Modern inorganic chemistry

290 GROUP V!

Liquid sulphur dioxide as a solvent

Liquid sulphur dioxide is a solvent for a number of substances, for

example iodine, sulphur, some sulphites, potassium iodide and

sulphur dichloride oxide, SOC1 2 (see below). The liquid can be

assumed to ionise slightly, thus:

2H 2 O^ H 3 O+ +OH~

Hence, for example, sulphur dichloride oxide behaves as an kacicT

and a sulphite as a *base' thus :

SOC1 2 + Na 2 SO 3 ~+2NaCli + 2SO'2

S02+ 4- 2Cr 2Na+ + SOi" -» salt solvent

acid base (insoluble)

Properties of sulphur dioxide

Sulphur dioxide is oxidised by chlorine in the presence of charcoal

or camphor to give sulphur dichloride dioxide (sulphuryl chloride),

SO 2 C1 2 ;

SO 2 + C1 2 -> SO 2 C1 2

Dioxides and peroxides oxidise it to yield sulphates:

PbO 2 + SO 2 -> PbSO 4

Na 2 O 2 + SO 2 -> Na 2 SO 4

Sulphur dioxide is an acidic oxide and dissolves readily in water,
and in alkalis with which it forms salts:

NaOH + SO 2 -> NaHSO 3

sodium hydrogensuiphite

2NaOH + SO 2 -» Na 2 SO 3 + H 2 O

sodium sulphite

Although sulphur dioxide, as a gas, is a reducing agent in the sense
that it unites with oxygen, free or combined (for example in dioxides
or peroxides) most of its reducing reactions in aqueous solution are
better regarded as reactions of 'sulphurous acid' (in acid solution),
or the sulphite ion (in alkaline solution).