Kinetics and mechanism of the reduction of Cr VI and Cr V by d-lactobionic acid

Abstract

The oxidation of d-lactobionic acid by Cr VI yields the 2-ketoaldobionic acid and Cr 3+ as final products when a 20-times or higher excess of the aldobionic acid over Cr VI is used. The redox reaction takes place through a complex multistep mechanism, which involves the formation of intermediate Cr IV and Cr V species. Cr IV reacts with lactobionic acid much faster than Cr V and Cr VI do, and cannot be directly detected. However, the formation of CrO 2 2+, observed by the first time for an acid saccharide/Cr VI system, provides indirect evidence for the intermediacy of Cr IV in the reaction path. Cr VI and the intermediate Cr V react with lactobionic acid at comparable rates, being the complete rate laws for the Cr VI and Cr V consumption expressed by: − d[ Cr VI ]/ dt={k I +k II [ H +]} [ lactobionic acid ] [ Cr VI ] , where k I=(4.1±0.1)×10 −3 M −1 s −1 and k II=(2.1±0.1)×10 −2 M −2 s −1; and −d[Cr V]/ dt={k III [ H +]+(k IV +k V [ H +]) [ lactobionic acid ]} [Cr V], where k III=(1.8±0.1)×10 −3 M −1 s −1, k IV=(1.1±0.1)×10 −2 M −1 s −1 and k V=(1.0±0.1)×10 −2 M −2 s −1, at 33 °C. The Electron Paramagnetic Resonance (EPR) spectra show that five-co-ordinate oxo-Cr V bischelates are formed at pH 1–5 with the aldobionic acid bound to Cr V through the α-hydroxyacid group.