Why does thiosulfate oxidize to sulfate in an acidic medium?

Question

Henry Antrim · Accepted Answer

Presumably because oxidation to sulfate (#S(VI+)#) is the thermodynamic product under these conditions.

Disproportionation of sodium thiosulfate in acidified aqueous solution is known to occur: #"Oxidation:"# #S_2O_3^(2-) + H_2O(l) rarr 2SO_2(g)+2H^(+) + 4e^-# #"Reduction:"# #S_2O_3^(2-) +6H^(+) + 4e^(-)rarr 2S(s)darr+3H_2O# #"Overall:"# #2S_2O_3^(2-) + 4H^(+) rarr 2SO_2(g) + 2S(s) + 2H_2O# Now this is a balanced chemical equation, but the equation should conform to reality, not vice versa. Under these conditions, I presume, that oxidation to #"sulfate ion"#, would be the preferred outcome, i.e. #"Oxidation:"# #S_2O_3^(2-) + 5H_2O(l) rarr 2SO_4^(2-)+10H^(+) + 8e^-# Note that I presume that sulphur dioxide #(S(IV+))# is fully oxidized to #S(VI+)# under the given conditions. #"Overall:"# #3S_2O_3^(2-) + 4H^(+) rarr 2SO_4^(2-) + 2S(s) + 2H_2O# Thus, I would assume that the thermodynamic result under these circumstances is the oxidation of sulfur to sulfate.

Natalie Anton · Answer

undefined Thiosulfate (S2O3^2-) oxidizes to sulfate (SO4^2-) in an acidic medium due to the action of an oxidizing agent, typically an acidified solution of potassium dichromate (K2Cr2O7). In the presence of acid, the thiosulfate ion is protonated to form sulfuric acid, which undergoes further oxidation to sulfate ions. The overall reaction involves the conversion of sulfur in thiosulfate from a lower oxidation state to a higher oxidation state.