KINETICS AND MECHANISM OF THE COMPLEX OXIDATION OF AMINOIMINOMETHANESULFINIC ACID BY IODATE IN ACIDIC MEDIUM

The reaction between iodate and aminoiminomethanesulfinic acid, NH2(NH)CSO2H (AIMSA), has been studied in acidic medium. The stoichiometry of the reaction in excess AIMSA is 2IO(3)(-) + 3AIMSA + 3H(2)O --> 3SO(4)(2-) + 3CO(NH2)(2) + 2I(-) + 6H(+) (eq 1), and the stoichiometry of the reaction in excess iodate is 4IO(3)(-) 5AIMSA + 3H(2)O --> 5SO(4)(2-) + 5CO(NH2)(2) + 2I(2) + +6H(+) (eq 2). In excess AIMSA and high acid concentrations the reaction shows an induction period and a transient formation of iodine, while in excess iodate concentrations iodine is produced and partially consumed, leaving a finite iodine concentration at the end of the reaction. The dynamics of the reaction is explained by a combination of three reactions: the first is the oxidation of AIMSA by iodate to give iodide, the second is the Dushman reaction which forms iodine from the iodate-iodide reaction, and the third is the reaction of iodine and AIMSA. The relative rates of these three reactions will determine the dynamics of the reaction. The oxidation of AIMSA with I-2 and I-3(-) was also investigated. The oxidation of AIMSA by I-2 and 1(3)(-) was found to be inhibited by acid because the oxidation of AIMSA by HOI is faster than that with molecular I-2. The reaction is also autoinhibitory because the product of the reaction, I-, combines with unreacted I-2 to form I-3(-) which is relatively inert toward AIMSA. A computer simulation study is performed to enhance the proposed mechanism.