So4 -2 ---------------- (s2o8 )-2 + 2e- (ox.) how it is possible as there is no change in the oxidation state of s from sulphate ion to thionate i.e. +6 to +6

Here in the product side (S2O8)2- the oxidation number while calculating the oxygen atom has -1 oxidation state and two oxygen atom has -2 charge and thus sulphur is going to have -1 oxidation state

so change in oxidation state is not in a sulphur atom but in oxygen atom

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