so4 -2 ---------------- s2o8 -2 + 2e- (ox.) how it is possible as there is no change in o. s. of s from sulphate ion to thionate i.e. +6 to +6

in S2O8 2- , there is a peroxy bond and while counting or calculating oxidation no. of sulphur, we take oxidation state of those oxygen atoms to be (-1) and so two O atoms with (-2) charge are going to (-1) oxidation state. so change in o.s. is in oxygen atoms not in the sulphur atoms. you can see the strusture of S2O8 2- ion given below having peroxy linkage in betweeen.

in S2O8 2- , there is a peroxy bond and while counting or calculating oxidation no. of sulphur, we take oxidation state of those oxygen atoms to be (-1) and so two O atoms with (-2) charge are going to (-1) oxidation state. so change in o.s. is in oxygen atoms not in the sulphur atoms. you can see the strusture of S2O8 2- ion given below having peroxy linkage in betweeen.