General properties and spontaneous decomposition of peroxodisulphate

Peroxodisulphuric acid, H2S2O8, is a strong acid whose second pK is below zero (Kolthoff and Miller ). Under the conditions normally employed in peroxodisulphate oxidations (aqueous solution, pH 1) the ion 8208 is the dominant species. The ion is a powerful two-electron oxidising agent with a redox potential of — 2.01 V. In the majority of its reactions the primary step is the formation of sulphate radical-ions, either by spontaneous fission of the peroxide bond, or by attack on a substrate X, i.e. 

The highly reactive sulphate radical-ion may attack the solvent or a substrate present in the solution. The spontaneous decomposition of peroxodisulphate in aqueous solution has been widely studied it is, in effect, the oxidation of the solvent by peroxodisulphate. It will be considered here because the steps involved are of importance in the mechanisms of oxidation of organic compounds by peroxodisulphate. The decomposition has the stoichiometry 

Early work established that the kinetics of decomposition are first order with respect to peroxodisulphate, and in a detailed study Kolthoff and Miller showed that the rate equation includes an acid-catalysis term, viz. 

The values of ko obtained by Kolthoff and Miller for the reaction in 0.1 M sodium hydroxide in the range 50-90 °C are given in Table 5. 

In their discussions of the decomposition, Tsao and Wilmarth , and Wilmarth and Haim, propose the following mechanism for the uncatalysed reaction (based on earlier suggestions by Kolthoff and Miller ) 

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