KINETICS AND MECHANISM OF THE OXIDATION OF TANNIC ACID BY POTASSIUM TRISOXALATOFERRATE (III) IN AQUEOUS HYDROCHLORIC ACID MEDIUM

Abstract

The kinetics of the redox reaction between tannic acid (TA) and potassium trisoxalatoferrate (III) trihydrate, (K3[Fe(C2O4)3].3H2O) has been studied in aqueous hydrochloric acid medium. The stoichiometry of the reaction determined by the mole ratio method was observed to be 1:1. The kinetic results showed first order dependence with respect to complex and tannic acid concentrations. The experimental data for the reaction system are consistent with the following rate law:where a = 2.0 × 10-3 mol-1 dm3 s-1 and b = 2.1 × 10-3 mol-2 dm6 s-1 at 420 nm and 27.0 ± 1.0 ºC. The rate of reaction was found to increase with increase in [H+] in the acid range used, but decreased with increase in ionic strength of the reaction medium. While the rate of reaction was observed to decrease with increasing dielectric constant of the reaction medium, the addition of CH3COO-, HCOO-, Ni2+and K+ catalyzed the rate of the reaction. The Michaelis-Menten plot of 1/kobs versus 1/[oxidant] gave a straight line with an insignificant positive intercept. The addition of acrylamide to the partially oxidized reaction mixture did gave a gel in the presence of excess methanol. Based on the experimental data, a plausible mechanism involving outer sphere electron transfer has been proposed for the redox reaction between tannic acid and potassium trisoxalatoferrate (III) trihydrate.