A Kinetic Study of the Reaction of Colloidal Manganese Dioxide with Perchloric Acid in Neutral Aqueous Solution

A dark brown colloidal MnO2 has been prepared by the reduction of KMnO4 by formic acid in neutral aqueous solution at 25 degrees C. The colloid was characterized using spectrophotometric and coagulation methods. Colloidal MnO2 was found to react spectrophotometrically with HClO4 in neutral aqueous solution at 25 degrees C. Attempts were made for kinetic analysis of the concentration-time profiles of MnO2 species. The decay profiles of MnO2 appear essentially exponential in nature. Addition of KI salt in the product mixture indicates that the end product contained Mn-species in the oxidation states higher than + 2. The average oxidation state of the Mn-species during the reaction determined iodimetrically suggests the formation of intermediate Mn-species with oxidation state higher than + 2. The rate of decay of the average oxidation state was slower than the rate of decay of MnO2. This is attributed to secondary reactions leading to formation of the final product species Mn2+. The reaction followed neither first order nor second order kinetics. A mechanism for the reaction is proposed.