Modern inorganic chemistry

292 GROUP VI

This reaction is a useful test for a sulphite or for moist sulphur

dioxide, which turns 'dichromate paper' (filter paper soaked in

potassium dichromate) from yellow to green.

3. Sulphites are oxidised by chlorine water and solutions con-
   taining chloric(I) (hypochlorous) acid or the chlorate(I) (hypochlorite)
   ion

C1 2 + SO§~ + H 2 O ~» 2CP 4- SOr + 2H +

or

ocr + sor -»cr + soj-

4. Iron(III) is reduced to iron(II) by sulphites:

2Fe3+ -h SOI" 4- H 2 O -> 2Fe2+ + 2H -f SO^

In the presence of strong reducing agents the sulphite ion acts as
an oxidising agent; some examples are:

1. The oxidation of hydrogen sulphide to sulphur:

2H 2 S 4- SO^ + 2H+ -> 3H 2 O + 3S|

2. In strongly acid solution, substances which are normally
   reducing agents reduce sulphur dioxide solution or sulphites, for
   example iron(II) and zinc:

4Fe2+ 4- SOr + 6H+ -»4Fe3+ 4- S| + 3H 2 O

2SOl~ + Zn -f 4H+ -* S 2 Oj" -f Zn2+ + 2H 2 O

(dust) (dithionite)

If a solid sulphite is heated with zinc dust (or carbon) the sulphite

is reduced to sulphide:

Na.SO, + 3Zn -* Na.S + 3ZnO

Uses

The reducing action of sulphurous acid and sulphites in solution
leads to their use as mild bleaching agents (for example magenta and
some natural dyes, such as indigo, and the yellow dye in wool and
straw are bleached). They are also used as a preservative for fruit and
other foodstuffs for this reason. Other uses are to remove chlorine
from fabrics after bleaching and in photography.