Effect of dissolved oxygen on the oxidation of dithionate ion. Extremely unusual kinetic traces.

The effect of dissolved oxygen on the acid-dependent disproportionation of dithionate ion (S(2)O(6)(2-)) and its oxidation with a number of reagents (I(2), Br(2), I(3)(-), Br(3)(-), IO(3)(-), BrO(3)(-), IO(4)(-), MnO(4)(-), Cr(VI), Ce(VI), and H(2)O(2)) were studied. Dithionate ion is remarkably inert at room temperature, and heating to elevated temperatures (50-90 degrees C) was necessary to observe detectable reaction rates in all of these processes. It was confirmed that dithionate ion is never oxidized directly; its redox reactions are zeroth-order with respect to the oxidizing agent and proceed through disproportionation and subsequent fast oxidation of the sulfur(IV) formed. The effect of dissolved oxygen is attributable to its reaction with sulfur(IV) produced in the disproportionation. This autoxidation occurs only with a catalyst. Cerium(III) and iodide ions were shown to catalyze the autoxidation, and their effect on the kinetic traces was studied. In a few cases, highly unusual kinetic traces, such as straight lines with sharp break points, inverted parabolas, and a combination of these, were detected and successfully interpreted in terms of simple kinetic and stoichiometric considerations.