Kinetics of arsenite-ferricyanide reaction in an alkaline medium

Abstract

The kinetics of the oxidation of arsenite to arsenate by ferricyanide has been studied in an alkaline medium. At low alkali concentrations the reaction is very slow and the reaction rates have therefore been determined in highly alkaline media (OH − > 10 −2M). The overall kinetics are first order in ferricyanide, arsenite and hydroxide ions. Addition of neutral salts viz. KCl and K 2SO 4 has a pronounced accelerating effect. Addition of ferrocyanide has no effect while methanol considerably lowers the reaction rate. The temperature coefficient of the reaction has been found to be unusually low. The energy and entropy of activation have been obtained as 8·2 Kcal and −42·8 cal. deg −1 mole −1 respectively. A mechanism has been proposed which involves the primary equilibria between various arsenite species and the hydroxide ions and is followed by the rate determining bimolecular reaction between a tribasic arsenite ion and ferricyanide.