Chemical Science International Journal

Reduction of 3,7-Bis(dimethylamino) phenazothionium Chloride by Benzenethiol in Aqueous Nitric Acid Medium: A Mechanistic Approach

S. O. Idris *

Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria

J. O. Suleman

Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria

J. F. Iyun

Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria

A. A. Osunlaja

Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria

*Author to whom correspondence should be addressed.

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Abstract

The kinetics of the reduction of 3,7-bis(dimethylamino)phenazothionium chloride (MB⁺) by benzenethiol in aqueous solution has been investigated in the acid range 0.3≤[H⁺  ≤ 0.9 mole dm^-3, ionic strength (µ) of the reaction medium in the range: 0.4 ≤ µ ≤ 1.0 mole dm^-3 (NaNO₃) and temperature, T = 29ºC. The reaction is first order in both [oxidant] and [reductant] and display an inverse order acid dependence with an overall reaction that conforms to the rate law:

 [MB⁺]  = (Kk₄[H⁺]^-1 + k₆)([MB⁺][ C₆H₅SH])

at [H⁺] = 0.3 mole dm^-3 and µ = 1.0 mole dm^-3 (NaNO₃). The rate of the reaction increased with increase in the ionic strength and decrease in dielectric constant of the reaction medium. Added HCOO^- and Cl^- accelerated the rate of the reaction. Spectroscopic investigation did not reveal the formation of any stable reaction intermediate. Test for free radical was positive and a plausible mechanism consistent with these observations has been proposed.

 

Keywords: 3,7-bis(dimethylamino)phenazothionium chloride, benzenethiol, kinetics, stoichiometry, Michaelis-Menten plot, outersphere

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