Thiosulfate occurrence

General procedure for conversion of p-toluenesulfonates into iodides The ester (1 mole) and dry Nal (1.2 moles) are heated in dry acetone at 100° for 2-5 h in a pressure flask or a bomb-tube. Reactive p-toluenesulfonates also react in ethyl methyl ketone, acetonylacetone, or diethylene glycol those of lower primary alcohols react even at room temperature. Occurrence of reaction is recognized by precipitation of sodium p-toluenesulfonate this is eventually filtered off and washed with acetone. After removal of the solvent, water is added and the iodide is taken up in ether and washed therein with thiosulfate solution and with water. 

The complex tetraiodomercurate(II) ion [Hgl4 ] is formed. It is a back titration. The excess iodine is titrated with a thiosulfate solution. We notice that mercury(I) is oxidized into mercury(II), an oxidation state that it exhibits in the ion complex tetraiodomercurate. Simultaneously, one molecule of iodine is reduced to two iodide ions. We notice also that the iodide ions formed are not free after the reaction, as in a classical direct iodometry, but are complexed. Now, we must wonder why this reaction is possible from a thermodynamic standpoint. There is indeed, at first sight, a sort of contradiction between the occurrence of this reaction and the values of the standard potentials of systems under study ... 

Increasing occurrence of secondary reactions, with oxidation of thiosulfate to sulfate and sulfuric acid... 

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