Modern inorganic chemistry

GROUP VI 293

SULPHITES AND HYDROGENSULPHITES

When a saturated solution of sulphur dioxide is titrated against
approximately 2 M sodium hydroxide solution the following pH
curve is obtained (Figure 10.4):

, 4

12

10

Q.

8 10 12 14 16 18 20
cm^3 2M No OH
Figure 10.4. Titration of 25 cm^3 of saturated aqueous sulphur dioxide with 2 M sodium
hydroxide at 298 K

The reaction completed on addition of 7.9 cm^3 is

SO 2 (aq) + NaOH -» NaHSO 3

sodium

hydrogensulphite

and after 15.8cm^3

SO 2 (aq) 4- 2NaOH ^ Na 2 SO 3 4- H 2 O

sodium sulphite

Evaporation and crystallisation of the sodium sulphite solution gives
crystals of the heptahydrate Na 2 SO 3 .7H 2 O. However, on evapora-
tion of the hydrogensulphite solution, the solid obtained is chiefly
sodium pentaoxodisulphate(IV) (smetabisulphite') Na 2 S 2 O 5 , and
contains little if any of the hydrogensulphite. However, the hydrogen
sulphite ion is obtained when the solid redissolves in water:

Na 2 S?O H 2 O HSO