Why does potassium permanganate and sodium bisulfite form a brown precipitate?

Answer 1

The brown precipitate is MnO2, it is the product of the following redox in neutral environment:

For each half reaction:

MnO4- + 4H++ 3e- → MnO2 + 2H2O REDUCTION

HSO3- + H2O → SO42- + 3H+ + 2e- OXIDATION

Overall: Permanaganate half reaction x2 + Bisulfite half reactionx3

2MnO4- + 3HSO3- + 8H+ + 3H2O + 6e- → 2MnO2 + 3SO42- + 4H2O + 9H+ + 6e-

Cancelling like terms:

2MnO4- + 3HSO3- → 2MnO2 + 3SO42- + H3O+

Note that all species are balanced, all charges are balanced and the brown precipitate MnO2 is accounted for. The reaction also predicts that acid is formed in the process.

Answer 2

When potassium permanganate reacts with sodium bisulfite, a complex of manganese dioxide is formed, which appears as a brown precipitate. This reaction occurs due to the reduction of the permanganate ion (MnO4-) to manganese dioxide (MnO2) by the bisulfite ion.