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 stabilityIII
II
IIII
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
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
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.
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.
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
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.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
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
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
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
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---69Table 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
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
Table 4.15 The % distribution of cadmium chloro species at varying
[Cn
inKN03 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 hangingdrop 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
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
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
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
[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
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
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
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,
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;
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.
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
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.
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
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
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
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
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
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Figure 1.1
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a
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
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.
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
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 tothe 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
andcoe
r.
They also foundthat 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
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 sedimenttests 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 concentrationwere 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
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
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 ofcadmium were of major significance only at high pH values.
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) carbonatocomplexes were identified with (CU(C03)23- and(Cu(C03
D
being the major species. Fourchloro 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
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(C03h2-
(M=
metal) complexes of similar stability whereas Cd(11) and Zn(11) formed only MC03
°
ofmuch lower stability.
In their polarographic study of complexes formed by Cu, Cd, Pb and Zn with formate ion,
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.).
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
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.
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 andcadmium 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 samplesfrom 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 farthe 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
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
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.
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 usedto 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
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
CdS04and 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
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
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