SO2 + H2O ==> H2SO3
Sulphurous acid, H2SO3, is a weak dibasic acid, known in the form of its salts (e.g. sodium sulphite).
Sulphurous Acid, H2SO3, is a weak dibasic acid, known in the form of its salts.
Sulphurous acid is unstable and has never been isolated as a pure compound.
Sulphurous acid is formed when sulphur dioxide is dissolved in water.
SO2 + H2O ==> H2SO3
Sulphurous acid is unstable and has never been isolated as a pure compound.
It may be better represented by the following reactions.
SO2 + H20 ===> HSO3(ion) + H(ion)
HSO3(ion) ===> H(ion) + SO3(ion)
This is known as the ionisation of Sulphurous acid.
Both the bisulfite ion, HSO3(ion), and the sulphate ion, SO3(ion) exist, for salts of both are well known.
Examples of the above are :
2NaOH + SO2 ===> Na2SO3 + H2O
Na2SO3 + H2O + SO2 ===> 2NaHSO3
The solution when heated in a sealed tube at 150 deg.C. deposits sulphur.
3H2SO3 ===> 2H2SO4 + H2O + S
Sulphurous acid can be oxidised by the use of strong oxidising agents.
Oxidising of Sulphurous acid by Oxygen
2H2SO3 + O2 + 4H2O ===> 4H20(ion) + 2SO4(ion)Sulphurous acid solution is slowly oxidised by atmospheric oxygen to sulphuric acid.
Oxidising of Sulphurous acid by Permanganate ions
When Sulphurous acid is added to permanganate ion which is coloured purple, SO2 will decolourise the MnO4(ion) when it is reduced to the colourless Mn(ion).
2MnO4(ion) + 5H2SO3 + 4H2O ===> 2Mn(ion) + 4H3O(ion) + 5SO4(ion) + 3H2O
Sulphurous acid is a strong reducing agent.
The solution has bleaching properties :
Moistened wool, straw for hats, and other materials injured by chlorine are bleached on
exposure to sulphur dioxide or the fumes of burning sulphur.