ABSTRACT
The kinetics of the redox reactions of malachite green, MG+ with these oxy-anions: Bromate (BrO3‾), Metabisulphite (S2O5²⁻), Sulphite (SO₃²⁻) and Chlorite (ClO₂‾) in aqueous acidic medium were studied under the following experimental conditions:
i. [H⁺] = 1.0 10⁻² mol dm⁻³ (H₂SO₄), I = 0.5 mol dm⁻³ (Na₂SO4) and T = 28 ± 1°C for MG+ – BrO₃‾ system.
ii. [H+] = 5.0 10⁻³ mol dm⁻³ (HCl), I = 0.5 mol dm⁻³ (NaCl) and T = 28 ± 1°C for
MG+ – S₂O₅²⁻ system
iii. [H+] = 5.0 10⁻³ mol dm⁻³ (HCl), I = 0.1 mol dm⁻³ (NaCl) and T = 27 ± 1°C for
MG+ – SO₃²⁻ system
iv. [H⁺] = 1.0 10⁻² mol dm⁻³ (H₂SO₄), I = 0.3 mol dm⁻³ (Na₂SO4) and T = 26 ± 1°C for MG+ – ClO2‾ system
The stoichiometries of the reactions were found to be 2:1 for MG+ – BrO3‾ system, 1:1 for MG+ – S2O52⁻, MG+ – SO3²⁻ and MG+ – ClO₂‾ systems. The order of the reaction was one with respect to both oxidant and reductant in all the systems. Increase in reaction rate with increasing acid concentration [H+] was observed for MG+ – BrO3‾ and MG+ – ClO₂‾ systems while a converse effect was observed for MG+ – S2O52⁻ system and the rate of reaction for MG+ – SO3²⁻ system was independent on [H+]. The influence of [H+] on the rates of reactions showed two parallel pathways for S2O52- and ClO2‾ systems, one pathway for BrO3‾ system and no pathway was available for the SO3²⁻ system.
The reactions conformed to the following rate equations:
(a [H+]2)[MG+][BrO3‾]
where a = 3.32 10-1 dm9 mol-3 s-1
(b + c [H+]-1)[MG+][S2O52-]
where b = 9.80 10-2 dm3 mol-1 s-1 and c = 1.38 10-3 s-1
[H+]0 [MG+][SO32-] and
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(d + e [H+])[MG+][ClO2¯]
where d = 0.11 dm3 mol-1 s-1 and e = 8.40 10-2 s-1
Increase in ionic strength increased the rate of reaction for all the systems except for the MG+-ClO₂‾ system where the reaction rate was not affected by changes in ionic strength of reaction medium. Added ions inhibited the rate of reactions for BrO3‾ and S2O52- systems and had no effect on the rate of reaction for the ClO2‾ system. The rate of reaction for the SO32- system was catalysed by added anions. Polymerisation test suggested the absence of free radicals in the ClO2‾ system only. Spectroscopic investigations and Michaelis-Menten plot showed no evidence of intermediate complex formation in all the reactions except for MG+ – ClO2‾ an evidence of intermediate complex formation was noticed by a shift in λmax from 620-600 nm. Outer-sphere mechanism is proposed for all the systems except MG+ – ClO2‾ system where the reaction is believed to proceed via inner-sphere mechanism.
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TABLE OF CONTENTS
Cover page i
Fly Leaf ii
Title page iii
Declaration iv
Certification v
Dedication vi
Acknowledgement vii
Abstract viii
Table of Contents x
List of Figures xiv
List of Tables xviii
Abbreviations xx
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Electron Transfer Reactions 1
1.2 Types of Electron Transfer Reactions 2
1.3 Points of Mechanistic Interest in Electron Transfer Reactions 3
1.4 Mechanisms of Electron Transfer Reactions 4
1.4.1 Inner-sphere mechanism 4
1.4.2 Outer-sphere mechanism 4
1.5 Justification of the Research Work 5
1.6 Aim and Objectives of the Research Work 5
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CHAPTER TWO
2.0 LITERATURE REVIEW 6
2.1 Uses and Reactions of Malachite Green 6
2.2 Reactions of Bromate Ion 8
2.3 Reactions of Metabisulphite Ion 9
2.4 Reactions of Sulphite Ion 11
2.5 Reactions of Chlorite Ion 12
CHAPTER THREE
3.0 Materials and Methods 15
3.1 Materials 15
3.1.1 Preparation of malachite green solution 15
3.1.2 Preparation of potassium bromate solution 16
3.1.3 Preparation of sodium metabisulphite solution 16
3.1.4 Preparation of sodium sulphite solution 16
3.1.5 Preparation of sodium chlorite solution 16
3.1.6 Preparation of standard sulphuric acid solution 16
3.1.7 Preparation of standard hydrochloric acid solution 16
3.1.8 Preparation of sodium carbonate solution 17
3.1.9 Preparation of sodium sulphate solution 17
3.1.10 Preparation of sodium chlorite solution 17
3.1.11 Preparation of other salt solutions 17
3.2 Methods 17
3.2.1 Stoichiometric study 17
3.2.2 Kinetic measurements 18
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Page
3.2.3 Effect of [H+] on the reaction rate 19
3.2.4 Effect of ionic strength and dielectric constant of the reaction
medium on the reaction rate 20
3.2.5 Effect of added ions on the reaction rate 20
3.2.6 Test for intermediate complex formation 20
3.2.7 Test for the presence of free radicals 21
3.2.8 Product analysis 21
CHAPTER FOUR
4.0 RESULTS 22
4.1 Stoichiometry 22
4.2 Determination of Order of the Reactions with Respect to the Reactants 27
4.3 Effect of Hydrogen Ion Concentration on the Rates of the Reactions 40
4.4 Effect of Ionic Strength of the Reaction Medium on the Reaction Rate 47
4.5 Effect of Changes in Dielectric Constant of the Reaction Medium on the
Reaction Rate 47
4.6 Effect of Added Ions on the Reaction Rate 55
4.7 Test for Intermediate Complex Formation 55
4.7.1 Test for the presence of free radicals 55
4.7.2 Michaelis-Menten plot 55
4.7.3 Spectrophotometric test 78
4.8.0 Products analysis 78
CHAPTER FIVE
5.0 DISCUSSION 83
5.1 Malachite Green – Bromate Ion System 83
5.2 Malachite Green – Metabisulphite Ion System 86
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5.3 Malachite Green – Sulphite Ion System 89
5.4 Malachite Green – Chlorite Ion System 91
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 94
6.0 Summary and Conclusion 94
6.1 Recommendation 95
REFERENCES 96
CHAPTER ONE
1.0 INTRODUCTION
Inorganic Chemistry is a branch of Chemistry that deals with the behaviour of inorganic materials which is fundamental to the study of other areas of chemistry. It involves interpreting, correlating and predicting the properties and structures of these materials. Areas of study include; co-ordination chemistry, organometallic chemistry, acid-base chemistry, study of non-aqueous solvents and chemistry of elements other than carbon (House, 2008).
Kinetics of a reaction helps in determining the rate at which a reaction mixture approaches the state of equilibrium, yielding a certain reaction product (Asperger, 2003). A reaction mechanism is a detailed stepwise process involving molecules, atoms, radicals or ions that occur consecutively and culminates in the overall reaction observed (Cooke, 1979).
The knowledge of kinetics and mechanism of a reaction is fundamental to the adequate understanding of a wide area of science and technology. This ranges from the interplay between metabolic processes, where the intricate control of the rate of enzymatic processes is vital to the overall wellbeing of biological systems through the industrial synthesis of both fine and heavy chemicals. Hence, the understanding of the inherent processes and interactions is fundamental for the optimization of reaction conditions (Arnaut et al., 2006; Hamza, 2011).
1.1 Electron Transfer Reactions
Electron Transfer Reactions (ETR), are reactions of the oxidative-reductive type in which an electron is transferred from one specie (reductant) to another (oxidant). A loss of electron is accompanied by an increase in oxidation state to give an oxidized specie while a
2
gain of electron is accompanied by a decrease in oxidation state, giving rise to a reduced specie (Asperger, 2003).
Electron transfer reactions are important chemical processes as they play central roles in many biological, physical and chemical reactions (Finnegan et al., 2002). Reactions of metal ion complexes often involve electron transfer or ligand substitution or both.
The stability and reactivity of an ion in any oxidation state is greatly influenced by the presence of ligands in this reaction. A particular oxidation state may be said to be stable only when its redox reactions involve an unfavourable free energy change or the activation energies for the intramolecular electron transfer processes are too large (Sutin, 1968; Taube, 1968; Chaffee and Edwards, 1970; Bugaje, 2006; Abdulsalam, 2015). When these two factors are favourable, a redox process is spontaneous.
1.2 Types of Electron Transfer Reactions
a) Homonuclear (Self) Exchange Reactions
These are reactions which occur between two identical centres existing in different oxidation states. The reactants and products are the same and hence have the same concentrations. The free energy change for such a reaction is mainly due to mixing and is approximately zero. There is no net chemical change and Keq= 1 (Sharpe 1982; Anweting, 2012). Examples of these reactions are given in Equations (1.1) and (1.2);
V2+ + *V3+ V3+ + *V2+ (1.1)
*Fe(CN)62+ + Fe(CN)63+ *Fe(CN)63+ + Fe(CN)62+ (1.2)
where * is an isotopic label
b) Heteronuclear (Cross) Exchange Reactions
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This involves the transfer of electrons between different metal ion centres and hence products of reaction are chemically distinct from the reactants (Cooke, 1979). The net change in free energy in most cases is less than zero and Keq ≠ 1.
Examples of these reactions are given in Equations (1.3) and (1.4):
2Fe2+ + Tl3+ 2Fe3+ + Tl+ (1.3)
Co(NH3)5X2+ + Cr2+ + 5H+ CrX2+ + Co2+ + 5NH4+ (1.4)
Redox reactions can also be classified as complementary and non-complementary reactions
In complementary redox reactions, the oxidant and reductant both have equal changes in oxidation state and stoichiometry is always 1:1, whereas a non-complementary redox reaction results to an unequal change in oxidation state with an unbalanced stoichiometry.
Examples of such reactions are given below:
*Co(NH3)63+ + Co(NH3)62+ *Co(NH3)62+ + Co(NH3)63+ (1.5)
Sn(II) + Tl(III) Sn(IV) + Tl(I) (complementary) (1.6)
2V2+ + Br2 2V3+ + 2Br‾ (non-complementary) (1.7)
where * is an isotopic label
1.3 Points of Mechanistic Interest in Electron Transfer Reactions
The following points are of great importance in the study of electron transfer (redox) reactions:
a) The stoichiometry of the reaction and the composition of the activated complex.
b) Whether the reaction is accompanied by transfer of electrons, atoms or other species.
c) The number of electrons transferred in each single step for a multi-step reaction.
d) The closeness of approach of reactants in the activated complex prior to electron transfer.
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e) The comparison of the rate of electron transfer with rate of ligand substitution.
f) The importance of acid-base catalysis when observed. Could it be rationalized in terms of reactants, products or transition state.
g) Identification and isolation of reaction products and intermediates if any.
1.4 Mechanisms of Electron Transfer Reactions
1.4.1 Inner-sphere mechanism
This type of mechanism involves penetration into the inner-coordination sphere of reactants with the formation of a bridged activated intermediate (Candlin et al., 1968; Anweting, 2012). The mechanism normally follows three distinct steps; substitution to form a bridge between oxidant and reductant, actual electron transfer and separation of the products often with transfer of the bridging ligand (Taube et al., 1953; Abdulsalam, 2015). A typical inner-sphere reaction between Co(III) and Cr(II) is shown below:
[Co(NH3)5Cl]2+ + [Cr(H2O)6]2+ + 5H2O
[Co(H2O)6]2+ + [Cr(H2O)5Cl]2+ + 5NH3 (1.8)
1.4.2 Outer-sphere mechanism
In this mechanism, the coordination shells of the complexes or metal ion remains intact, during the course of electron transfer. Outer-sphere electron transfer is generally enthalpically less favorable than inner-sphere electron transfer because the interaction through space between the redox centers in outer-sphere electron transfer is weaker than the interaction through the chemical bridge present in the inner-sphere mechanism. By the same token, outer-sphere electron transfer is usually entropically more favorable than inner-sphere electron transfer as the two sites involved do not have to go through the ordering processes associated with the formation of a bridge (Mcnaught and Wilkinson, 1997; Abdulsalam, 2015). Such a
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mechanism is established when rapid electron transfer occurs between two substitution-inert complexes as shown in the examples below:
[Fe(CN)6] 4- + [Mo(CN)8] 3- [Fe(CN)6] 3- + [Mo(CN)8]4- (1.9)
[Fe(CN)6] 4- + [ IrCl6] 2- [ Fe(CN)6] 3- + [IrCl6] 3- (1.10)
1.5 Justification of the Research
Malachite green (MG⁺) dye has been widely used for the dyeing of leather, wool, jute and silk, in distilleries, as a fungicide and antiseptic in aquaculture industry to control fish parasites and disease (Zhang et al., 2008). In spite of the outlined uses of this dye, literature reveals no report on the kinetics and mechanism of its redox reactions with these oxyanions; BrO3⁻, S2O52-, SO32- and ClO2⁻. Data generated and the subsequent mechanisms proposed will assist in the better understanding and more efficient utilization of the reactions of this dye and also complement the much needed kinetic information which may throw more light on the staining properties, dye fastness and the redox properties of this dye.
1.6 Aim and Objectives of the Research
The aim of this research work is to study the kinetics of the redox reactions of malachite green with BrO3⁻, S2O52-, SO32- and ClO2⁻ in acidic medium and propose mechanisms for these reactions. This aim would be achieved through the following objectives:
a. determining the stoichiometry of the redox reaction,
b. estimating the rate constants as well as obtain the order of the reaction,
c. monitoring the effect of changes in acid concentration, ionic strength, dielectric constant and added ions on the reaction rates
d. testing for intermediate complex formation and presence of free radicals
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e. qualitative analysis of the products of reactions and
f. proposal of plausible mechanisms for the reactions.
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