1. Alkaline permanganate and hydrogen sulfite

2. In the HSO3- ion, sulfur is in the +4 oxidation state.
Sodium hydrogen sulfite is a white solid which dissolves in water to form a colourless solution.

3. Sodium hydroxide solution is added to make the solution alkaline.

4. Purple potassium permanganate solution is added.
The purple solution turns green.

5. MnO4- + e- → MnO42-

6. The alkaline solution is acidified by adding hydrochloric acid.
The green solution turns brown, then orange, and finally colourless as the excess HSO3- further reduces the green MnO42- to brown MnO2 and then colourless Mn2+.

7. Finally we add barium chloride solution, which reacts to form a white precipitate. This confirms that the HSO3- has been oxidised to SO42-.
The solution had to be acidified first, because Ba(OH)2 is only slightly soluble.

8. MnO e- → MnO42-
purple green
HSO H2O → SO H e-
2MnO HSO H2O → 2MnO SO H+