Modern inorganic chemistry

340 GROUP VII: THE HALOGENS

HALATE(V) SALTS

Generally the solubility of a given metal halate decreases from
chlorate(V) to iodate(V) and many heavy metal iodates(V) are
quantitatively insoluble. Like their parent acids, the halates(V) are
strong oxidising agents, especially in acid solution: their standard
electrode potentials are given below (in volts):

x = Cl Br I

Acid solution:

XO 3 ~(aq) + 6H 3 O+ + 6e~ -> X"(aq) + 9H 2 O +1.45 +1.67 +1.19

Alkaline solution:

XO 3 "(aq) 4- 3H 2 O + 6e~ -* 6OH~(aq) + X" 4-0.62 +0.61 +0.26

Unexpectedly we find that the bromate(V) ion in acid solution
(i.e. effectively bromic(V) acid) is a more powerful oxidising agent
than the chlorate(V) ion, C1OJ. The halates(V) are thermally
unstable and can evolve oxygen as one of the decomposition
products. Potassium chlorate(V), when heated, first melts, then
resolidifies due to the formation of potassium chlorate(VII) (per-
chlorate):
4KC1O 3 -* 3KC1O 4 + KC1

but a further, stronger heating will make the chlorate(VII) de-

compose, evolving oxygen:

KC1O 4 -» KC1 + 2O 2

The decomposition of potassium chlorate(V) is catalysed by
manganese(IV) oxide, MnO 2 , and oxygen is evolved on heating the
mixture below the melting point of the chlorate(V),
The ability of the solid chlorates(V) to provide oxygen led to
their use in matches and fireworks. Bromates(V) and iodates(V)
are used in quantitative volumetric analysis. Potassium hydrogen
diiodate(V), KH(IO 3 ) 2 , is used to standardise solutions of sodium
thiosulphate(VI) since in the presence of excess potassium iodide
and acid, the reaction

IOJ + 51" + 6H+ -» 3I 2 + 3H 2 O

occurs quantitatively. The liberated iodide is then titrated using the
thiosulphate solution of which the concentration is required: