand character of the adsorptional layer:

bonded molecules of hydroxamic acid are surrounded

by residual basic amino-groups. This environment can

promote the dissociation of BPHA [31].

Since no theoretical relationships between equili-

brium constants of reactions that take place in AL and

ionic strength of liquid media (I) have been developed,

we used empirical equations. For the studied interval

of ionic strength linear dependencies were observed:

for SilochromeÿHA : pKa � 5:4� 2:0I;

correlation coefficient r � 0:97; (8)

for AerosilÿHA : pKa � 5:25� 2:2I; r � 0:92:

(9)

The closeness of coef®cients in Eqs. (8) and (9)

evidences that there are no signi®cant differences

between protolytic properties of BPHA anchored on

Silochrome and Aerosil. With an increase of the ionic

strength, a difference between protolytic abilities of

grafted and native BPHA molecules decreases. Such

phenomenon was also observed earlier for other

grafted reagents [9]. Probably, since a high ionic

strength compensates surface charge and decreases

thickness of a double electric layer enough to reduce

interactions between neighboring grafted groups, with

increasing I the properties of ®xed reagents move

closer to those in native state.

It should be noted that reaction (6) describes the

simplest hypothesis about the state of counter-ions

near the charged surface groups, namely a strong

®xation. One can consider another limiting case that

assumes unlimited movement of the counter-ions in a

gel ®lm near an adsorbent surface [32]:

HQ � K� � H� � K� � Qÿ: (10)

This case can be included into the model instead of

reaction (6). The modi®ed model takes into account

that reaction (10) has been tested. It was revealed that

this model fails to ®t experimental data properly as

residual variances s20 were increased in 6±8 times in

comparison with the initial model. Hence, it was

concluded that the initial model of strong ®xation

of counter-ions describes better a real situation of

complex formation in AL. It may well be that strong

®xation of counter-ions is caused by (1) chemical

structure of pendant groups and (2) topography (lat-

eral inhomogeneity) of SiO2±BPHA surface. Since

hydrophilic hydroxamic functional groups produce

hydrogen bonds with surface silanoles and/or residual

amino-groups, lipophylic spacers generate arched

structures on the surface (Fig. 2). The hydrophobic

nature of the spacers can prevent migration of counter-

ions along the silica surface from one bonded group to

another. Additionally, since SiO2±BPHA has cluster

distribution of bonded groups, counter-ions may

hardly migrate from one cluster to another.

4.3. Immobilized coordination compounds resulted

from chemisorption of metal ions from aqueous

media

As it was expected from chemical structure of

bonded groups, SiO2±BPHA has high adsorption af®-

nity to Fe(III) and V(V) ions. Fig. 3 illustrates adsorp-

Fig. 2. Structure of the adsorption layer.

Fig. 3. Adsorption isotherms of metal ions on Silochrome±BPHA

from aqueous solution. Conditions: V�25 ml, a(SiO2±

BPHA)�0.3 g, initial concentrations of metal salts 10ÿ4 mol lÿ1,

t�208C.

V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21 17

tion isotherms for metal ions from solutions with

different pH. Iron as well as vanadium ions can be

selectively adsorbed from water solution at pH�1.5±

2. In contrast, Cu2�, Zn2� and Pb2� ions are adsorbed

only from pH 2.5±3.0. The rest of the studied metal

ions except Mn2� can be ®xed on SiO2±BPHA at

pH>6.0±6.5 only. High af®nity of SiO2±BPHA

towards V(V) and Fe(III) ions together with intensive

color of surface complexes is a good basis for applica-

tion of SiO2±BPHA as an optical sensor.

The models which adequately ®t experimental data

for chemisorption of Mn(II), Co(II), Ni(II), Cu(II),

Zn(II), Cd(II), Pb(II), Fe(III) and V(V) ions on SiO2±

BPHA have been found. The random errors in pH

(0.02), pM (0.05) or equilibrium concentrations of

metal ions [M] (5%) were accounted for in computa-

tion of statistical weights wk and residual variances s20.

The validity of proposed models was controlled by at

least two independent measurements of the system

parameters: pH and pM, for example. The model was

accepted if complexes with the same compositions

and similar estimations of calculated stability con-

stants were suggested from different data. Thus, the

model for chemisorption of Cu2� ions on Silochrome±

BPHA at I�1 mol lÿ1 was validated because stability

constants calculated from electrochemical measure-

ments (log �11�4.80�0.10) agreed with data obtained

from spectrophotometric (log �11�4.70�0.15) and

from pH-metric data (log �11�5.0�0.4).

For the majority of the systems studied only a single

®xed complex with equimolar M and Q composition

was found. Supposing a strong ®xation of counter-

ions, one can present the equations of chemisorptional

reactions for Co(II), Ni(II), Cu(II), Zn(II), Cd(II),

Pb(II) and Fe(III) as follows:

Mx� � Qÿ � �xÿ 1�Clÿ ��11MQClxÿ1�x � 2 or 3�:

(11)

For V(V) ions the corresponding equations are

written in the form

VO3� � 2Qÿ � Clÿ ��21VOQ2Cl; (12)

VOQ2Cl� H2O�K VO�OH�Q2 � H� � Clÿ: (13)

The results of calculations are presented in Table 3.

It should be noted that the determined concentration

stability constants are conditional [33] with respect to

concentration of the background electrolyte (the nota-

tion Mx� symbolizes a set of possible species

[M(H2O)hClc](xÿc)�, differing in stoichiometric

indices h and c). Hence, with variations of the ionic

strength, the concentration stability constants are

changed depending on the yield of different species

[M(H2O)hClc](xÿc)� and the variation of activity coef-

®cients of reagents.

Relationship between pKa, log �11 and equilibrium

constants of native BPHA in water±dioxane mixtures

(dioxane volume fraction 50%) at 258C (�1 Table 3)

was found to be close to the linear dependence

(Fig. 4). From this relationship it was concluded that

immobilization decreases stability of the complexes,

as log �(surf.)�log �(sol.)ÿ2.9.

Table 3

Stability constants of metal complexes on the Silochrome±BPHA

obtained from aqueous media

Metal

ion

I

(mol lÿ1)

log �11 s20 log �1

[13]

Mn2� 1 �2 0.8 5.08

Co2� 1 3.4�0.2 0.8 5.68

Ni2� 1 2.2�0.2 1.1 5.92

Cu2� 0.1 6.5�0.2 1.1 9.46

0.5 4.5�0.2 2.3

1 4.80�0.10 1.2

2.5 6.3�0.2 1.7

Cd2� 0.1 4.1�0.2 0.5

Pb2� 0.1 5.76�0.12 2.0

Fe3� 0.1 10.04�0.08 0.2

VO3� 0.1 log �21�19.67�0.15

log K�ÿ1.8�0.2 2.0

Fig. 4. Relationship between stability constants of fixed complexes

and non-immobilized analogs in dioxane±water solutions.

18 V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21

Complexes of SiO2±BPHA with studied divalent

metal ions have poor color intensity. For copper

complexes some absorbance in the near-ultraviolet

region can be detected due to charge-transfer band.

Two other weak bands with maxima at 700 and

800 nm can also be present in the spectra. The band

at 700 nm seems to be caused by d±d transition in a

surface complex [Cu(NH2�SiO2)2(H2O)4]2� formed

due to the reaction of Cu2� with residual amino-

groups on SiO2±BPHA surface (compare with

[Cu(NH3)2(H2O)4]2� that absorbs at 690 nm [34]).

The second band at 800 nm can be attributed to a

copper complex with bonded hydroxamic acid

(CuQCl) with possible tetrahedral geometry [35].

The intensity of absorbance at 700 nm changes only

slightly with pH, whereas absorbance at 800 nm

increases with increasing pH. This observation agrees

with the calculated yield of immobilized complexes

with hydroxamic acid and con®rms proposed assign-

ments for absorbance bands.

For Fe(III) the only complex with composition

FeQCl2 was found on Silochrome±BPHA. In UV±

Vis spectra for Silochrome±BPHA after Fe(III) treat-

ment an intensive band with maximum at 500 nm is

observed whereas the native BPHA forms three com-

plexes: FeQ2�, FeQ�2 and FeQ3 with absorbance

maxima at 510, 470 and 440 nm, respectively [37].

The concentration of adsorbed Fe3� ions (CFe) does

not affect the position of absorbance maximum,

whereas the Kubelka±Munk function (F) [36] deter-

mined at 500 nm,

F � �1ÿ R�2=2R; (14)

where R is the fraction of re¯ected light, is linearly

dependent on CFe (r�0.99). This fact supports our

conclusion about formation of a surface complex with

equimolar Fe3� and BPHA content. The dark-red

color of iron bonded complex was found to be stable

for more than four weeks.

Similarly to other immobilized complexes, the iron

complex is less stable (Table 3) than the native one (at

258C in aqueous solution log �1 (Fe3�, Qÿ)�11.39

[37]).

The chemisorptional equilibrium of vanadate ions

was studied from acidic solutions within pH 1±5 and

calculated with the model of chemical reactions. It

was found that compositions of immobilized com-

plexes are the same as for those in solution [14]. Red

color complexes with absorbance maximum at

500 nm were found for V(V) chemisorbed on Silo-

chrome±BPHA from acidic solutions (1.5<pH<3.2).

As distinct from Fe(III) chemisorption, the Kubelka±

Munk function for vanadium immobilized complexes

do not increase linearly with increasing V(V) concen-

tration. This can be an evidence for multi-step form-

ation of bonded vanadium complexes with different

compositions. That is why an application of SiO2±

BPHA as an active matrix for the determination of

vanadium is problematic. When vanadium adsorption

was performed from solutions with pH 4±5, slightly

yellow complexes were formed on SiO2±BPHA. It can

be explained by binding V(V) in a form of poly-

vanadate ions. The color of the immobilized red

complexes is more stable than in the case of native

vanadium complexes. For bonded complexes it

remains unchanged for at least two weeks.

For all metal ions studied thermodynamical stabi-

lities of the ®xed complexes are less than those for

native analogs (Fig. 4). This situation could hinder

the application of SiO2±BPHA as adsorbent. For-

tunately, the grafting increases the BPHA dissociation

constant (Fig. 4). These two effects compensate each

other.

4.4. Grafted complexes resulted from chemisorption

of metal salts from acetonitrile solutions

We studied a chemisorption of FeCl3, KVO3, NbCl5and CuCl2 from diluted solutions. The dissociation of

those salts in acetonitrile is negligible. For the series of

Fig. 5. Fitting adsorption isotherms with Eq. (4). (1) Sorption of

CuCl2 on Aerosil±HPHA; (2) sorption of NbCl5 on Aerosil±BPHA;

(3) sorption of CuCl2 on Silochrome±BPHA.

V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21 19

adsorption isotherms obtained an analysis of data

revealed the chemisorption to conform to the Lang-

muir equation. An excellent ®t of the data with Eq. (4)

was obtained, Fig. 5. The calculated parameters are

given in Table 4.

To propose a scheme of chemisorption some facts

should be taken into account. First, according to the

Langmuir model, the interaction of adsorption centers

with entity to be ®xed results in the formation of only

one product rather than grafted complex and species in

solution. Second, as evident from the results of cal-

culations (Table 4), each adsorption center on the

SiO2±BPHA surface contains, as a rule, more than

one grafted group. On the other hand, the grafted

complexes isolated from acetonitrile and aqueous

media demonstrate very similar electronic spectra.

This is an evidence of similar metal ion environment

in surface complexes obtained from different media.

Thus, it should be concluded that not all functional

groups from one adsorption center are covalently

bonded with metal ion, whereas the rest of the groups

do not interact chemically with metal salt. This

phenomenon results from the cluster distribution of

the reagents anchored to surface. Being formed, a

surface metal complex shields neighboring BPHA

molecules from a solution phase. Ultimately, the

following schemes of chemisorption are in line with

the Langmuir model and the results of electronic

spectroscopy:

KVO3 � �HQ�2�s2K��VO2�H2O�Q2�ÿ; (15)

MClx � HQ��1H��MQClx�; (16)

where M is Fe, Cu or Nb, x�2 for Cu, x�3 for Fe and

x�5 for Nb.

The determination of equilibrium parameters from

Eq. (4) by using linear-squares method is not perfectly

correct as the main conditions of a linear regression

analysis [22] are disturbed. More proper estimations

can be obtained by means of solving the non-linear

regression problem (2) using a software program

CLINP 1.0 (Table 4).

5. Conclusions

1. BPHA molecules cover the modi®ed silica

surfaces inhomogeneously. This feature of the

materials affects the protolytic and complexing

properties of bonded reagent manifesting itself in

increasing the dissociation constants and in

association of bonded BPHA molecules.

2. Chemisorption of metal salts leads to the for-

mation of anchored complex compounds with

compositions similar to those of native analogs

in solutions. Stability constants of surface com-

plexes are less than those for homogeneous

ones.

3. Iron and vanadium ions can be selectively adsorbed

from acidic aqueous solutions with invoking inten-

sive coloring of SiO2±BPHA. Linear correlation

between the Kubelka±Munk function and Fe(III)

content on a sorbent surface can be used for solid

phase spectrophotometric determination of iron

ions.

Table 4

Determination of equilibrium parameters for complex compounds obtained by chemisorption of metal salts from acetonitrile media

Metal salt Carrier Handling data with Eq. (4) Non-linear regression applied to Eq. (2)

(program CLINP 1.0)

log �1 log �2 t(Q)/� ra q log �1 log �2 s20

KVO3 Silochrome 5.89�0.12 1.84 0.98 2 6.00�0.08 15

Aerosil 4.14�0.15 5.75 0.995 6 4.63�0.12 6.5

NbCl5 Silochrome 3.74�0.10 0.90 0.99 1 3.81�0.03 0.8

Aerosil 4.14�0.10 3.31 0.998 3 3.90�0.06 1.5

CuCl2 Silochrome 3.93�0.08 3.04 0.993 3 3.93�0.04 0.2

Aerosil 4.15�0.07 2.21 0.99 2 4.05�0.06 1.9

FeCl3 Aerosil 4.4�0.2 2.07 0.96 2 4.63�0.15 15

ar is the correlation coefficient.

20 V.N. Zaitsev et al. / Analytica Chimica Acta 379 (1999) 11±21

Acknowledgements

The research described in this publication was made

possible in part by grant no. 94-252 from International

Association for the Promotion of Cooperation with

Scientists from the Independent States of the Former

Soviet Union (INTAS). V. Zaitsev and Yu. Kholin

thank International Soros Science Education Program

for the support through grant nos. APU 073032, APU

063110 and APU 073114.

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