Modern inorganic chemistry

GROUP VII: THE HALOGENS 339

Halic(IH) acids, HXO 2

Only chloric(III) acid, HC1O 2 , is definitely known to exist. It is

formed as one of the products of the reaction of water with chlorine

dioxide (see above). Its salts, for example NaClO 2 , are formed

together with chlorates(V) by the action of chlorine dioxide on

alkalis. Sodium chlorate(III) alone may be obtained by mixing

aqueous solutions of sodium peroxide and chlorine dioxide:

2C1O 2 + Na 2 O 2 -> 2NaClO 2 + O 2 t

A solution of the free acid may be obtained by using hydrogen

peroxide, instead of sodium peroxide.

Chloric(III) acid is a fairly weak acid, and is an oxidising agent,

for example it oxidises aqueous iodide ion to iodine. Sodium

chlorate(III) (prepared as above) is used commercially as a mild

bleaching agent; it bleaches many natural and synthetic fibres

without degrading them, and will also bleach, for example, oils,

varnishes and beeswax.

Chlorates(III) disproportionate on heating, or on boiling the

aqueous solution, thus:

3C1O2 -> 2C1OJ + Cl~

chlorate(V) chloride

Halic(V) acids

Chlorine, bromine and iodine form halic(V) acids but only iodic(V)
acid, HIO 3 , can be isolated. Solutions of the chloric(V) and bromic(V)
acids can be prepared by the addition of dilute sulphuric acid to
barium chlorate(V) and bromate(V) respectively, and then filtering
(cf. the preparation of hydrogen peroxide). These two acids can also
be prepared by decomposing the corresponding halic(I) acids, but
in this case the halide ion is also present in the solution.
Attempts to concentrate chloric(V) and bromic(V) acids beyond
certain limits lead to decomposition which may be violent.
lodic(V) acid is prepared by oxidising iodine with concentrated
nitric acid:

312 4- 10HNO 3 -» 6HIO 3 + lONOt + 2H 2 O

The iodic acid(V) and some diiodine pentoxide separate out and the
iodic(V) acid is purified by recrystallization from hot water.
All the halic(V) acids are strong acids and their salts are not
appreciably hydrolysed in aqueous solution. They are also powerful
oxidising agents (see below).