GROUP VII: THE HALOGENS 319
obtained. The bromine may be freed from chlorine by bubbling it
through iron(III) bromide solution, which retains the chlorine. Last
traces of bromine from the process can be removed by passing over
moist iron filings. Bromine is a dark-red heavy liquid, boiling point
332 K, appreciably volatile at ordinary temperatures. It is soluble
in organic solvents, for example chloroform, and they can be used
to extract bromine from aqueous solutions (see Tests, p. 349).
IODINE
Iodine occurs to a minute extent (less than 0.001 %) in sea water, but
is found in greater concentration, combined in organic form, in
certain seaweeds, in oysters and in cod livers. Crude Chile saltpetre,
or caliche contains small amounts of sodium iodate, NaIO 3 , from
which iodine can be obtained (see below). Some insoluble iodides.
for example tiiose of silver and mercury(II), occur in Mexico. Iodine
is found in the human body in the compound thyroxin in the thyroid
gland; deficiency of iodine in diet causes enlargement of this gland
(goitre).
Iodine is rarely prepared in the laboratory; the method used is
the oxidation of an iodide by manganese(IV) oxide and sulphuric
acid, for example with sodium iodide:
2NaI + MnO 2 + 3H 2 SO 4 -> MnSO 4 + 2NaHSO 4 + I 2 + 2H 2 O
The iodine distils off and can be collected on a cooled surface. It
may be purified by sublimation in vacuo.
This reaction is also used on a large scale, to obtain iodine from
seaweed. The ash from burnt seaweed ("kelp^1 ) is extracted with
water, concentrated, and the salts other than iodides (sulphates and
chlorides) crystallise out. The more soluble iodides remain and the
liquor is mixed with sulphuric acid and manganese dioxide added;
the evolved iodine distils off and is condensed.
Most iodine produced commercially comes from the sodium
iodate(V) remaining after sodium nitrate has been crystallised from
Chile saltpetre. The iodate(V) is first reduced to iodide by blowing
sulphur dioxide into the solution (or by addition of sodium sulphite):
ioj + ssor ->r + ssoj-
More iodate is then added, and with the sulphuric acid formed
(or added if sodium sulphite is used), iodine is liberated :
IO + 51" + 6H+ -* 3I 2 4- 3H 2 O