The substituted thiourea, 4-methyl-3-thiosemicarbazide, was oxidized by iodate in acidic medium. In high acid concentrations and in stoichiometric excess of iodate, the reaction displays an induction period followed by the formation of aqueous iodine. In stoichiometric excess of methylthiosemicarbazide and high acid concentration, the reaction shows a transient formation of aqueous iodine. The stoichiometry of the reaction is: 4IO 3
Iodine formation is due to the Dushman reaction that produces iodine from iodide formed from the reduction of iodate: IO 3 Ϫ ϩ 5I Ϫ ϩ 6H ϩ : 3I 2 (aq) ϩ 3H 2 O (B). Transient iodine formation is due to the efficient acid catalysis of the Dushman reaction. The iodine produced in process B is consumed by the methylthiosemicarbazide substrate. The direct reaction of iodine and methylthiosemicarbazide was also studied. It has a stoichiometry of 4I 2 (aq
The reaction exhibits autoinhibition by iodide and acid. Inhibition by I Ϫ is due to the formation of the triiodide species, I 3 Ϫ , and inhibition by acid is due to the protonation of the sulfur center that deactivates it to further electrophilic attack. In excess iodate conditions, the stoichiometry of the reaction is 8IO 3 Ϫ ϩ 5CH 3 NHC("S)NHNH 2 ϩ H 2 O : 4I 2 ϩ 5SO 4 2Ϫ ϩ 5CH 3 NHC("O)NHNH 2 ϩ 2H ϩ (D) that is a linear combination of processes A and B.