KINETICS AND MECHANISMS OF THE REDOX REACTIONS OF MALACHITE GREEN WITH SOME OXY-ANIONS IN AQUEOUS ACIDIC MEDIUM
KINETICS AND MECHANISMS OF THE REDOX REACTIONS OF MALACHITE GREEN WITH SOME OXY-ANIONS IN AQUEOUS ACIDIC MEDIUM
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Date
2016-10
Authors
IMAM, MADINA
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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 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.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES,
AHMADU BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF A
MASTER OF SCIENCE DEGREE IN INORGANIC CHEMISTRY
DEPARTMENT OF CHEMISTRY
FACULTY OF SCIENCE
AHMADU BELLO UNIVERSITY,
ZARIA, NIGERIA
Keywords
KINETICS,, MECHANISMS,, REDOX REACTIONS,, MALACHITE GREEN,, OXY-ANIONS,, AQUEOUS ACIDIC MEDIUM,