Potassium dichromate ($K_2Cr_2O_7$) is a strong oxidising agent in acidic medium. It oxidises sulphite ions ($SO_3^{2-}$) from sodium sulphite into sulphate ions ($SO_4^{2-}$). In this process, the orange-coloured dichromate ion ($Cr_2O_7^{2-}$) is reduced to the green-coloured chromium(III) ion ($Cr^{3+}$).

The chemical equation is: $Cr_2O_7^{2-} + 3SO_3^{2-} + 8H^+ \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$

Since the reaction medium is acidified (usually with sulphuric acid), the chromium(III) ions exist as chromium(III) sulphate, $Cr_2(SO_4)_3$ .