Kinetics and Mechanism of Reduction of Iron(III) Kojic Acid Complex by Hydroquinone and L-Cysteine

The effect of pH on the kinetics of reduction of iron(III) kojic acid complex by hydroquinone (H2Q) and L-cysteine (L-Cys) was studied in the pH range of 2.34 - 4.03 for H2Q and 3.04 – 5.5 for L-cysteine at ionic strength of 0.5 M and at 35oC. The pseudo-first order rate constants for the reduction of Fe(KA)3 by L-cysteine and hydroquinone increase linearly with increasing reductant concentration, indicating first-order kinetics in reductant concentration. However, whereas the rate of reduction by H2Q increases with increasing pH, an opposite trend was observed in the case of reduction by L-cysteine. Plausible rate laws and mechanisms have been proposed in line with these observations. Activation parameters (∆H# and ∆S#) were evaluated for the reduction of iron (III) kojic acid complex by cysteine and the values obtained are 35.25 kJmol-1, -141.4 JK-1mol-1 and 28.14 kJmol-1 , 161.2 JK-1mol-1 for pH 4.5 and 3.52 respectively.

Atim Sunday Johnson, Zahid Hussain, Muhammad Perviaz, Syed Arif Kazmi, Ededet Akpan Eno and Offiong E. Offiong