Oxyhalogen−Sulfur Chemistry: Oxidation of Hydroxymethanesulfinic Acid by Chlorite

The reaction between chlorite and hydroxymethanesulfinic acid, HOCH 2 SO 2 H, (HMSA) has been studied in the pH range 4−8. The reaction is very complex and has a variable stoichiometry that is dependent on ratio of the oxidant to the reductant. In excess HMSA, the stoichiometry is ClO 2 - + HOCH 2 SO 2 H → SO 4 2- + HCHO + Cl - + 2H + while in excess ClO 2 - the stoichiometry is 3ClO 2 - + 2HOCH 2 SO 2 H → 2SO 4 2- + 2HCOOH + 4H + + 3Cl - . The reaction is very fast and is characterized by a short induction period (≈1 s) which is followed by a rapid and autocatalytic ClO 2 production. The first stage in the oxidation is the cleavage of C−S bond to form SO 4 2- and HCHO. In the presence of excess oxidant the HCHO is oxidized to HCOOH. After prolonged standing (72 h or more), the HCOOH can be oxidized to CO 2; giving a third possible stoichiometry. The rate-determining step is a 2-electron oxidation of HMSA by ClO 2 - to give the sulfonic acid HOCH 2 SO 3 H. A 19-reaction mechanism is used to simulate the reaction. There is reasonable agreement between the experiments and simulations.