Modern inorganic chemistry

294 GROUP VI

Alternatively these salts can be prepared by first saturating a known

volume of alkali with sulphur dioxide, giving a solution of the

hydrogensulphite, from which sulphite can be prepared by the

addition of a second equal volume of alkali.

Properties

The redox properties have already been considered. A number of
reactions of soluble (alkali metal) sulphites are noteworthy:

1. On boiling a solution of a sulphite with sulphur a thio-
   sulphate(VI)* is formed, and sulphur 'dissolves':

SOi~ + S -> S 2 Or (e.g. Na 2 S 2 O 3 )

thiosulphateCVI)

Sodium thiosulphate is an important reducing agent used in

volumetric analysis for the estimation of iodine:

I 2 + 2S 2 Or ~+2F + S 4 Oi-

tetrathionate ion

It is used as the Tixer' in photography under the name 'hypo'.

2. Addition of barium chloride precipitates white barium sulphite:

Ba2+ + SOr-»BaSO 3 l

Barium sulphite is soluble in dilute hydrochloric acid unlike

barium sulphate which is insoluble. Hence this reaction, and the

evolution of sulphur dioxide on addition of an acid, distinguishes a

sulphite from a sulphate.

3. Sodium hydrogensulphite, when freshly prepared, reacts with
   aldehydes to form crystalline addition compounds, for example

H

HSO7 + CH.C

O

ethanal

(acetaldehyde)

/

H

CH,C—OH

SO 3

This reaction is used in organic chemistry to separate an aldehyde
from, for example, an ester.

• The thiosulphate ion has the structure [S=SO 3 ]^2 ~; the oxidation state of the
  central sulphur atom is + 6.