Kinetics and mechanism of the reduction of chromium(VI) by D-ribose
Abstract
The reaction between CrVI and D-ribose in aqueous perchloric acid, studied by application of the initial-rates method, is first order in both oxidant and reductant, whereas the apparent order of H+ is a little lower than 2. At [HClO4]=0.366 M and 25.0°C the reaction with monomeric CrVI is 12.7 times faster than that with dimeric CrVI. Under the same acidity conditions, the apparent activation parameters are ΔH0,=40.1±0.6 kJ mol-1 and ΔS0,=-155±2 J K-1 mol-1. At low [CrVI]0 the reaction is catalyzed by MnII. The catalytic reaction pathway is zero order in CrIV and first order in both D-ribose and H+. At high [CrVI]0 the reaction is inhibited by MnII because it traps the intermediate CrVI. In the presence of MnII the formation of MnIII as an intermediate has been detected. Mechanisms for the uncatalyzed and catalyzed reaction pathways are proposed.