Abstract
The formation and disappearance of a stable Cr(V) intermediate can be conveniently seen by spectrophotometric studies. The formation of Cr(V) follows first order kinetics with respect to [Cr(VI)] and second order kinetics with respect to [citric acid]. The rate increased markedly with increasing [HClO4]. Second order dependence of the reaction shifts to first order in the presence of [Mn(II)]. On the other hand, the rate shows a first order dependence on the [citric acid] for the decomposition of Cr (V). Role of Mn(II) ion suggests that the reduction follows the sequence Cr(VI) --> Cr(V) --> Cr(III) without passing through Cr(IV) intermediate stage. The rate laws governing the reduction of Cr(VI) by citric acid-Mn(II) ion system over a wide range of conditions are: rate = k(1)K(1)K(2) [Mn(II)](2) [citric acid] [Cr(VI)]/[H+] and k(2) [Cr(V)-citric acid complex] for the formation and decomposition of Cr(V), respectively. Based on the kinetic data three probable mechanisms have been proposed for the reaction.