Alkaline permanganate and hydrogen sulfite

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.

Sodium hydroxide solution is added to make the solution alkaline.

Purple potassium permanganate solution is added.

The purple solution turns green.

MnO4- + e- → MnO4

2-

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 MnO4

2- to brown MnO2 and then colourless Mn2+.

Finally we add barium chloride solution, which reacts to form a white precipitate. This confirms that the HSO3

- has been oxidised to SO4

2-.

The solution had to be acidified first, because Ba(OH)2 is only slightly soluble.

MnO4- + e- → MnO4

2-

purple green

HSO3- + H2O → SO4

2- + 3H+ + 2e-

2MnO4- + HSO3

- + H2O → 2MnO42- + SO4

2- + 3H+