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# A dismutation reaction

## The thiosulfate ion $S_2O_3^{2-}$ in acid medium

### Reaction between sodium thiosulfate and sulfuric acid:

There exist two half-reactions of thiosulfate in acid medium: (1) $4{S_2}O_3^{2-}$ $..$ $\longrightarrow$ $S_8$ (Product: Sulfur, a yellow precipitate) The oxidation number of the $S$ atoms varies from $4\cdot 2\cdot 2$ $=$ $16$ to $8\cdot 0$ $=$ $0$, therefore this is a reduction with gain of $16e^-$: $4S_2O_3^{2-}$ $+$ $16e^-$ $+$ $24H^+$ $\longrightarrow$ $S_8$ $+$ $8H_2O$ (2) $S_2O_3^{2-}$ $..$ $\longrightarrow$ $2 SO_4^{2-}$ The oxidation number of the $S$ atoms varies from $2\cdot 2=4$ to $2\cdot 6=12$, therefore this is an oxidation with loss of $8e^-$: $S_2O_3^{2-}$ $-$ $8e^-$ $+$ $5H_2O$ $\longrightarrow$ $2SO_4^{2-}$ $+$ $10H^+$ Now the complete equation : $4S_2O_3^{2-}$ $+$ $16e^-$ $+$ $24H^+$ $\longrightarrow$ $S_8$ $+$ $8H_2O$ $S_2O_3^{2-}$ $-$ $8e^-$ $+$ $5H_2O$ $\longrightarrow$ $2SO_4^{2-}$ $+$ $10H^+$  |  $\cdot 2$ $6S_2O_3^{2-}$ $+$ $4H^+$ $\longrightarrow$ $S_8$ $+$ $4SO_4^{2-}$ $+$ $2H_2O$

As the thiosulfate ion is oxidised and reduced simultaneously, it is said to dismutate.