Kinetics and Mechanisms of the Redox Reactions of Crystal Violet With Some Oxyanions in Aqueous Acidic Medium

Kinetics and Mechanisms of the Redox Reactions of Crystal Violet With Some Oxyanions in Aqueous Acidic Medium

Chapter One

Aim

This research work aims to study the kinetics and mechanism of the redox reaction of crystal violet with oxidant BrO ^–, IO ^–, ClO^– , and S O ^2- as oxyanions in an acid medium.

Objectives

The above aim would be achieved through the following objectives:

determining the stoichiometry of the redox reaction,
estimating the rate constant as well as obtain the order of the reaction,
monitoring the effect of changes in acid concentration, ionic strength and added ions on the reaction rates,
testing for intermediate complex formation and free radicals and
deducing a plausible mechanism and assigning operative mechanism for the

CHAPTER TWO

LITERATURE REVIEW

Reactions of Crystal Violet

Crystal violet (Gentian violet) with a molecular formular of C₂₅H₃₀N₃Cl is a triphenyl methane dye that is antimicrobial (Hall et al., 1966). It is used as a bacteriostatic agent in medical solutions (Bale, 1981), and to treat skin infections by Staphylococcus aureus (Saji et al., 1995).

Crystal violet is a cationic dye. Cationic or basic dyes are dyes in which the chromophore is on the cation (Parameswaran et al. 1974). It is used as a dye and also possesses antibacterial, antifungal and anthelmintic properties which make it useful in dentistry (Gorgas et al. 1901). The dye differs from other triphenylmethanes in that the amino groups are methylated or substituted. Crystal violet is readily soluble in water and highly stable, with the absorption maximum of _max 590 nm. It is very important in biological stains for the study of bacteria and related microorganisms. The dye is also used as a constituent of culture media, as indicator and for laboratory diagnosis of disease (Docampo and Moreno, 1990).

The kinetic studies of the reaction of crystal violet (CV⁺) and chlorate (ClO ^–) was carried out in aqueous acidic medium, at a temperature of 32 + 1^oC; I = 0.50 mol dm^-3 (NaCl), [H⁺] = 1.00 x 10^-2 mol dm^-3 (HCl) (Mohammed et al., 2011). In the stoichiometry, one mole of the dye was consumed by two moles of ClO ^–. The reaction was found to be first order in both the dye and the oxidant. The rate of redox reaction showed dependence on acid concentrations.

Rate law for the reaction has been proposed as:

The second order rate constant for the crystal violet –CIO ^– reaction was found to be 3.88 dm³ mol^-1s^-1. The rate of reaction displayed zero salt effect and was not affected by changes in dielectric constant of the reaction medium. Added anions and cations catalysed the reaction. Results of the Michaelis-Menten analysis gave no evidence of intermediate complex formation. Based on the results obtained experimentally, the outersphere mechanism was proposed for the reaction.

The kinetics and mechanism of the reaction of crystal violet by peroxomonosulphate (oxone) has been reported by Kranti (2011). The reaction was studied under pseudo first order condition at constant temperature of 25+ 0.1^oC. The stoichiometry was found to be one mole of crystal violet to two moles of oxone. The reaction was not affected by changes in the dielectric constant of the reaction medium and there was no evidence for the formation of free radicals in the mechanism of the reaction.

Reactions of metabisulphite ion

Metabisulphite ion or pyrosulphite ion is a good reducing agent whose compounds are used as preservative and antioxidant in food (EUFIC, 2007). Concentrated sodium metabisulphite can be used to remove tree stumps and it is a primary ingredient in campden tablets, used for wine and beer making to inhibit the growth of wild yeasts, bacteria and fungi (Miline, 2005). Some brands contain 98% sodium metabisulphite, and cause degradation of lignin in the stumps, facilitating its removal (OSHA, 2008).

In the study of the kinetics of reduction of rosaniline with sodium metabisulphite, one mole of rosaniline was consumed by one mole of the metabisulphite ion (Onu and Iyun, 2001). Second order rate constant was determined and the effect of acid on the rate of reaction showed two pathways, one which is acid dependent and the other which is acid independent.

CHAPTER THREE

MATERIALS AND METHODS

Materials

All solutions were prepared with distilled water except otherwise stated. Analytical grade reagents were used throughout this work without further purification. Hydrochloric acid was used to investigate the effect of hydrogen ion on the reaction for the crystal violet- metabisulphite, crystal violet-bromate and crystal violet-periodate systems, while perchloric acid was used for crystal violet-hypochlorite system. Sodium chloride was used to maintain a constant ionic strength for each run in the crystal violet- metabisulphite, crystal violet-bromate and crystal violet-periodate systems, while sodium perchlorate was used for crystal violet- hypochlorite system. Sodium metabisulphite, potassium bromate, sodium periodate and sodium hypochlorite were the redox reagents used.

CHAPTER FOUR

RESULTS

Stoichiometry

The results of the stoichiometric investigations indicate that one mole of crystal violet is consumed by one mole of metabisulphite ion, two moles of crystal violet consumed three moles of bromate ion, while one mole of crystal violet consumed two moles each of periodate ion and hypochlorite ion respectively. The titration curves from which the stoichiometries of each of the systems were determined are presented in Figures (4.1-4.4).

CHAPTER FIVE

DISCUSSION

Crystal violet- metabisulphite ion system

The results of the stoichiometric investigations indicated that one mole of crystal violet was consumed by one mole of metabisulphite. This agrees with the stoichiometries observed in the reactions of S₂O ^2- with triphenylmethane dye (Onu and Iyun, 2001) and basic fuchsin (Lawal, 1997). However, stoichiometry of 1:3 was established for the reduction of methylene blue with metabisulphite (Babatunde et al., 2013).

The order of the reaction was found to be one in both the oxidant and reductant respectively. Thus the reaction is second order overall and the second order rate constant k₂ was determined to be k₂ = (23 ± 0.45) dm³ mol^-1 s^-1. Similar order was obtained with respect to the reductant concentration by earlier workers (Gupta et al., 1987, Lawal, 1997, Onu and Iyun 2001 and Babatunde et al., 2013). The order of zero in the [reductant] was observed in the reaction of Fe₂O⁴⁺ and S₂O ^2-(Idris et al., 2005), thus the k₁ evaluated from the slopes of the pseudo-first order plots were constant irrespective of the concentration of S₂O ^2-.

The studies on the effect of hydrogen ion concentration, [H⁺], on the rate of the reaction showed that increase in hydrogen ion concentration decreased the rate of reaction with negative slope of 0.97 (Figure 4.15). Least square plot of k₂ versus [1/H⁺] had intercept (Figure 4.19), this showed two parallel reaction pathways: the acid independent and the inverse acid dependent pathways. The inverse acid pathway shows that there is a pre-equilibrium step before the rate determining step in which a proton is lost. This means that the two rate- controlling steps are preceeded by a rapid equilibrium for which the equilibrium constant is small, and both the forms, protonated and deprotonated, are reactive (Gupta and Gupta 1984). Similar acid dependence pathway has been reported on the oxidation of S₂O ^2- by basic fuchsin (Lawal, 1997) and Reduction of triphenylmethane dye by S₂O ^2-(Onu and Iyun, 2001). The result in this differs from Babatunde et al., 2013 where enhancement in the rate was obtained by increase in hydrogen ion in the reduction of methylene blue with S₂O ^2-.

where a = 2.27 dm³ mol^-1 s^-1, b = 0.86 s^-1, c = 9.22 dm³ mol^-1 s^-1 d = 1.16 dm⁹ mol^-3 s^-1, e = 1.49 s^-1, f = 1.8 x 10^-3 dm³ mol^-1 s^-1 and g = 6.16 dm⁶ mol^-2 s^-1.

The second order rate constants for the various systems were observed as follows: Crystal violet-S₂O ^2- reaction, (k = 23.57 ± 0.45 dm³ mol^-1 s^-1)

Crystal violet-BrO ^– reaction, (k 38.47 ± 0.26 dm³ mol^-1 s^-1) Crystal violet-ClO^– reaction, (k₂ = 52.53 ± 0.41 dm³ mol^-1 s^-1) Crystal violet-IO ^– reaction, (k = (6.89 ± 0.032) x 10^-2 s^-1)

All reactions were sensitive to change in ionic strength except the crystal violet-ClO^– which shows no dependent on ionic strength. Except in crystal violet-ClO^– system where added anions increase the rate of reaction and crystal violet-IO ^– where anions had no effect on the rate of reaction, added anions inhibited the rate of reactions for the rest of the systems.

Evidence of intermediate complex formation was suggested for the crystal violet-IO ^– reactions by the shift in λ_max from 585-620 nm and no shift was observed for the other systems. Free radicals were not observed for all the systems. Thus the results obtained in this study show that crystal violet-S₂O ^2-, crystal violet-BrO ^– and crystal violet-ClO^– reactions could be said to be occurring through the outersphere mechanism, while the crystal violet-IO ^– reaction occurs via the innersphere mechanism. Therefore such mechanisms were proposed for the reactions respectively.

CHAPTER SIX

SUMMARY, CONCLUSION AND RECOMMENDATION

Summary

The kinetics study of the redox reactions of crystal violet with some oxyanions (S₂O ^2-, BrO ^–, IO ^– and ClO^–) in aqueous acid solution was carried out. The stoichiometry of 1:1 was observed for CV⁺-S₂O ^2-, 2:3 for CV⁺-BrO ^– and 1:2 for CV⁺-IO ^– and CV⁺-ClO^– systems respectively.

The reactions were second order at constant [H⁺] for all the systems except that of crystal violet with IO ^– which was first order. Apart from the crystal violet-S O ^2- system which showed inverse first order hydrogen ion dependence pathways, acid dependent pathways was observed for the crystal violet ClO^–, whereas acid dependent and acid independent pathways were observed for crystal violet–IO ^– and crystal violet-BrO ^– systems respectively.

The reactions are therefore in conformity with the following rate equations:

Conclusion

On the basis of the results of these investigations, absence of and/or presence of kinetic and/or spectroscopic evidence for complex formation prior to electron transfer and non- conformity of the results with Michaelis-Menten plots, rationalization of previous results, outersphere mechanism is postulated for the CV⁺-S₂O₅^2-, CV⁺-BrO ^– and CV⁺-ClO^– reactions, whereas, innersphere mechanism is postulated for CV⁺-IO ^– reaction.

Recommendation

It is recommended that:

further studies should be carried out on the activated parameters of these reactions,
thorough analysis to identify the organic product should be made, and
more systems involving other oxidants and reductants should be studied for more information on the chemical characteristics of crystal

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