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

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.