Cerium(III) catalysis in acid bromate oxidation of citric acid in the presence of bromo-complex-forming metal ions

Abstract

Cerium(III) ion-catalysed bromate oxidation of citric acid in aqueous acetic acid media containing HClO 4 and mercury(II), a bromo-complex-forming metal ion, exhibits first-order in [bromate] and less than unit order in [citric acid] and [cerium(III)]. Reaction rate increases linearly with an increase in [HClO 4 ]. However, an opposite effect is observed with [H 2 SO 4 ] and [NaHSO 4 ]. D 2 O and Hg(OAC) 2 have insignificant effect on the rate of reaction. The catalytic effect of Ce III ion is displayed by its complex forming ability and in the proposed mechanism it is assumed that acid bromate reacts with the Ce'-citric acid binary complex, whose formation (Kc) is precedent to the bromate-cerium(III) reaction, and results a ternary complex (K f ), which subsequently decomposes (kd) into products with the regeneration of Ce1. The effects of ionic strength and dielectric constant of the media are also in accordance with the proposed mechanism. The reaction constants (Kc, K f and kd) involved in the envisaged mechanism are evaluated. There is a good agreement between the observed and the calculated rate constants under different experimental conditions.