GROUP VI 291
'SULPHUROUS ACID'The solution obtained when sulphur dioxide dissolves in water has
long been thought to contain unionised sulphurous acid, H 2 SO 3 ,
but more probably contains hydrated sulphur dioxide (cf. NH 3
solution, p. 217). The solution behaves as a dibasic acid, i.e.
SO 2 (aq) -f 2H 2 O ^ H 3 O+ HSO 3 ~ :
K = 1.6 x 10"^2 moll-^1 at 298 KHSO 3 ~ 4- H 2 O ^ H 3 O* 4- SOi' :
Ka = 6.2 x 1(T^8 mol 1~ i at 298 K.The sulphite ion, SOf ~, has a pyramidal structure and the short
S — O bond length suggests the presence of double bonding, i.e.t \Two important redox potentials for reduction by sulphur dioxide
in aqueous solution are :Acid:SOj-(aq) H 3 O+ -¥ 4 2e~ -»
H 2 SO 3 '(aq) -h 5H 2 O : £^ = 4- 0.17 V
Alkali: SO|~(aq) -f H 2 O -f 2e~ ->
SO| ~ (aq) -h 2OH " (aq) : E^ = - 0.93 V.
Some important reducing reactions are given below ; for simplicity,
the reducing entity is taken to be SOl~ in all cases.
- Sulphites react with molecular oxygen (or air) to give sulphates,
a reaction catalysed by certain ions (for example Fe^2 + , Cu^2 + ,
arsenate(III) ion, AsOl") and inhibited by, for example, phenol,
glycerol and tin(II) ions, Sn^2 + : - Sulphites react with oxidising agents, for example mangan-
ate(VII) and dichromate(VI) :
4-Cr 2 Of~
orange+ 3H 2 O
-h 4HO
green