ABSTRACT
The kinetics of the redox reactions of 2,6-dichlorophenol indophenol (DCPIP) by metabisulphite ions (S2O52-), sulphite ions (SO32-), hydrazine (N2H4) and hydrogen peroxide (H2O2) at 28 ± 1 °C was studied at λmax of 600 nm in aqueous medium under pseudo-first order conditions at I = 0.2 mol dm-3 (NaCl). The stoichiometry of 1:1 was obtained for all the reactions except DCPIP – S2O52- which was 2:1. The reactions were first order in [S2O52-], [N2H4], [H2O2] and zero order with respect to [SO32-]. The reactions conform to the following rate equations:
d[DCPIP]= k2 [DCPIP] [S2O52-]
dt
where k2 = 16.24 ± 0.03 dm3 mol-1 s-1
d[DCPIP]= k1[DCPIP]
dt
where k1 = 1.79 ± 0.02 dm3 mol-1 s-1
d[DCPIP]= k2 [DCPIP] [N2H4]
dt
where k2 = 0.13 ± 0.02 dm3 mol-1 s-1
d[DCPIP]= k2[DCPIP] [H2O2]
dt
where k2 = 3.01 ± 0.02 dm3 mol-1 s-1. The rates of reactions displayed negative salt effect for DCPIP – S2O52- reaction, positive salt effect for DCPIP – N2H4 reaction and zero salt effect for DCPIP – SO32- and DCPIP – H2O2 reactions. Added cations catalysed the reaction of DCPIP – S2O52- but had no significant effect in the reactions of DCPIP – SO32-, DCPIP – N2H4 and DCPIP – H2O2. While added anions had negligible effect for all the reactions except DCPIP – N2H4 reaction. There was no evidence of formation of intermediate complex of significant stability for any of the reactions.
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Free radicals were not detected in all the reactions. Based on the results obtained in these studies, outer-sphere mechanism was proposed for the reactions of DCPIP with S2O52-, SO32- , H2O2 and inner-sphere mechanism for the reaction of DCPIP with N2H4.
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TABLE OF CONTENTS
Cover page i
Fly leaf ii
Title page iii
Declaration iv
Certification v
Dedication vi
Acknowledgements vii
Table of Contents viii
List of Tables xi
List of Figures xiii
Abstract vxi
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Statement of the Research Problem 3
1.2 Justification 3
1.3 Aim and Objectives 3
CHAPTER TWO
2.0 LITERATURE REVIEW 5
2.1 Reactions of 2,6-Dichlorophenol Indophenol 5
2.2 Reactions of Metabisulphite Ions 6
2.3 Reactions of Sulphite Ions 7
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2.4 Reactions of Hydrazine 8
2.5 Reactions of Hydrogen Peroxide 9
CHAPTER THREEE
3.0 MATERIALS AND METHODS 10
3.1 Materials 10
3.1.1 Preparation of stock 2,6-dichlorophenol indophenol solution 10
3.1.2 Preparation of 0.4 mol dm-3 sodium metabisulphite solution 10
3.1.3 Preparation of 0.2 mol dm-3 sodium sulphite solution 10
3.1.4 Preparation of 0.1 mol dm-3 hydrazine hydrate solution 11
3.1.5 Preparation of 0.1 mol dm-3 hydrogen peroxide solution 11
3.1.6 Preparation of 2.0 mol dm-3 sodium chloride solution 11
3.1.7 Preparation of salts solutions 11
3.2 Methods 11
3.2.1 Stoichiometric studies 11
3.2.2 Kinetic measurements 12
3.2.3 Effect of ionic strength and dielectric constant of reaction medium
on the reaction rate 12
3.2.4 Effect of added ions on the reaction rate 13
3.2.5 Test for intermediate complex 13
3.2.6 Free radical test 13
3.2.7 Products analysis 13
CHAPTER FOUR
4.0 RESULTS 15
4.1 Stoichiometry 15
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4.2 Determination of Order of the Reactions with Respect to the Reactants 19
4.3 The Effect of Ionic Strength of the Reaction Medium on the Reaction Rate 32
4.4 The Effect of the Changes in Dielectric Constant of the Reaction Medium on
Reaction Rate 35
4.5 The Effect of Added Ions on the Reaction Rate 35
4.6 Test for Intermediate Complex 58
4.6.1 Michaelis-Menten plot 58
4.6.2 Test for free radicals 58
4.6.3 Spectrophotometric test 58
4.7 Products Analysis 66
CHAPTER FIVE
5.0 Discussions 67
5.1 2,6-Dichlorophenol Indophenol – Metabisulphite System 67
5.2 2,6-Dichlorophenol Indophenol – Sulphite System 71
5.3 2,6-Dichlorophenol Indophenol – Hydrazine System 73
5.4 2,6-Dichlorophenol Indophenol – Hydrogen Peroxide System 77
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 81
6. 1 Summary and Conclusion 81
6.2 Recommendation 82
REFERENCES 83
CHAPTER ONE
1.0 INTRODUCTION
Inorganic chemistry is the chemistry of all elements in the periodic table and the compounds they form based on the concept of structure, bonding and reactivity (Cox, 2004). Areas of research in inorganic chemistry includes organometallic chemistry, synthetic inorganic chemistry, cluster chemistry, bioinorganic chemistry, industrial chemistry, kinetic and mechanistic inorganic chemistry (Miessler and Tarr, 2003).
In the last few decades, studies of electron transfer reaction in inorganic chemistry have developed tremendously both theoretically and experimentally as witnessed by large number of research articles (Gennady et al., 2013). In chemical system, mechanism of electron transfer gives kinetic explanation of elementary steps by which the overall chemical reaction change occurs (Espenson, 2002). The mechanisms of such reactions have been generally grouped as either the inner-sphere mechanism; that involve formation of bridging ligand between the reacting species centre prior to electron transfer or the outer-sphere mechanism; that involve direct transfer of electron between reacting species without change in the coordination sphere of the species (Jagannadham, 2012).
2,6-Dichlorophenol indophenol (DCPIP) with a molecular formular of C12H7Cl2NO2 is a quinone imine dye that can be synthesized as a dark green powder, stable, odourless and freely soluble in water (Cabello et al., 2009). Several kinetic process for its oxidation have been used for the determination of chemical reductants in food, as biological sample for making microscopic object more clearly visible than they would be when unstained and in pharmaceutical industries because of it chemical stability, systemic deliverability and membrane permeability (Cabello et al., 2009).
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Metabisulphite ion (S2O52-) is an inorganic compound with a wide range of application. It is a good reducing agent used as antioxidant, antimicrobial agent in variety of foods and drugs, antichloride and as sulphonating agent (Quanxi et al., 2015; Ivana et al., 2011), it is also used in wine and beer making to inhibit the growth of wild yeasts, bacteria and fungi (Miline, 2005).
Sulphite ion (SO32-) is an inorganic compound that has a powerful reducing ability. Its sulphur atom is double-bonded to one oxygen atom and single-bonded to two other oxygen atoms that carries a formal charge of -1, together accounting for the -2 charge on the ion. It has a resonance equivalent structure due to the resonance hybrid involving pπ-pπ S-O bonding (Catherine and Alan, 2008). It is used in pharmaceutical industries, paper industries, textile industries, as oxygen scavenger in water treatment and also as preservative additive in food (Hassan and Neil, 2012).
Hydrogen peroxide can act both as oxidising and reducing agent. But, it is only in the presence of strong oxidising agents it act as reducing agent. It is the simplest peroxide that contain O-O single bond (Goor et al., 2007). It is used as a bleaching agent, disinfectant, catalyst, in paper and pulp industries and water pollution control as well as biological and medicinal applications (Onu et al., 2015; Hill, 2001).
Hydrazine, a good reducing agent, is a colourless liquid at room temperature. The compound and it derivatives have been used in agriculture as pesticide, deoxygenation of boiler water, chemical industries, pharmaceutical drugs, polymerization catalyst, fuel rocket, explosive and as mono propellants in gas turbine generators (Jean-Pierre and Paul, 2002).
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1.1 Statement of the Research Problem
Dyes are coloured and ionizing compounds which have affinity toward the substrate to which it is being applied (Booth, 2000). DCPIP is used for the determination of ascorbic acid in food, as biological stain for the study of bacteria and related microorganism and in pharmaceutical industry (Cabello et al., 2009). It also serves as good medium for disruption of cell proliferation, tissue angiogenesis and inflammation by directly or indirectly targeting specific cellular factors when delivered enclosed in specific nanoparticles (Fadee et al., 2010). Despite these uses, the direct assessment of the redox activities of the dye with respect to the medium involved in control of cancer cells and proliferation is lacking (Wondrak, 2009; Fry et al., 2005; Kumar and Acharya, 1999). Available literature collaborates the fact that not much have been done on the kinetics and mechanism of its redox reaction.
1.2 Justification
The kinetic data that would be generated from these studies will complement much needed kinetic information for quantitative comparison of various electron transfer reactions involving the dye, its uses as well as its handling. Furthermore, the kinetic data generated and the proposed mechanisms would contribute to conceptual development of mechanism of redox reactions of the dye.
1.3 Aim and Objectives
The research is aimed at studying the kinetics of the redox reactions of 2,6-dichlorophenol indophenol with S2O52-, SO32-, H2O2 and N2H4 in aqueous medium and propose mechanisms for these reactions.
The objectives of the study are to:
i. determine the stoichiometry and order of the redox reactions,
ii. establish the rate law,
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iii. investigate the effect of changes in ionic strength, added ions and dielectric constant on the reaction rates,
iv. test for intermediate complex formation, free radicals and
v. analyse the reaction products.
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