Peroxodisulphate ion also called persulphate and peroxydisulphate

The reduction by Ag is the most widely investigated example although in recent years several other reductants have been used. The series of oxidations effected by the Ag -SjOg couple are referred to in the section on Ag(II) and Ag(III). In general the rate of disappearance of persulphate is independent of the concentration and, to some extent, the nature of the substrate, viz. 

The various alternative mechanisms for the production of Ag(III) and Ag(II) are given in Section 4.1.1. Reaction without added substrate follows the kinetics  

The observation of Fronaeus and Ostman that the first-order decay coefficient of persulphate in the presence of Ag is quite unchanged by adding cerous ions (which are oxidised) indicates the decomposition of persulphate and the redox process to have the same rate-determining step. These workers obtain values for ki and 2 of (8 + 3)xlO sec and (3.75 + 0.17)x 10 respectively, at 25 °C, and they prefer the mechanism 

It is pertinent that S20g accepts an electron generated by pulse radiolysis of water to give optically detectable S04 within 1.5 x 10 sec .  

The ability of the stable free radical diphenylpicrylhydrazyl (DPPH) to act as an efficient trap for reactive radicals such as 804 and OH- has been utilised by Bawn and Margerison in their examination of the Ag -S20g couple. The disappearance of the intensely coloured DPPH gave excellent zero-order kinetics the rate as a whole was identical with that found by Fronaeus and Ostman and 2 was given by 3.1 x 10 exp(—17.9x lO /RT) l.mole sec A Sengar and Gupta have also determined Arrhenius parameters for this reduction and have compared them with those for some redox processes (Table 23). 

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