Peroxodisulphate oxidation

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 possible types of chain mechanisms for peroxodisulphate oxidation have been classified by Wilmarth and Haim according to the dominant initiation and termination steps, and the relative importance of sulphate radical-ions and hydroxyl radicals in the propagation steps. Some of the rate equations corresponding to the different types of mechanisms are the same, so the observation of a particular rate equation does not always permit a unique mechanism to be inferred. In certain cases the nature of the chain initiation step can be deduced from the effect of a free-radical scavenger on the reaction rate. Thus in the oxidation of 2-propanol, the addition of allyl acetate reduces the rate to that observed for the spontaneous decomposition of peroxodisulphate, indicating that the chain initiation step is the same as the rate-determining step of the spontaneous decomposition, viz. the fission of peroxodisulphate into sulphate radical-ions. 

As noted above, many peroxodisulphate oxidations are catalysed by silver... 

In the following sections the peroxodisulphate oxidations of organic compounds are classified according to the nature of the substrate. Unless otherwise stated the solvent is water. 

Mercaptans are oxidised to disulphides by peroxodisulphate. Eager and Winkler studied the kinetics of the oxidations of n-butyl, n-octyl, and n-dodecyl mercaptans in acetic acid/water solvent (80 ml acid-I-20 ml water) and found first-order kinetics with respect to peroxodisulphate. The first-order rate coefficient increases with increase of mercaptan concentration, and reaches a limit at about 5 x 10 Af mercaptan. A decrease in the first-order rate coefficient with increase of the initial peroxodisulphate concentration was observed and attributed to a salt effect. Eager and Winkler suggested a mechanism involving sulphate radical ions. Levitt proposed a mechanism involving sulphur te-troxide, but there is no evidence for its formation in peroxodisulphate oxidations. 

Nydahl F. 1978. On the peroxodisulphate oxidation of total nitrogen in waters to nitrate. Water Res. 12 1123-1130. 

It can also be precipitated in a hydrated form by the oxidation of a manganese(II) salt, by, for example, a peroxodisulphate ... 

Peroxomonosulphate and peroxodisulphate have also been used to oxidize sulphoxides to sulphones in good yields at room temperature. Potassium persulphate (KHS05) readily oxidizes a range of sulphoxides to sulphones at 0°C in yields greater than 90%, in the presence of hydroxy, keto and alkene groups82-84. The mechanism is similar to that observed for other peroxy species, as discussed above. Peroxomonosulphate oxidation has been used as an analytical procedure for the estimation of dimethyl sulphoxide84. 

Potassium peroxodisulphate (K2S2Og) also oxidizes sulphoxides to sulphones in high yield, either by catalysis with silver(I) or copper(II) salts at room temperature85 or in pH 8 buffer at 60-80 °c86-88. The latter conditions have been the subject of a kinetic study, and of the five mechanisms suggested, one has been shown to fit the experimental data best. Thus, the reaction involves the heterolytic cleavage of the peroxodisulphate to sulphur... 

Peroxodisulphates are particularly dangerous oxidants because of their reactivity. 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schwarz HA, Bielski BHJ (1986) Reactions of H02 and 02 with iodine and bromine and l2 and I atom reduction potentials. J Phys Chem 90 1445-1448... 

However, although the oxidation process was ascribed to either direct reaction on the electrode surface, or mediated by peroxodisulphate and other inorganic reagents electrogenerated at the anode surface, the linear decrease of the Faradic yield down to zero in the investigated range of concentration was interpreted as an indication of a process under diffusive control. This leads to the conclusion that the oxidative degradation of the compounds essentially occurred at the electrode interface. 

Hydrogen peroxide (a) or ammonium peroxodisulphate (b), when added to a solution of tetrahydroxoplumbate(II) forms a black precipitate of lead dioxide by oxidizing bivalent lead to the tetravelent state ... 

Benzidine acetate test (DANGER THE REAGENT IS CARCINOGENIC) A neutral or weakly acetic acid solution of a peroxodisulphate converts benzidine into a blue oxidation product. Perborates, percarbonates, and hydrogen peroxide do not react. Chromates, hexacyanoferrate(III) ions, permanganates, and hypohalites react similarly to peroxodisulphates. 

Method A This procedure involves the removal of the bromide and iodide (but not chloride) by oxidation with potassium or ammonium peroxodisulphate in the presence of dilute sulphuric acid. The free halogens are thus liberated, and may be removed either by simple evaporation (addition of water may be necessary to maintain the original volume) or by evaporation at about 80° in a stream of air. 

Fronaeus and Ostman have provided strong evidence for the presence of one, and only one, radical capable of oxidizing cerium(III) per molecule of peroxodisulphate decomposed thermally. They therefore propose that the ratedetermining initial step is... 

Oxidations of thiocyanate ion by nitrous and nitric acids and by peroxomono-sulphate and peroxodisulphate are considered under the oxidants (p. 293 and 350). Oxidation by hydrogen peroxide and by iodine will be considered here. 

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... 

---