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+
