ABSTRACT
The kinetics and mechanisms of the redox reaction of crystal violet (hereafter CV+) was studied with some oxyanions (S2O52-, BrO3-, IO4- and ClO-) in aqueous acidic medium. The stoichiometries of the reactions were found to be 1:1 for CV+- S2O52-, 2:3 for CV+- BrO3-, and 1:2 for CV+- IO4- and CV+- ClO- systems. The order of the reactions was one with respect to both oxidant and reductant in CV+- S2O52-, -BrO3- and -ClO- reactions respectively. For the CV+- IO4- system, the order of the reaction was one with respect to the [CV+] and zero with respect to [IO4-]. Studies on the influence of [H+] on the rates of reactions point to two parallel pathways for S2O52-, BrO3-, IO4- reactions and one reaction pathway for ClO- reaction. The reaction conformed to the following rate equations:
(a + b [H+]-1)[CV+][S2O52-]
a = 2.27 dm3 mol-1 s-1 and b = 0.86 s-1
(c + d [H+] 2)[CV+][BrO3-]
c = 9.22 dm3 mol-1 s-1 and d = 1.16 dm9 mol-3 s-1
(e + f [H+])[CV+]
e = 1.49 s-1 and f = 1.8 x 10-3 dm3 mol-1 s-1
(g [H+])[CV+][ClO-]
g = 6.16 dm6 mol-2 s-1
the rate of the reaction displayed positive salt effect for CV+- BrO3- and CV+- IO4- reactions and negative salt effect was observed for CV+- S2O52- reaction. In the case of CV+- ClO- reaction, increase in ionic strength has no effect on the rate of reaction. Added anions inhibited the rate of reactions of CV+- S2O52- and CV+- BrO3- but increased that of CV+- ClO-, while added anions has no effect on the rate of reaction for CV+- IO4-. Added cations had no effect on the rate of reactions of all the systems except for CV+- BrO3- where the rate of the reaction decreased with increase in cation concentration. Spectroscopic investigations and Michaelis-Menten plot showed no evidence of intermediate complex formation in all the reactions except for CV+- IO4- where evidence of intermediate complex was noticed by shift in λmax from 585 to 620 nm. Outersphere mechanism was proposed for all the systems except CV+- IO4- system where the reaction was believed to proceed via innersphere mechanism.
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TABLE OF CONTENTS
Contents Page
Cover page i
Title page ii
Declaration iii
Certification iv
Acknowledgement v
Dedication vi
Abstract vii
Table of Content viii
List of Tables xii
List of Figures xiii
Abbreviations xvi
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Kinetic studies 2
1.2 Electron transfer reactions 2
1.3 Oxidation-reduction in inorganic reactions 3
1.4 Outersphere mechanism 3
1.4.1 Consideration for outersphere mechanism 4
1.5 Innersphere mechanism 4
1.5.1 Consideration for innersphere mechanisms 5
1.6 Probable ways of electron transfer reactions 5
1.6.1 Complimentary reactions 5
1.6.2 Non-complimentary reactions 6
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1.7 Justification 6
1.8 Aim 8
1.9 Objectives 8
CHAPTER TWO
2.0 LITERATURE REVIEW 9
2.1 Reactions of crystal violet 9
2.2 Reactions of metabisulphite ion 11
2.3 Reactions of periodate ion 13
2.4 Reactions of bromate ion 16
2.5 Reactions of hypochlorite ion 18
CHAPTER THREE
3.0 MATERIALS AND METHODS 20
3.1 Materials 20
3.1.1 Crystal violet 20
3.1.2 Preparation of sodium metabisulphite solution 21
3.1.3 Preparation of potassium bromate solution 21
3.1.4 Preparation of sodium periodate solution 21
3.1.5 Preparation of sodium hypochlorite solution 21
3.1.6 Preparation of standard solution of hydrochloric acid 21
3.1.7 Preparation of 0.1 mol dm-3 Na2 B4O7. 10 H2O (Borax) 22
3.1.8 Preparation of standard solution of perchloric acid 22
3.1.9 Preparation of standard sodium perchlorate solution 22
3.1.10 Preparation of standard sodium chloride solution 22
3.1.11 Preparation of sodium carbonate solution 22
3.1.12 Preparation of salt solutions 23
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3.2 Methods 23
3.2.1 Stoichiometric studies 23
3.2.2 Kinetic measurements 23
3.2.3 Effect of [H+] on the reaction rate 25
3.2.4 Effect of ionic strength and dielectric constant of the reaction
medium on the reaction rate 25
3.2.5 Effect of added ions on the reaction rate 26
3.2.6 Test for intermediate complex 26
3.2.7 Test for free radicals 26
3.2.8 Product analyses 27
CHAPTER FOUR
4.0 RESULTS 28
4.1 Stoichiometry 28
4.2 Determination of order of the reactions with respect to the reactants 29
4.3 Effect of hydrogen ion concentration on the rates of the reactions. 49
4.4 The effect of ionic strength of the reaction medium on the reaction rate 58
4.5 The effect of changes in dielectric constant of the reaction medium on the
reaction rate 62
4.6 Effect of added ions on the reaction rate 62
4.7 Test for intermediate complex 79
4.7.1 Test for free radicals 79
4.7.2 Michaelis-Menten plot 79
4.7.3 Spectrophotometric test 79
4.8 Products analyses 89
CHAPTER FIVE
5.0 DISCUSSION 90
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5.1 Crystal violet- metabisulphite ion system 90
5.2 Crystal violet-bromate ion system. 93
5.3 Crystal violet- hypochlorite ion system 97
5.4 Crystal violet-periodate ion system 99
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 104
6.1 Summary 104
6.2 Conclusion 105
6.3 Recommendation 106
References 107
CHAPTER ONE
1.0 INTRODUCTION
Inorganic chemistry is concerned with the properties and behaviour of inorganic compounds, which include metals, minerals and organometallic compounds. Areas of research interest in inorganic chemistry include organometallic chemistry with bias toward catalysis, coordination chemistry and the biochemical role of metals (Purcell and Kotz, 1977). In all these areas of interest, the focus has been on reaction kinetics especially electron transfer or redox reactions. Knowledge of these reactions constitutes an inevitable pre-requisite to the understanding, development and eventual effective control of a wide area of science and technology (Iyun, 1982).
Reactions of metal ion complexes often involve ligand substitution or electron transfer or both. Electron transfer (ET) is one of many pathways by which redox reaction can occur, and this is thought to provide a low energy pathway for redox reaction. A typical example is the oxidation of both metal ions and non-metallic substrate by chromium(VI) ion (Beatle and Height, 1972).
In redox reactions, the stability and reactivity of an ion in any oxidation state is greatly influenced by the presence of ligands. 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). When these two factors are favourable, a redox process is spontaneous and it is often accompanied by changes in the oxidation state of at least two of the
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reactants. The investigation of the mechanism of a large number of these electron transfer reactions has been reported (Sutin 1962, Sykes, 1966; Wilkins, 1974, Burgess, 1978).
1.1 Kinetic studies
Chemical kinetics deals with the rates of chemical reactions and the factors affecting rates of reaction. Kinetics study is important in providing essential evidence as to the mechanisms of chemical processes. In chemical reactions, there are two basic questions that must be answered; does the reaction want to go? This has to do with chemical thermodynamics and if the reaction wants to go, how fast will it go? This has to do with chemical kinetics (Zuckerman, 1986).
1.2 Electron transfer reactions
Electron transfer reactions play a central role in physical, chemical and biological processes. Because of the ubiquity of electron transfer processes, the study of electron transfer reactions, perhaps more so than that of any other area of chemistry is characterized by a strong interplay of theory and experiment (Zuckerman, 1986), nonetheless the importance of electron transfer in transition metal redox chemistry has been recognized (Wilkinson, 1987).
The work of Taube (1967) in redox systems unequivocally demonstrated the transport of electron from reductant to oxidant. This discovery certainly added many important features in the syntheses of coordination complexes and organometallics. It is such a subject, which has manifestations in almost all walks of life. Oxidation-reduction reaction needs at least two reactants, one capable of gaining electrons (oxidant) and the other capable of losing electrons
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(reductant). Redox reactions are the basis for numerous biochemical pathways and cellular chemistry, biosynthesis, and regulation (Shapiro, 1972).
1.3 Oxidation-reduction in inorganic reactions
Oxidation-reduction reaction may involve one or more electron transfers. Depending upon the number of electrons transferred between oxidant and reductant, the reaction may proceed in one or more steps. Electron transfer reactions may occur by either of two mechanisms: outersphere mechanisms and innersphere mechanisms (Banerjee, 1993).
1.4 Outersphere mechanism
In this mechanism, the coordination shells of the complexes or metal ion remains intact, during the course of electron transfer. Outersphere electron transfer is generally enthalpically less favorable than innersphere electron transfer because the interaction through space between the redox centers in outersphere electron transfer is weaker than the interaction through the chemical bridge present in the innersphere mechanism. By the same token, outersphere electron transfer is usually entropically more favorable than innersphere 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). Such a mechanism is established when rapid electron transfer occurs between two substitution-inert complexes.
[Fe(CN)6] 4-+ [Mo(CN)8] 3- [Fe(CN)6] 3-+ [Mo(CN)8]4- (1.1)
[Fe(CN)6] 4- + [ IrCl6] 2- [ Fe(CN)6] 3- + [IrCl6] 3- (1.2)
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1.4.1 Considerations for outersphere mechanisms
The reactants in an outersphere mechanism must get close together for tunneling to occur, bond lengthening and shortening must occur and Franck-Condon principle must be obeyed such that electronic transitions (and electron transfer) occur on a far shorter time scale than molecular vibrations (nuclear motion). This means that electron transfer will only occur when the reactants are distorted to the appropriate geometry for the products, that is, this imposes an electronic barrier on the rate of electron transfer (Mcnaught and Wilkinson, 1997).
1.5 Innersphere mechanism
An innersphere mechanism is one in which the reactant and oxidant share a ligand transitorily in their inner or primary co-ordination spheres forming a bridged intermediate activated complex. The discoverer of the innersphere mechanism was Henry Taube (Taube, 1967), who was awarded the Nobel Prize in Chemistry in 1983 for these pioneering studies. A particularly historic finding is summarized below:
[Co(NH3)5Cl] 2++ Cr(H2O)6] 2+ + 5 H2O
[Co(H2O)6] 2+ + [Cr(H2O)5Cl]2+ + 5NH3 (1.3)
The electron being transferred across a bridging group. An example is given below
[CoCl (NH3)5]2+ + [Cr(H2O)6 ]2+
[Co(NH3)5(H2O)]2+ + [CrCl(H2O)5]2+ (1.4)
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1.5.1 Considerations for innersphere mechanisms
The innersphere mechanisms normally obey 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 and Meyers, 1953).
1.6 Probable ways of electron transfer reactions
Complementary and Non-complementary reactions are types of electron transfer reactions as depicted by Basolo and Pearson (1967) and Sharma et al., (1988).
.
1.6.1 Complementary reactions
The oxidant and reductant change their oxidation state by an equal number of units. These are termed as complementary electron transfer reactions (Malik et al., 1996). Complementary reaction can be explained in term of; one equivalent–one equivalent reaction, in which there occurs the transfer of one electron from one species to the other. These simple reactions serve as models for more complicated systems and their study has proved invaluable in developing and understanding of the electron transfer in solution (Taube, 1959).
Ce(III) + Co(III) Ce(IV)+ Co(II) (1.5)
Two-equivalent–two-equivalent reactions in which there occurs the transfer of two electrons from one species to the other (Taube, 1957; Harkness and Halpern, 1959):
U(IV) + Tl(III) U(VI) + Tl(I) (1.6)
Sn(II) + Hg(II) Sn(IV) + Hg(0) (1.7)
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A large number of complementary reactions have been explained by assuming the formation of bridged activated complexes between the oxidant and the reductant for the facile transfer of electron through the bridging ligand.
1.6.2 Non-complementary reactions
The oxidant and the reductant change their oxidation states by a different number of units. These are termed as non-complementary electron transfer reactions (Wiberg, 1965). Most of the non-complementary reactions proceed via elementary steps each involving one electron transfers. The most commonly observed kinetic scheme as illustrated by Wiberg, (1965) is shown below.
Cr(VI) + Fe(II) Cr(V) + Fe(III) (1.8)
1.7 Justification
Crystal violet or Gentian violet (also known as hexamethyl pararosaniline chloride) is a triphenylmethane dye that is useful in multiple areas of human endeavour. In medicine, crystal violet has antibacterial, antifungal and anthelmintic properties (Docampo and Moreno, 1990). In biology, it is used as a histological stain, particularly grams method for classifying bacteria (Hall et al., 1966). It is also used as an alternative to fluorescent stains, which made it popular as a means of avoiding UV-induced DNA destruction when performing DNA cloning in vitro. In forensics, gentian violet was used to develop fingerprints. It is also used as a tissue stain in the preparation of light microscopy sections (Henneman and Kohn, 1975).
In laboratory, solutions containing crystal violet and formalin are often used to simultaneously fix and stain cells grown in tissue culture to preserve them and make them
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easily visible, since most cells are colourless. It is also sometimes used as a cheap way to put identification markings on laboratory mice since many strains of lab mice are albino so the purple colour stays on their fur for several weeks (Henneman and Kohn, 1975). Industrially, it is used to dye paper and as a component of navy blue and black inks for printing, ball-point pens and ink-jet printers. It is also used to colourize diverse products such as fertilizers, anti-freezes, detergents, and leather jackets.
Therefore, potential human exposure to crystal violet could result from the consumption of treated fish and from working in the dye and aquaculture industries and also in the laboratories (Anderman and Clifton, 1993). Coloured dyes are not only carcinogenic, but also disturb life processes of living organisms in water. It is needed to remove these dyes before throwing them into receiving water bodies. Possible treatments such as chemical oxidation, electrochemical degradation and adsorption are used for this purpose (Whebi et al., 2010, Alamddine and Jamal, 2009, Rammel et al., 2011). In view of the above outlined important uses of this dye, hence there is need for further research in this area. Kinetics and mechanisms of the dye with oxyanions (ClO3-, HSO5-, Cr2O72-) have been studied (Mohammed et al., 2011; Kranti, 2011; Mohammed and Komolafe, 2010). The present work deals with the kinetics and mechanisms of the dye with oxyanions (BrO3-, IO4-, ClO- and S2O52-) in aqueous acidic medium.
The kinetics data generated and the subsequent mechanisms proposed will assist in the better understanding and more efficient utilization of the reactions of crystal violet and also complement the much needed kinetic information which may throw more light on the staining properties, dye fastness and the electron transfer properties of this dye with the aim of explaining and subsequently improving their 7hysic-chemical properties.
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1.8 Aim
The aim of this research work is to study the kinetics and mechanism of the redox reaction of crystal violet with oxidant BrO3-, IO4-, ClO- and S2O52- as oxyanions in acid medium.
1.9 Objectives
The above aim would be achieved through the following objectives:
a. determining the stoichiometry of the redox reaction,
b. estimating the rate constant as well as obtain the order of the reaction,
c. monitoring the effect of changes in acid concentration, ionic strength and added ions on the reaction rates,
d. testing for intermediate complex formation and free radicals and
e. deducing a plausible mechanism and assigning operative mechanism for the reaction.
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