6The 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+.
7Finally 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.
8MnO e- → MnO42-purple greenHSO H2O → SO H e-2MnO HSO H2O → 2MnO SO H+