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THE DETERMINATION

OF STABILITY CONSTANTS

OF

CADMIUM

COMPLEXES

WITH

SELECTED

LIGANDS

AND

MODELLING

THE

LIKELY

SPECIATION

OF

CADMIUM IN LAKE BOGORIA, KENYA

By

NJAGI NJOMO

I'

A thesis submitted in partial fulfilment of the requirement for the degree of

Masters of Science of Kenyatta University.

Kenyatta University

JULY, 2003

Njorno, Njagt <The>determination of stability

III

II

IIII

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DECLARATION

This thesis is my original work and has not been presented for a degree in any other university.

NJAGI NJOMO

We as university supervisors confirm that the work reported in this thesis was carried out by the candidate.

-==:G~~---...

~~----Prof H.M. Thairu Academic Division

Jomo Kenyatta University of Agriculture and Technology

Dr. C.O. Onindo

Department of Chemistry Kenyatta University

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ACKNOWLEDGEMENT

I would like to express my unreserved thanks and gratitude to my supervisors, Prof H.M. Thairu, Dr. F.W. Maloba and Dr. C.O. Onindo for their guidance in this work. Their motivating advices, constructive criticism and stimulating suggestions made me to shape this work to its current state.

Special thanks goes to Prof K.H. Schr<I>der of Norwegian University of Science and Technology for his financial assistance, fruitful discussion and contribution to this work. The financial assistance I received through him and Prof Thairu to attend and present papers at the regional symposium on the chemistry of salt lakes at Makerere University, Uganda and later at two different international conferences in Kenya and Tanzania, gave me a great exposure and inspiration.

I am deeply grateful to KAAD of Germany for granting me a scholarship which saw me go I'

through the second year of my study. For this I must thank Prof Stanley Waudo, Director of board of postgraduate studies, Kenyatta University for assisting me to get the scholarship.

A lot of thanks to the teaching and technical staff of chemistry department, Kenyatta University for their constant encouragement and help in one way or another. I also

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acknowledge with thanks the assistance given to me by the staff at: KIRDI, department of mines and geology and the ministry of water research laboratories.

A lot of thanks also goes to my colleagues: M. Mwihaki, B.E. Ndinya, B. Omusiro, G. Muriithi and P. Maina for their comfort, encouragement and company throughout the study period.

Last but not least to all my friends and family members who in one way or another contributed to and/or made my study possible.

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DEDICATION

To my parents Daniel Njomo Kaburaci and Eunice Wakaria Njomo, wife Mary Muthoni, sons Kinyua and Ndumbi for the encouragement and assistance they have given me throughout my academic career.

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ABSTRACT

This thesis is a report of a speciation study of the complexes formed by cadmium with selected ligands found to be dominant in an inland salt lake, Lake Bogoria. Water from Lake Bogoria was analysed using standard analytical techniques to obtain the stoichiometric concentrations of the major ions. Differential pulse anodic stripping voltammetry (DPASV) and hanging drop amalgam voltammetry (HDA V) were then used to measure oxidation peak potential shifts of cadmium in presence of the inorganic ligands carbonate, bicarbonate, chloride, fluoride and hydroxide. The peak potential shifts were then used to calculate the stability constants of cadmium complexes at the ionic strength of the lake using DeFord-Hume graphical method. The determined stability constants were then used to model the speciation of the lake using Incenzy method.

Three carbonato species CdC03°, Cd(C03)22- and Cd(C03)34- were identified in both the KN03 aqueous and lake water media using data from DP ASV while only two species, "CdC03 ° and Cd(C03)l- were identified with data from HDA V. With DP ASV data, four

bicarbonato complexes, CdHCO/, Cd(HC03)20 , Cd(HC03)3- and Cd(HC03)/- were shown to exist in both the KN03 aqueous and lake water media while three bicarbonato species; CdHCO/, Cd(HC03)20 Cd(HC03)3- were obtained using data from HDA V. DPASV data showed existence of six hydroxo complexes, CdOW, Cd(OH)2°, Cd(OH)3-, Cd(OH)/-, Cd(OH)s3- and Cd(OH)64- in the lake water medium and four complexes

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CdOW, Cd(OH)2° , Cd(OH)3- and Cd(OH)/-in the KN03 aqueous medium. With HDAV, five complexes were identified. These were; Cd(OH)+, Cd(OH)20, Cd(OH)J, Cd(OH)/- and Cd(OH)s3-. Five chl oro species were found to exist in the KN03 aqueous medium using data from both the DPASV and HDAV. These were

cacr,

CdCho, CdCl,", CdCI/- and CdCls2-.Three fluoro complexes CdF+, CdF20 and CdF3- were found using data from both techniques. The study has shown that the stabilities, relative proportions and distribution of the various species vary quite significantly with the composition of the matrix mixture.

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Table of Contents

Page

1'itle page ---i

I>eclaration ---ii

Acknowledgement ---iii

I>edication ---v

Abstract ---vi

I.ist of tables ---xi

List of figures --- xiv

Chapter 1---1

Introduction---1

1.1 General background ---1

1.1.1 Definition of the term speciation ---2

1.1.2 Significance of speciation studies---3

I' 1.1.3

T

oxicity and toxicology of cadmium---

---

-

---

-

-

-

-

-

---

-

--

-

----

6

1.2 Objectives ---8

1.3 Justification---8

1.4 Thelocality ofLake Bogoria ---9

Chapter 2---12

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2.1 Introduction ---12

2.2 'Ioltarnrnetry ---13

2.3 Review of speciation studies in the determination of stability constants ---14

2.4 Theory of metal complex formation ---31

2.4.1 Complex formation equilibria --- 32

2.4.2 Equilibria of mononuclear complexes ---33

2.5 Theoretical framework for voltarnrnetric determination of stability constant of metals --- 35

.2.5.1 Lingane' sequation --- 36

2.5.2 Derivation of Deford - Hume equation for evaluating stability constants of single ligand complexes ---37

2.5.3 Application of Deford - Hume equation in the study---42

2.5.4 Characteristics of the F-Functions ---44

Chapter 3 ---46

Experimental work --- 46

I' 3.1 Introduction ---46

3.2. Sampling ---46

3.2.1 Preparation of plastic containers for sampling ---46

3.2.2 Collection of water samples from the lake ---47

3.2.3 Pre-treatment of samples ---48

3.3 Analysis of lake water ---48

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3.3.1 Introduction ---48

3.3.2 Determination of cations ---49 3.3.2.1 Determination of magnesium, aluminum, calcium, cadmium, potassium, sodium and total iron ---49

3.3.2.2 Instrumental conditions for AAS --- 50

.3.3.3 Determination of anions-- --- 51

3.3.3. 1 Determination of fluoride--- 51

3.3.3.2 Determination of chloride --- 52

3.3.3.3 Determination of sulphate ---53

3.3.3.4 Determination of nitrates --- 56

3.3.3.5 Determination of nitrites --- 57

3.3.3.6 Determination of phosphate --- 58 3.3.3.7 Determination of carbonates and bicarbonates ---59

3.4 General preparations for polarographic work ---60

I' 3.4.1 Cleaning of the polarographic cell ---69

3.4.2 Cleaning and filling of the capillary ---61

3.4.3 The reference electrode ---62

3.4.4 Counter electrode ---62

3.4.5 Supporting electrolyte ---63

3.4.6 Deaeration of electrolyte and test solutions---63

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3.4.7 Preparation of the oxygen scrubbing system---64

3.4.8 Neutralization of the lake water ---65

3.4.9 Suppression of precipitation ---66

3.4.10 Solutions for polarographic work ---67

3.4.11 Instrumental and laboratory conditions ---67

3.5 Experimental procedure for hanging drop amalgam voltammetry (HDAV) ----68 Chapter 4 ---69

Results, discussion and conclusion ---69

4.1 Introduction---69

4.2 Results of lake analysis ---69

4.2.1 Ionic balance ---71

4.3 Results of polarographic work --- 72

4.3.1 Calculations ofF-Functions and the determination of stability constants ---72

4.4 Calculations of species distribution ---77

I' 4.4.1 The incenzy method --- 77

4.5 Distribution of single ligand complexes ---78

4.5.1 Carbonato complexes ---78

4.5.2 Bicarbonato complexes ---84

4.5.3 Hydroxo complexes ---89

4.5.4 Chloro complexes ---93

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4.5.5 Fluoro complexes ---96

4.6 The speciation of some selected systems ---99

4.7 Conclusion and recommendations ---1 03

References ---106

Appendix A---115

Appendix B ---126

LIST OF TABLES

Table 3.1

.

Summary of Instrumental settings for AAS ---51 Table 4.1 The concentration of selected water variables---69

Table 4.2 Ionic balance --- 71

Table 4.3 Data for calculation of F-functions for hydroxo

Cd2+ complexes ---73

Table 4.4 F-Functions for hydroxo complexes of cadmium in

Lake water medium (data from DP ASV)---7 5

Table 4.5 I'

The stability constants of mononuclear single ligand complexes

determined at the ionic strength ofthe lake water---76

Table 4.6 The %distribution of cadmium carbonato species at varying

[CO/OJ in lake Water medium (data from DPASV)---81

Table 4.7 The %distribution of cadmium corbanato species at varying

[C032-] in KN03 aqueous medium (data from DPASV)---82

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Table 4.8

Table 4.9

Table 4.10

Table 4.11

Table 4.12

Table 4.13

Table 4.14

The %distribution of cadmium carbonato species at varying

[C032-]inKN03 aqueous medium (data from hanging

drop amalgam voltammetry) ---83

The % distribution of cadmium bicarbonato species at varying [HC03-] in (data from DPASV) ---86

The %distribution of cadmium bicarbonato species at varying

[HC03-] inKN03 aqueous medium (data from hanging

drop amalgam voltammetry)---87

The %distribution of cadmium bicarbonato complexes at varying

[HC03-] inwater medium (data from DPASV)---88

The %distribution of cadmium hydroxo species at varying [OH-] inKN03 aqueous medium (data from DPASV)---90 The % distribution of cadmium hydroxo species at varying

[OH-] inKN03aqueous medium (data from hanging

drop amalgam voltammetry ---91

The %distribution of cadmium hydroxo species at

varying [OH-] in lake water medium (data from DPASV) ---92

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Table 4.15 The % distribution of cadmium chloro species at varying

[Cn

in

KN03 aqueous medium (data from DP ASV) ---94

Table 4.16 The % distribution of cadmium chloro species at varying

[Cn

in KN03 aqueous medium (data from hanging

drop amalgam voltammetry) ---95

Table 4.17 The %distribution of cadmium fluoro species at varying

[F] in KN03 aqueous medium (data from hanging

drop amalgam voltammetry) ---97

Table 4.18

Table Al

Table A2

Table A3

Table A4

The % distribution of cadmium fluoro species at varying [F] in

KN03 aqueous medium (data from DP ASV) ---98

Solution mixtures for the polarographic determination

of stability constants of cadmium - carbonato

complexes in KN03 aqueous medium---115

Solution mixtures for the polarographic determination

of the stability constants of cadmium - hydroxo

complexes in KN03 aqueous medium---115

Solution mixtures for the polarographic determination

the stability constants of cadmium - bicarbonato

complexes in KN03 aqueous medium ---116

Solution mixtures for the polarographic determination

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Table AS

Table A6

Table A7

Table A8

,..Table AlO

Table All

Table A12

of the stability constants of cadmium - chloro

complexes inKN03aqueous medium ---116 Solution mixtures for the polarographic determination

ofthe stability constants of cadmium - fluoro

complexes inKN03aqueous medium ---117 Solution mixtures for the polarographic determination

of the stability constants of cadmium - carbonato

complexes in lake water medium ---117

Solution mixtures for the polarographic determination

ofthe stability constants of cadmium - hydroxo

complexes in lake water medium ---118

Solution mixtures for the polarographic determination

ofthe stability constants of cadmium - bicarbonato

complexes in lake water medium ---118

F-Functions for carbonato complexes of cadmium in

KN03aqueous medium (data from DPASV) ---119

F-Functions for carbonato complexes of cadmium in

KN03 aqueous medium (data from hanging drop

amalgam voltarnmetry) ---120

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Table Al3

Table A14

Table A15

Table A16

Table A17

Table A18

Table A19

Table A20

in lake water medium (data from DPASV) ---120

F-Functions for hydroxo complexes of cadmium

in KN03 aqueous medium (data from DPASV)---121

F-Functions for carbonato complexes of cadmium in

KN03 aqueous medium (data from hanging drop

amalgam voltammetry )---121

F-Functions for carbonato complexes of cadmium

in lake water medium (data from DPAS V) ---122

F-Functions for bicarbonato complexes of cadmium

inKN03 aqueous medium (data from DPASV) ---122

F-Functions for bicarbonato complexes of cadmium in

KN03 aqueous medium (data from hanging drop

amalgam voltammetry ---123

F-Function for bicarbonato complexes of cadmium

In lake water medium (data from DPASV) ---123

F-Functions for chIoro complexes of cadmium

in KN03 aqueous medium (data from DP ASV) ---124

F-Functions for chIoro complexes of cadmium in KN03 aqueous

medium (data from hanging drop amalgam voltammetry )---124

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Table A2l F-Functions for fluoro complexes of cadmium in

KN03 aqueous medium (data from DPASV)---125

F-Functions for fluoro compleses of cadmium inKN03 aqueous

medium (data from hanging drop amalgam voltammetry) ---125

LIST OF FIGURES Table A22

Fig. 1.1

Fig. 2.1

Fig. 4.2

Fig. 4.3

Fig. 4.4

'-Fig.4.5

Fig. 4.6

Fig. 4.7

The position of Lake Baringo and other lakes ofKenya--- -11

F-functions plots for Cu (II) carbonato complexes in KN03 (aq)

at [Cu2+] =6.3x10-5M ---4 2

Plots ofF-Functions Vs concentration of hydroxide

ions in lake water medium (data from DPASV) ---75

The % distribution of cadmium carbonato species atvarying

[C032-] in lake water medium (data from DP ASV)---81

The % distribution of cadmium carbonato species at varying

[C032-] in KN03 aqueous medium (data from DPASV)---82

The.% distribution of cadmium carbonato species at varying

[C032-] in KN03 aqueous medium (date from hanging

drop amalgam voltammetry) ---83

The % distribution of cadmium bicabonato species at varying

[HC03-] in KN03 aqueous medium (data from DPASV)---86

The % distribution of cadmium bicarbonato species at varying

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[HC03-] inKN03 aqueous medium (data from hanging

drop amalgam voltammetry---87

The % distribution of cadmium bicarbonato species at

varying [HC03-]in lake water medium (data from DPASV)---88

The % distribution of cadmium hydroxo species at varying

[OH-]inKN03 aqueous medium (data from DPASV)---90

The % distribution of cadmium hydroxo species at varying

[Off] inKN03 aqueous medium (data from hanging

drop amalgam voltammetry )---91

The %distribution of cadmium hydroxo species at varying

[Off] in lake water medium (data from DP ASV)---92

The % distribution of cadmium chloro species at varying

[CninKN03aqueous medium (data from DPASV)---94

The %distribution of cadmium chloro species at varying

[Cn inKN03aqueous medium (data from hanging

drop amalgam voltammetry---95

Fig. 4.14 The %distribution of cadmium fluoro species at varying Fig. 4.8

Fig. 4.9

Fig. 4.10

Fig. 4.11

Fig. 4.12

Fig. 4.13

Fig. 4.15

[F] inKN03aqueous medium (data from hanging

drop amalgam voltammetry)---97

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inKN03 aqueous medium (data from DP ASV)---98

Plots ofF-Functions Vs [COl-] inKN03 aqueous medium

(data from DP AS V)---126

Plots ofF-functions Vs [C032-] in lake water medium

(data from DPAS V)---126

Plots ofF-functions Vs [C032-] inKN03aqueous medium

(data from hanging drop amalgam voltammetry )---127

Plots ofF-functions Vs [HC03-] inKN03 aqueous medium

(data from DP ASV)---127

Plots F-functions Vs [HC03-] inKN03aqueous medium

(data from hanging drop amalgam voltammetry )---128

Fig. B6 Plots ofF-functions Vs [OH-]in lake water medium Fig. Bl

Fig. B2

Fig. B3

Fig. B4

Fig. B5

(data from DPAS V)---128

Plots of'F-functions Vs [OH-]inKN03 aqueous medium

(data from DP AS V)---129

Plots of F-functions Vs [Cn in lake water medium

(data from DPASV)---129

Fig. B9 Plots ofF-functions Vs [Cn inKN03 aqueous medium

Fig. B7

Fig. B8

(data from hanging drop amalgam voltammetry )---130

Fig. BI0 Plots ofF-functions Vs [F] inKN03 aqueous medium

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Fig. Bll

Fig. B12

Fig. B13

Fig. B14

Fig. B15

(data from DP AS V)---13 0 Plots ofF-functions Vs [F] inKN03aqueous medium

(data from hanging drop amalgam voltammetry )---131 Stripping Polargrams for cadmium-hydroxo complexes

inKN03aqueous medium (data from hanging drop

amalgam voltammetry )---132 Stripping polargrams for cadmium-hydroxo complexes

inKN03aqueous medium (data from hanging drop

amalgam voltammetry) ---133 Stripping polarograms for cadmium hydroxo complexes

inKN03aqueous medium (data from hanging drop

amalgam voltammetry) ---134 Stripping polargrams for cadrnium-chloro complexes

inKN03aqueous medium (data from hanging drop

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CHAPTER!

Introduction

1.1 General background

Considerable research effort in the fields of biological and environmental science has focused

on the chemistry of natural water systems. The term "natural water" is often used to refer to

an actual system of some complexity, consisting basically of three phases:an aqueous solution

phase, one or more solid phases and most often a gas phase. The real system may be

inhomogeneous overall but may have sufficiently characterized homogenous regions. The

aqueous solution phase is composed of a variety of substances, inorganic and organic in

nature. Among the inorganic species present are trace metals whose functions in the human

body and environment, have been increasingly recognized. Trace metals may exist in water

reversibly bound to inorganic anions or to organic compounds, or they may,in a few cases,be

present as organometallic compounds containing carbon to metal bonds. These species often

have vastly different solubilities, transport properties and biological effects from the

aquo-/' metal ions themselves.1-3

The chemistryof natural waters has received much attention. Much of it has been centered on

the determination of buffer capacities, stability constants of both organic and inorganic

complexes, qualitative and quantitative trace elements, speciation and metal species

distribution 4,5. This thesis is a report of a speciation study of the complexes formed by

cadmium with selected ligands found to be dominant in an inland salt lake, Lake Bogoria,

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divided into four inter-related chapters. Chapter one discusses and defines the term speciation

and the significance of speciation studies. The chemistry of cadmium in relation to its

environmental toxicity and toxic effects is discussed and an account given on the study area.

Chapter two reviews the previous literature on speciation studies and discusses the theoretical

treatment of metal complex equilibria. Chapter three details the various experimental

preparations for the voltamrnetric work. In chapter four, results of analysis and calculations

,

are discussed and an attempt to model some equilibria of cadmium complexes given. The

conclusion and the recommendations of the study are also discussed.

1.1.1 Definition of theterm speciation

The term chemical speciation has no conventional definition attached to it andvarious workers

have assigned various meanings to it. The term is generally used to distinguish between measuring the total metal concentration of an element and the concentration of each of its

chemical forms.Nriagu"defines the term metal speciation as all the possible chemical forms of

a metal that may occur in different environments, while Florence" uses the term speciation to

refer to the determination of individual physicochemical forms of an element which together

make up it's total concentration in a sample. The individual physicochemical forms may

include particulate matter, simple hydrated metal ions or dissolved forms such as simple

inorganic species and organic complexes. David et al.8 defines speciation as the identification

of inorganic, organometallic or organic components actuallypresent in the environmentwhile

Schnfder" defines the term as the determination of the species present in an undistorted

system The commission of the European communities and community bureau of reference'?

has developed a broad definition of the term It defines speciation as either the process of

identifying and quantifying the different defined species, forms or phases present in a material;

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or the description of the amount and kinds of these species, forms or phases present in a system In this work the term speciation should be taken to mean the characterization, quantification and distribution of chemical species.

1.1.2 Significance of speciation studies

The concern and attention that is attributed to the environmental occurrence and fate of toxic metals is justified with respect to the hazards they cause to man. Therefore, to protect the ecosystems upon which our health and lives depend, there is a need to understand the different natural and chemical processes that may affect the type of equillibria found in different ecosystems. This way, adequate models may be developed that would help in predicting the effect of changes (for example, the addition of contaminants) on these ecosystems. Data derived from speciation studies would be useful in developing such a model.

Proliferation of heavy metal contaminants in the environment has focused attention on their determination and characterization. Below their threshold levels, many metal ions have essential functions to all sorts of biological organisms including man. Among hazardous environmental chemicals, certain heavy metals and metalloids have gained particular significance and priority due to their toxicity". In primary focus are the toxic heavymetals Cd, Pb and Hg, which belong to the class of first order priority in ecotoxicology. A number of others, for example, Cu, Cr, Zn.,Co, V, Ni, Se, Sn, et cetera, which have essential functions for living systems at low concentrations exert toxicity above their respective threshold levels.

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environmental compartments via respiration and the continental and marine food chains.Due

to their non-biodegradability, these metals can be persistent and insidiouslypoisonous. They

tend to accumulate in various vital organs e.g. kidney, liver,intestinal tract, lungs, renal tract

and brain.Some, for example Pb and also Cd maybe deposited in bones. From these deposits,

the toxic metals may also be re-mobilised under certain metabolic conditions. Some display

carcinogenic effects due to detrimental interaction with nucleic acid. 3,12 - 14. One can

distinguish between two groups of sources for the input of toxic metals to the environment:

the natural and anthropogenic sources. The natural sources include weathering of respective

minerals from respective geological deposits, volcanism and from various geographical

positions at the seafloor. The contributions of these natural sources though significant cannot

be controlled. Nevertheless, their contributions to the toxic metal levels in various

environmental compartments have to be explored, monitored and quantitatively understood. The anthropogenic sources include, effluent from domestic use, sewage, vehicle traffic

emissions and agricultural activities. It is increasingly being realized that the distribution,

mobility, bioavailability, bioaccumulation and toxicity of metals depends not simplyon their

total metal concentrations but criticallyon their chemical and physical associations whichthey

undergo in natural systems. Changes in environmental conditions, whether natural or

I'

anthropogenic can strongly influence the behaviour of both essential and toxic elements by

altering the forms in which they occur. Some of the more controlling factors include pH,redox

potential and availability of reactive species such as complexing ligands (both organic and

inorganic), particle surfaces for adsorption, and colloidal matter". In order to comprehend the environmental chemistry of an element, it is necessary to characterize in full the properties of

all its various forms under the diverse range of conditions possible in natural systems. Whilst

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important forms of an element in order to understand the transformations between forms that

can occur, and to infer from such information the likely environmental consequences.

Laboratory experiments designed to measure the concentration of heavy metals toxic to

aquatic organisms will have little meaning unless the chemical forms of the metal in the test

water is known." Chemical speciation is thus a discipline which is of relevance to scientists

with different backgrounds. Chemists, biologists, soil and sediment specialists, physicists and

specialists in various aspects of nutrition and medicine,all require this type of information.

An investigation of the speciation of metals in natural waters has relied on three different basic

approaches; 17 - 20

• The study of the behaviour and reactions of metals in a simulated water system This is

usually done at a constant ionic strength and constant temperature. Stabilityconstant

data for a wide range of metal complexes have been calculated from such studies.

• The application of theoretical thermodynamic modeling techniques to predict the

distribution and transformation of chemical species. The use of thermodynamic data to

predict trace metal speciation is an important facet of the study of trace elements.

Despite a number of problems, notably inaccuracies of, and gaps in stabilityconstant

data, thermodynamic calculations of equilibrium species distributions provide an essential theoretical basis for speciation analysis.

• The determination of species, or groups of species in real samples using analytical

techniques such as anodic stripping voltammetry, potentiometry, ion-selective

electrode, flow injection analysis,et cetera.

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1.1.3 Toxicity and toxicology of cadmium

Cadmium is a highly toxic metal. It was chosen for this study due to its growmg

environmental and toxic concerns to aquatic life and to man. The study intended to establish

some of the chemical forms in which cadmium could exist in Lake Bogoria. This was done

taking cognizance of the fact that the chemical,physical and geochemical reactions of a metal

as well asits physiological or toxicological effects in a medium depends largelynot on its total

metal concentration but on the various physicochemical forms in which it exists. Below is a

brief description of cadmium,its toxicityand toxicology.

The discovery of cadmium (relative atomic mass and atomic numbers are 112.40 and 48

respectively) as a distinct element was made bythe German chemist Stromoever in 181721.

Cadmium is the second member of the group lIb triad (Zn, Cd, Hg) in the periodic

classification of elements with an electronic configuration of 1s22S22p63s23p63d1o4S24p6

4d1o 5s2.The stable state of cadmium in the natural environment is Cd2+.Cadmium is silvery

white and ductile with a faint blue tinge. It has characteristics that are mid-way compared to

zinc and mercury.Cd2+ has high polarizing ability which imparts moderate covalence in bonds

,

-and high affinityfor sulfhydryl groups, leading to increased lipid solubility, bioaccumulation

and toxicity. Zinc has been shown to be an essential element while cadmium on the other had,

is a highlytoxic metal with no known fimction in animal metabolism22,23Cd is aninhibitor of

sulfhydryl enzymes. It also has affinity for other ligands in cells such as hydroxyl, carbonyl,

phosphatyl, cysteinyl and histidyl side chains of proteins, purines and porphyrin and canalso

disrupt pathways of oxidative phosphorylation". Cadmium accumulates in livers and kidneys

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protein that acts as the transport protein for Cd. Since the metabolism of cadmium is closely

related to zinc metabolism, metallothionein binds and transports both cadmium and zinc though in many vital enzymatic reactions cadmium seems to displacezinc which aggravates Zn

deficiency causing disruption or cessation of Zn activities. Cadmium interacts and or competes

with other metals in biological processes. For example,in animal studies,high dietarylevels of

this element have been shown to depress copper uptake and to change the distribution of

copper in tissues. Studies have shown that rabbits fed on Cd develops a hyperplastic bone

marrow and a hypochromic microcytic anaemia similar to that induced by iron deficiency'<".

Exposures to Cd occur via the respiratory tract, though bad hygienic practice may result on

some gastro-intestinal absorption. Inhalation of fumes or dust containing Cd and its compounds

primarily affects the respiratory tract but there are subsequent systemic effects as well. Some

hours after exposure, a dryness of the throat, a sense of constriction and difficulty in breathing

are experienced. There may be headache, vomiting and muscle cramps. In fatal cases, a

pulmonary edema, acute inflammatory changes in the kidney and fatty degeneration of the liver occur. People exposed to Cd environments for long develop emphysema of the lungs,mild liver

damage, some dental changes and impairment of the sense for smell,that is,anosmia.Ingestion

I'

of Cd compounds produces symptoms suggestive of food poisoning, for example, nausea,

salivation, vomiting followed by diarrhoea with abdominal discomfort and pain which may

appear almost suddenly or can be delayed for a few

hours29,30.

These description shows that cadmium is a highly toxic substance and it's distribution in the

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study in areas where it mayor may be suspected to occur so as to infer the various

physicochemical forms in which it might exist. Of concern is the distribution of cadmium in

Lake Bogoria. The lake is economically important in that it is a sanctuary for hundreds of

flamingoes that migrate from Lake Nakuru. These attract hundreds of both local andforeign

tourists thus earning the country a lot of foreign exchange. Further, a game park that

accommodates wild animals such as kudus, impalas, zebras, klip springer, leopards, et cetera

surrounds the lake. Their health and survival would depend on how well the environmental

pollution of the lake and lake region is controlled. During the rainy season, effluentsfrom

farms are washed into the lake and this may affect the quality of the lake water, and by

extension the health of the animals that depend on the lake. It is against this background that

lake Bogoria was chosen for this study.

1.2 Objectives

The objectivesof this study were to:

• determine the stoichiometric composition ofLake Bogoria andits ionicstrength.

• determine the stabilityconstants of cadmium complexeswith selectedinorganicligands

and model the speciation of cadmium in the lake.

1.3 Justification

The data obtained from the study can be applied in pollution control, treatment of water and

wastewater effluent into the lake, transportation, bioavailability and toxicity of cadmiumto

(29)

1.4 The locality of Lake Bogoria 31-33

Lake Bogoria, formerly Lake Hannington is a soda lake and occupies a spectacular trough,

which is 18km long, and upto 5Km wide. The cliffs of the Siracho escarpment rising over

700m above the lake surface bound it on the East Side.The lake is situated in the Ng

elesha-Solai region ofBaringo/ Koibatek districts of the Rift ValleyProvince of Kenya It stands ata

grid reference of 0°10' - 0°02' and 36°04' - 36°07'. It lies in an area of great topographic

diversity, in the spectacular part of the Baringo basin, a section of the Kenyan rift valley

system, physiographically known as the Gregory rift. The Gregory rift is upto 30 km wide,

extending from Nginyang in the north-west at about 880m above sea level upto the Dispei

plateau south ofLake Bogoria, where the rift floor elevation rises to 1800m

The area around the lake is characterized byhot springs, fumeroles and geysers, which occur

extensively.There are 140 hot springs, which have been recorded in 8 areas aroundthe lake.A

large homogeneous bodyofwarm water (about 35°C) underlines this area including Majimoto

and Liboi, which are sodium bicarbonate type springs. The main source of steam seems to be

at the south end of the lake (Mwanasis Peninsula) where numerous hot springs and geysers

feature. The steam has a high carbon dioxide content, with some methane and hydrogen

sulphide.

According to Renout et al.32, Lake Bogoria and Lake Baringo were one time one lake.

However, during the quartenary period, the lakes became separated owing to either normal

regression or delta propagation. Minor faulting and subsidence in the Baringo depression

(30)

The mmes and geology department (Kenya), indicate that the lake Bogoria water is

homogeneous, enriched in sodium carbonate, chloride and fluoride.The average annual rainfall

varies between 1140 mrn to less than 500mm. Many small earth darns and boreholes tapping

the aquifers in the plateau phonolites support the ranching industry around the lake. Though

most of the rivers towards the rift valley floor may have their water recharged with ground

water wherever they cross minor faults, their water is used for irrigation (for example,at Liboi

plains). This form ofland use has over the last few decades changed the catchment area of the

lakes reducing the amount of water flowing into them and thus resulting in the reduction of

their sizes.

To the north of the lake, there is a seasonal swamp into which the rivers that feed the lake

flow. The Waseges River running between the Siracho and Laikipia escarpments on the lake's

eastern side feeds into the Kisibor swamp while the Ndolaita running parallel to the lake on its

western side feeds into the Liboi swamp. On the southern side, the lake receives water from

(31)

"'\ .",.

) 1 '\ \. '\ ') \ \ 1 ) ( <I: o

::> /

/ ./

( /

400E

ETHIOPIA

/ /

'.<

, ..,....

/ '-._

-( Turkana ''-'_'-'''- <, __.r'

/

~L 8aringo

" L. Bogoria

(

I r.n

I

0

I ~

I

l>

I

r I

I

l>

I

I 0

I

I

l

"-\ DLNokuru

'L Elementoito ~L. Noivo stio

o

100 200 Km

1:::1===±::I ====::JI

<,

Figure 1.1

The p

o

s

i

tion

of

L

ak

e B

a

r

i

n

g

o a

n

d

o

th

e

r

l

akes of Ken

y

a

(32)

CHAPTER 2

Literature Review

2.1 Introduction

Interest in chemical speciation and its procedures is expanding rapidly as a wide spectrum of

the scientific community recognizes that assessment of health hazards, toxicity and

bio-availability of heavy metals must be based on levels of specific chemical forms, rather than

on total element levels. A lot of work on metal complexes has been done with a variety of

analytical techniques. Most of the common experimental methods are regularly reviewed

and a number of scientists have contributed greatly in the refinement of concepts, theories

and methods in use today"?'.

The potential for disturbing existing equilibria conditions during speciation analysis is high,

particularly during the sampling stage. The various chemical forms co-exist in equilibrium or

quasi-equilibrium states and all stages of prescribed analytical procedures can be intrusive.

The choice of a particular method depends on whether it meets the basic requirements of

sensitivity, accuracy and selectivity to the specific forms of complexes in solution. The

choice of a procedure is further restricted by the fact that the total concentration of trace

elements present in a sample (for example, Cu, Pb, Cd, Zn, et cetera) is often near the

(33)

requirements, speciation procedures adopted, unlike other analytical methods, should not

disturb existing equilibrium conditions.

No attempt in this work has been made to review all the common experimental methods used in the determination of stability constants and/or metal species. A greater emphasis has been placed on the electrochemical methods with an emphasis on the voltammetric technique, polarography, which is the technique of choice for this work due to its inherent differential response to different metal species based on their reduction potential other methods are cited in cases where they have been used in specific studies related to speciation. In this chapter, a brief review of voltammetry is given plus a detailed review of

speciation work done elsewhere. A theoretical framework for voltammetric determination of stability constants of metals is also discussed.

~2 Voltammetry

Voltammetry comprises a group of electro analytical methods in which information about the analyte is derived from the measurement of current as a function of potential waveform applied to the electrode under conditions of complete concentration polarization. These include differential pulse polarography (DPP), differential pulse anodic stripping

voltammetry (DP ASV), square wave voltammetry, cyclic voltammetry, et cetera.

(34)

The main analytical application of polarography for many years was to measure total metal

ions of environmental importance (Cu, Cd, Hg, Pb, et cetera.) in water at very low

concentrations. Later it was found suitable in speciation studies because of its inherent

differential response to different metal species based on their reduction potentials. The

introduction of pulse techniques'" made polarographic techniques quite versatile. The

modern polarographic techniques compete with other refined analytical techniques such as

atomic absorption, molecular spectroscopy, and activation energy analysis in terms of

selectivity and sensitivity. The use of solid electrodes (for example, rotating platinum

electrodes in an organic medium, electrodes made of graphite or vitreous or porous

carbon, or of mercury deposited on carbon), anhydrous organic solvents, platinum thin

film electrode (PTFE) capillaries and of continuos flow micro cells; allthese features have

enlarged the field of application of modern polarographic techniques in chemistry,

medicine, geophysics, geochemistry, environmental studies, et cetera. The rest of this

section deals with the review of specific speciation studies that have so far been done using

the various speciation methods. The review focuses on the techniques and/or the type of

metal or non-metal complexes formed.

2.3 Review of speciation studies in the determination of stability constants

Formation constants of the bivalent metal ions of copper with acetyl acetone ions were

studied by Izatt et al.54 using potentiometric titrations. He obtained the stability constants,

log KJ and log K2, for the two copper acetyl acetone complexes formed as 8.22 and 6.73

(35)

Grande" has reported that when salmonid fishes are exposed to copper in Soft River

water, the toxicity of copper is reduced when humic substances are present. He attributed this to the complexation of copper with these humic substances. Low levels of N-containing organic acids with chelating properties have similarly been found, in a study

carried out by Sprague", to reduce the toxicity of copper salts. This shows that complex

formation can alter the toxic properties of metals.

Ernest et al.57 studied the chemical speciation of copper in both freshwater system and

seawater environments with the inorganic ligands Off, CO/- and Cr. They found that the predominant copper species in fresh water of pH 7.0 and total carbonate concentration of

lO-lM was CuC03 while Cu (C03

)l-

and Cu2+(aq) contributed relatively minor portions to

the total copper distribution among the ligands. However, they found that in seawater, the degree of complexation increased probably due to changes in pH, ionic strength of the water and/or the concentration of the ligands. From their values of stability constants, the

I'

predominant forms of copper were found to be

Cu

2+,

CuOW

and

coe

r.

They also found

that alkalinity and pH govern the copper speciation in absence of other complexing and adsorbing agents by formation of carbonato and hydroxo complexes.

Since speciation can affect bioavailability and toxicity of copper in aquatic systems,

accurate predictions of effects ofbio-available forms require detection and/or measurement of these forms. In order to make defensible estimates of the potential risks of metals in

(36)

sediments or water, it is essential to identify the fraction of total metal that is bio-available.

Deaver

et al

.

58 used DP ASV to measure bio-available copper in aqueous and sediment

tests with the amphipod,

Hyalella Azteca Saus

s

ur

e.

To develop an approach for measurements of bio-available copper, they used a copper sulphate solution

(CuS04.3Cu(OH)2.H20) in ten-day aqueous and sediment toxicity tests with the

H

y

alella

.

Azteca Saussure

.

Their tests encompassed ranges of pH (6.8 - 8.1), alkalinity (10 - 70 mg/L as CaC03) and hardness (10 - 70 mg/L as CaC03). Changes in copper concentration

were measured using atomic absorption spectroscopy (AAS) for total copper and

differential pulse anodic stripping voltammetry (DP ASV) for labile copper. The concentrations were evaluated relative to amp hipod survival. In the Ifl-day tests, they

found that total copper concentrations were not predictive of sediment toxicity, but

Hyalella Azteca Saussure

survival was explained from DP ASV measurements that indicated the level of the bioavailable fraction of the metal.

Stella

et al.

59 determined the speciation of copper in natural water from rivers Ticino and Po in Italy using the copper-ion selective electrode. They found copper to exist as CuOF',

CU(OH)22+,CuC03, and CuL: where L represents organic ligands such as humic acid and

fulvic acids. Using the values of stability constants given by Sillen and Martel60 they were

able to calculate the species distribution. The predominant inorganic species was found to

be CuC03 with the other inorganic species contributing a negligible fraction of the total

(37)

sulphides and soluble silicates was found to be the major analytical problem.

Rotating disk electrodes (RDE) voltammetry with a mercury thin-film electrode on a

glassy carbon substrate combined with anodic stripping voltammetry (ASV) using

square-wave, differential-pulse, and staircase waveforms was applied by Chakrabarti et al.61 to

direct determination of Cu(11) and Pb( 11) speciation in model solutions of Cu(

11)-nitrilotriacetic acid and Pb( 11)-11)-nitrilotriacetic acid complexes and in snow samples. They

found the dissociation rate constants of Cu(l1) and Pb(l1) complexes to be similar. Of

the three waveforms examined for their suitability in the study of Cu(l1) and Pb(11)

complexes in the above model solutions and in snow samples, staircase voltammetry gave

the most satisfactory estimates of dissociation rate constants and diffusion coefficients of

/

the metal complexes though the analytical sensitivity of square-wave voltammetry was

found to be two orders of magnitude higher than that of staircase voltammetry. They also

found that the RDE technique combined with ASV is capable of distinguishing labile and

non-labile complexes present in extremely low concentrations in aqueous solutions and in

precipitation samples. The RDE technique can do the above differentiation by virtue of its

ability to measure metal availability for reduction over a wide range of time scales, and

gives quantitative information about the extent of metal complexation, that is, it can

estimate the rate constants for the dissociation of metal complexes and the concentrations

of various metal complexes. The technique offers the additional, potential advantage of in

(38)

In his polarographic study of Cu speciation in Lake Elementaita water and a model water

system maintained at a constant ionic strength of O.27M with KN03, Gikandi62observed

two copper peaks when using the DPASV technique. He attributed the formation of the

two peaks to the step-wise reduction of Cu2+to Cu+ions. The peak for Cu2+was at

+O.38V and the one for Cu+ at -O.OIOV versus (saturated calomel electrode, SCE). He concluded that the copper complexes in the lake water and the artificial media studied

were Cu(1) complexes and that carbonato complexes contribute most while bicarbonato

complexes contribute least to the speciation of Cu( I) complexes in Lake Elementaita. He

Cu(HC03)2-, CU(OH)2- and Cu(OH)32- which were previously unreported. Mwaniki63on

the other hand studied the lead complexes in the same lake at an ionic strength of I.253M

and a pH of 10.2. He found that chloro and fluoro complexes of lead contribute most to

the speciation of lead at low pH but at higher pH, their contribution is minor. Maloba 64

also working on Lake Elementaita at an ionic strength of O.256M and the same pH as

above, reported the existence of four chloro complexes (CdCt, (CdCho, Cdf'l,', CdCli1

three carbonato complexes (CdC03°, Cd(C03)22-, Cd(C03)34-) and four hydroxo

I'

complexes (CdOW, Cd(OH)2°, Cd(OH)3- and Cd(OH)i) in both lake water and aqueous

media. Three fluoro complexes (CdF+, CdF2

°

and CdF3) were identified in aqueous media while in lake water only CdF+ and CdF2°were identified. He showed that cadmium in L. Elementaita exists mainly as carbonato and bicarbonato species while chloro complexes of

cadmium were of major significance only at high pH values.

(39)

Cu( 11) complexes with CU(C03)34- being the most predominant species at high carbonate

concentrations and CuC03

°

dominating at low concentrations. Two Cu( 1) carbonato

complexes were identified with (CU(C03)23- and(Cu(C03

D

being the major species. Four

chloro Cu(ll) complexes, (Cuct, CuCh, Cuf'l,' and CuCI/-) were identified with the

CuC142- contributing the most to copper speciation at the chloride level in the lake. Only

one Cu(l) chloro complex was found but its contribution to copper speciation in the lake

was quite significant. Of the two Cu(ll) hydroxo complexes identified, Cu(OHf was found to be the most abundant while CU(OH)2ocontributed the most to the speciation of

Cu among the hydroxo complexes identified. At the level of the hydroxide (very low) in

the lake, the hydroxo ligand was found to be of minor importance in the speciation of

copper, the ionic metal species being more abundant. Fluoro copper complexes were also found to be less important in the speciation of copper in the lake because of the weak

fluoride - copper binding. At the level of the fluoride in the lake, CuF+ and CuF20were

found to be important fluoro species. Of all the copper complexes identified, thiourea

complex Cu(CH4N2S)/+ was found to be the most abundant species.

Mixed ligand complexes of Cu( 11) with some amino acids (aspartic, glutamic or lysine

acids) and oxalic acid were studied by Shah et at.65 They used the extended form of

Deford-Hume expression ofFo(X) to calculate the stability constants ofCu(ll) complexes.

'\./\..

This was done at an ionic strength of 1.0M maintained with KN03 at a temperature of (30

± 1)OC. They found that all the complexes underwent a two electron reduction at the

dropping mercury electrode (DME). They also studied the steric effects (due to the size of

(40)

being larger than the oxalate ion offered a greater steric effect and their stability constant

values were lower. The oxalate mixture; (Cu(OX):CU(OX)22) had values oflog /31and log

/32equal to 5.7 and 9.3 respectively while the aspartate mixture (Cufasp)"; Cu(asp)2) had

higher values oflog /31and log /32equal to 8.6 and 15.5 respectively.

Rebello et al.67 determined lead in polluted tropical sea water of Guanabara bay (in Rio de

Janeiro) by anodic stripping voltammetry in the differential pulse and linear sweep modes at mercury film electrodes (MFE) and hanging mercury drop electrodes (HMDE). Values

found for total lead concentration ranged between 0.07 and 5.4 ppm. Guanabara bay represents a complex aquatic system in a tropical climate. Its area of about 400 km2 is surrounded by the highly populated and industrialized state of Rio de Janeiro. About 1000

m3 of fresh water per second runs through the bay. Rivers carrying industrial and domestic

wastes, confers a complex heterogeneous character to the bay waters.

Stability constants of some hydroxo and carbonato complexes ofPb(11), Cu(ll), Cd(ll)

and Zn(11) were determined in a simulated seawater by Bilinski et al.68 using DPP and

ASV methods. The water was maintained at a constant ionic strength of 0.102M using

KN03. They found that Pb(ll) and Cu(ll) formed only MC03

°

and M(C03

h2-

(M

=

metal) complexes of similar stability whereas Cd(11) and Zn(11) formed only MC03

°

of

much lower stability.

In their polarographic study of complexes formed by Cu, Cd, Pb and Zn with formate ion,

(41)

that these metals form weak but definite complexes of the type M(HCOOt, M(HCOO)2°,

M(HCOO)3- and M(HcOO)l- (M =Metal) with the formate ion at 25°C.

Ntale et al.70 studied the inorganic (chloro, fluoro and carbonato) complexation of lead in

lake Katwe using drop amalgam voltammetry. This was done using sodium perchlorate as

the supporting electrolyte at an ionic strength of 7.35M, the predetermined ionic strength

of the lake water. The results showed the existence of two lead chloride species with the formulae PbCI+ and PbCho; three fluoride species, PbF+, PbF20 and PbF3-; and two

carbonate species with the formulae PbC03° and Pb(C03)22-.

Hume et al." usmg polarography found that cadmium form complexes with the

thiocyanate ligand to give the species; CdSC~ ,Cd(SCN)2° ,Cd(SCN)3- and Cd(SCN)l

-. Arce et al.72studied mixed hydroxy-complexes of monoethanolamine with lead and with

cadmium in aqueous and aqueous methanol. They reported some previously unreported complexes; Cd(MEA)40H, Pb(MEA)OH and Pb(MEAh(OH)2.

Khurana et al.73 studied mixed ligand complexes of cadmium with oxalate and tartarate

ligands. They found that cadmium forms the following complexes Cd(Ox.)(Tar.h,

Cd(Ox.)(Tar.) and Cd(Ox.h(Tar.). In some other work involving cadmium - oxalate

-maleate complexes, these authors have reported the formation of the complexes

Cd(Ox.)(Mal.)2 and Cd(Ox.)2(Mal.).

(42)

Polarographic Study of the complex systems Cd-hexanedioate, Cd-Pentanediote and mixed

Cd-hexanediote-Pentanediote was carried out by Baghel et aC4 They found that cadmium

forms hexaco-ordinated complexes with each of the ligand individually and that three

mixed complexes [Cd(X)(Y)t; [Cd(X)(Y)2t and [Cd(X)2(Y)t are formed where X2

-and y2- stands for pentanedioate and hexanedioate ions respectively.

The nature of mixed ligand chelates of Cd(11) with bidentate ligands oxalic and salicyclic

acids were investigated by a polarographic technique by Dhuley et aC5. They observed

that Cd(11) forms two complexes with salicyclic acid; [Cd(Sal.)r and [Cd(Sal.j-], and

three complexes with oxalic acid; [Cd(OX)], [Cd~OX)2t and [Cd(OX)3t. They also

reported the formation of mixed ligand chelates [Cd(OX)(Sal.)r and [Cd(OX)(Sa\)2t.

In their study of Zn2+and Cd2+citrate complexes in aqueous solution using potentiometry

with the glass pH electrode at various temperatures, Sammartano et al." reported the

formation of the following species; Zn/Cit)', Zn(Cit)H, Zn(Cit)24- and Zn2(Cit)H2 for zinc

,-complexes while Cd(Cit)"2, Cd(Cit)H, Cd(Cit)24- and Cd(Cit)H2- were found for cadmium.

The study was carried out at various temperatures at ionic strength of 0.1 OOM maintained

using potassium nitrate as base electrolyte. In another communication", the same authors

have described experimental and a calculation procedure for the study of weak complexes

of alkali and alkaline earth metals by pH measurement techniques. They have reported an

algorithm for the calculation of formation constants together with a computer program in

(43)

Correla Dos Santos et al.78 studied cadmium complexes of amino acids in sea water

conditions by potentiometry and differential pulse polarography for the amino acids;

alanine, serine, valine and glutamic acid at 20°C and 25 °C in 0.700M NaCI04 solution as

support electrolyte. The stability constants were found to be of the same order of

magnitude since the chelating groups of the ligands are identical. The studies revealed that

the cadmium complexes with amino acids maybe more important in less saline freshwaters

where competition with chloride ion is lower.

Cukrowski " studied the ligand N-(2-hydroxyethyl) ethylenediamine (HEEN) with Cd(II)

by differential pulse polarography (DPP) at a fixed total ligand (L- T) to total metal (M- T)

concentration ratios and varied pH at 25°C and an ionic strength of 0.5M. The

polarographic experimental complex formation curve (ECFC) and calculated complex

formation curve (CCFC) were used for modeling of the metal-ligand system and the

refinement of stability constants. The ECFC, in which experimental parameters of

polarographic peaks are included (a shift in a peak potential and a variation in a peak

height) were found to be a characteristic function for a particular metal-ligand system

studied at a particular L- T: M- T ratio. The CCFC was a theoretical curve calculated for

the assumed metal-ligand model from mass-balance equations. The analytical model of

metal species formed is the one for which the CCFC fits best the ECFC. He reported four

cadmium complexes: CdL2+, CdLl+, CdL/+, and CdL2(OHt with their stability constants

in log. form being 5.08 ± 0.03, 9.44 ± 0.04, 1l.25 ± 0.05 and 12.06 ±0.03 respectively.

(44)

It was the first time that the complex CdL32+ was reported.

Viksha et

at.

80 has developed an improved method for the determination of lead and

cadmium in whole blood of mothers and their babies by stripping potentiometry (SP). The

method was validated using graphite furnace atomic absorption spectrometry (GFAAS) for

the simultaneous determination of lead and cadmium. Using the method, they found the

concentration of lead and cadmium in the whole blood of Polish mothers and their babies

to be about three times higher than their Swedish counterparts. This may be the result of

accumulation of lead and cadmium from polluted environment.

The extent of iron complexation by natural organic ligands in seawater was determined by

Van den Berg et al.81using catalytic cathodic stripping voltammetry (CSV). CSV was used

to take advantage of ligand competition between the added ligand, 1-nitroso-2-naphol

(NN), and natural ligands present in seawater. The conditional stability constant for the

complexation of iron by NN was calibrated for salinities between 1 and 36 using ligand

competition with EDTA. The values of log K/Fe(NN) (Valid for pH 6.9 seawater) were I'

found to vary linearly with log (salinity) according to log K/ Fe(NN i" -1.04

+

0.08 log

(salinity)

+

30.12

±

0.09. Preliminary measurements of iron complexing ligands in samples

from coastal and open oceanic origin revealed the presence of natural complexing ligands

(L) at concentrations higher than that of total dissolved iron. The stability constants for the

complexes were high, log

K

FeLfalling within the range of 18.8-21.2, indicating that by far

the greatest component (99%) of the dissolved iron occurs organically complexed at pH

6.9 seawater. Model calculations showed that it is possible that the organic fraction may be

(45)

less at a pH value near 8.

Meyer et al.82 have devised a method for a reliable and ultrasensitive determination of

inorganic ionic mercury using differential pulse anodic stripping voltammetry on a glassy

carbon electrode. Using the method, it was possible to determine mercury down to a

concentration of 5 x 10-14M, the lowest detection limit ever reported for a voltammetric

method. This was achieved by using a thiocyanate electrolyte and relatively long

deposition times. The mercury ions were stabilized in the solution by the formation of

strong thiocyanate complexes, which leads to a highly reproducible cathodic plating and

anodic dissolution of mercury.

In their study of iodine speciation, Tian et al." determined the vertical profiles of iodide,

iodate and total free iodine monthly for one year (from July 1993 to June 1994) at the

DYF AMED permanent station located in the northwestern Mediterranean Sea. Differential

pulse polarography and cathodic stripping square wave voltammetry was directly used to

determine dissolved iodate and iodide respectively. Iodate was found to be the

"

predominant species, ranging from 390 nM in surface waters to 485 nM in deep waters.

Iodide was found to be present in significant concentrations up to 80nM in surface waters

and from undetectable levels to several nanomolar

«

10 nM) in deep water. The concentration of total free iodine was found to be slightly lower in surface waters

(467-478 nM with an average of 472 nM) than in deep waters (475-486 nM with an average of

481 nM). Considerable variations in iodine speciation were observed. Iodide

(46)

to April (about 20 nM), increased up to 80 nM from may until November and then

decreased from December to February. Comparison between iodide abundance and

primary production demonstrates that the transformation from iodate to iodide in surface

waters is linked to the regenerated production. They concluded that the stability of

regenerated iodide in surface seawater makes iodide a potential indicator to evaluate new

production versus regeneration.

In their study of iodine speciation in a stratified water column (Salinity and dissolved

oxygen gradients) of the Rogoznica lake, Stipanicev et al." observed specific

concentration profiles of iodate, iodide and organo-iodine in the water column of the lake,

where both oxic and anoxic conditions occur. Iodate and iodide concentrations were determined directly in water samples by DPV and cathodic stripping square wave

voltammetry (CSSWV) respectively. The concentrations of 'labile' and 'stabile'

organo-iodide were determined by DPV in the samples pretreated with chlorine water and

UV-irradiation combined with hydrogen peroxide and followed by chlorine water addition

respectively. In the column which had pronounced biological productivity, high

"

concentrations of iodide (up to 0.87 mlvl) and significant percentages of 'labile' and

'stabile' organo-iodine, up to 37% and 30% respectively. They concluded that the

formation of 'labile' organo-iodine is primarily governed by chemical reactions with

dissolved sulphur forms and organic compounds as well as the remineralization processes

of ,stabile' organo-iodine.

(47)

zinc (II) by glycine in seawater. This was done in artificial seawater at an ionic strength of

0.7M maintained using sodium perchlorate and also in actual seawater from the open

pacific at the low levels of 10-6M zinc concentration. They found results that were valid

only for seawater containing glycine as the only organic ligand, or another ligand with

similar stability constants. They found that only for 2 x 10-4M do zinc - glycine complexes

begin to contribute to the speciation of zinc to the extent of two percent.

The dependence of reduced sulphur compounds on the concentration of oxygen and

organic matter, temperature and salinity in the water column of a small sea lake, Lake

Rogoznica, were analyzed by Ciglenecki

et al

.

86 Cathodic stripping voltammetry was used

to detect and quantify sulphur species. The lake was found to be rich in sulphur (up to 900

mM), especially elemental sulphur and sulphide. Anoxic water was found to contain

mainly sulphide (760 mM) while in the oxic layer only trace amounts of elemental sulphur

were found (4-30 mM). They also found a relatively high concentration (140 mM) of

elemental sulphur in the anoxic water too, which they ascribed to the presence of

polysulphide. The content and speciation of sulphur compounds in samples change with

time due to biotic and abiotic processes. They therefore measured all the samples fresh,

immediately after sampling. They also investigated the influence of different preserving

agents (formaldehyde, hydroxylamine and ascorbic acid) on electrochemical measurements

and the concentration of sulphur. The best results were obtained with formaldehyde, as the total concentration of sulphur remained unchanged in the sample within 7 days, while the

(48)

The oxidative electrochemistry of AI-catechol (catechol = 1,2-benzenediol) and AI-DASA

(DASA = 1,2-dihydroxyanthraquinone-3-sulphuric acid) complexes in aqueous solution

was studied by Downard et ai.87 using cyclic voltammetry and steady-state voltammetry at

the rotating disk electrode. Using speciation calculations they were able to determine the

nature and concentration of the metal complexes. Electrochemical response(s) could be

identified with particular species. For the AI-Catechol system, it was found that the

complexes [AI(Cat)3t, [AI(Cat)2L and [AI(Cat)2]OHf were electroactive, each with

EP(a)

=

0.27V versus saturated calomel electrode. The primary anodic processes for

[AI(DASA)t, [AI(DASA)2,t and [AI(DASA)3t are irreversible two-electron oxidation

of one coordinated ligand at EP(a) values 0.93, 0.92 and 0.80 V versus SCE, respectively.

They also found that the initial (reduced) forms of the catechol and DASA complexes were

inert on the experimental time scale and after oxidation, the ligand disassociated from the

metal centre and when the experimental time scale was sufficiently long, further oxidation

of the resultant lowered the stoichiometry complex observed for [AI(DASAht.

Gardiner" investigated the chemistry of cadmium in synthetic and natural waters using a

cadmium specific ion electrode. He found that cadmium complexed with the natural

inorganic complexes in natural waters to give the species CdOW, CdC03,

cac

r

CdS04

and Cd-humic complex with the following values of stability constants: log KCdOH+=9.06,

log K CdCO;=4.02, log KcdC/ =48 and log KCdS04 =220. A substantial proportion of the

total cadmium in natural water was found to exist in free form with humic substances

(49)

Sobey et al.89 determined an optimization of the differential pulse cathodic stripping

voltammetry (DPCSV) method and then used it to determine trace selenium. They

studied the influence of the different electrochemical parameters as well as of the nature of

the supporting electrolyte. In their optimized electrochemical conditions, H2S04 (0.1M)

gave the lower Se(IV) detection limit (25 ngl'). They carried out a study of the

interference brought about by natural water salinity on Se(IV) detected by differential

cathodic stripping voltammetry (DPCSV) and observed no disturbance. The presence of

fulvic acids was found to hinder Se(IV) measurements and the detection limit was found to

reach 750 ngl" in the presence of fulvic acids at a 5 ngl' concentration. They used the

DPCSV method to develop a selenium speciation scheme based on different

physico-chemical pre-treatments of samples followed by the determination of Se(IV). The scheme

was applied, with satisfactory results to selenium speciation of synthetic and natural waters

spiked with the selenium species, Se (IV), Se (VI) and selenomethionine with satisfactory

results.

Guerin et al.9{)have developed an anion exchange HPLC-ICP-MS procedure that allows

the simultaneous multi-elemental speciation analysis of arsenic, selenium, antimony and

tellurium. Four arseruc species (As-III, AS- V, monomethylarsonic acid and

dimethylarsonic acid), two selenium species (Se-IV and Se-IV) may be determined in a

(50)

Ugo et al.91 have developed a glassy carbon electrodes modified with coatings of

poly-]-methyl-3- (Pyrrol-l-ylmethyl) pyridinium], poly-MPP by electrochemical oxidation of

suitable monomer in acetonitrile solutions. They exploited the anion exchange properties

of the coated electrode in aqueous solutions for pre-concentrating and detecting the

anionic complex HgCIl-, which is the prevailing inorganic Hg2+ species in seawater and

other chloride media. They calculated the partition coefficients for the ion-exchange

equilibrium involved from voltammetric data and compared them with those obtained at

electrodes coated with Tosflex, a perfluorinated anion exchanger, and poly (vinylpyridine).

They also studied the selectivity of the poly-MPP coated electrodes towards the rejection

of copper interference. The optimization of experimental conditions allowed them to

develop a differential pulse voltammetric method for the determination of sub-micromolar

mercury concentrations with a detection limit of O.lnM. They applied the method to the

analysis of mercury in the pore-waters of tidal sediments of the lagoon of Venice (Italy),

and determined Hg2+ concentration values in the 70-80 nM range.

Different voltammetric methods are promising in speciation studies and utilisation of such

I'

methods is preferred when the concentrations of the species allow them to be used.

Special attention has to be given to accurate measurements of the potential, as this is a

critical parameter for the results. The most convenient method of investigation is to find

the shift of the half-wave potential or the peak potential when the actual solution is

compared with a solution without any complex-forming species. Knut Schrrbder'" has

introduced anodic amalgam voltammetry with in situ preparation of amalgams [hanging

Figure

Table Al3F-Functions for hydroxo complexes of cadmium
Fig. 4.2 ; Plots of F-functions Vs concentrationlake water medium (data from DPASV)
Table 4.9;The % distribution of cadmium bicarbonato species at
Table 4.10;
+7

References

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