378 THE TRANSITION ELEMENTS
THECHROMATES(Vl)
The chromates of the alkali metals and of magnesium and calcium
are soluble in water; the other chromates are insoluble. The
chromate ion is yellow, but some insoluble chromates are red (for
example silver chromate, Ag 2 CrO 4 ). Chromates are often isomorph-
ous with sulphates, which suggests that the chromate ion, CrOj",
has a tetrahedral structure similar to that of the sulphate ion, SO|~
Chromates may be prepared by oxidising chromium(III) salts; the
oxidation can be carried out by fusion with sodium peroxide, or
by adding sodium peroxide to a solution of the chromium(III) salt.
The use of sodium peroxide ensures an alkaline solution; otherwise,
under acid conditions, the chromate ion is converted into the orange-
coloured dichromate ion:2CrOt~ + 2H+ ^±Cralkali L (^2) 'Or 2l + H 2 O
and certain metal ions
The dichromate ion has the following geometrical structure (single
lines not necessary implying single bonds):
O O
I
Cr—O—Cr\
CT / \ O
O O
i.e. two tetrahedral CrO 4 groups joined by a common oxygen atom.
If a metal ion of an insoluble chromate is added to a solution
containing the dichromate ion, the chromate is precipitated; for
example with a soluble lead(II) salt:
2Pb^2 + + Cr 2 O?- + H 2 O -» 2PbCrO 4 l + 2H +
yellow precipitate
of lead chromate
Sodium dichromate is prepared on the large scale by heating
powdered chromite with sodium carbonate, with free access of air;
the sodium chromate first formed is treated with acid:
4FeCr 2 O 4 4- 8Na 2 CO 3 4- 7O 2 -> 8Na 2 CrO 4 + 2Fe 2 O 3 + 8CO 2 T
2Na 2 CrO 4 + H 2 SO 4 -> Na 2 SO 4 + Na 2 Cr 2 O 7 + H 2 O
Sodium sulphate crystallises out in hydrated form (common ion
effect) and is filtered off; on concentration, sodium dichromate is
obtained. For analytical purposes, the potassium salt. K 2 Cr 2 O~.
is preferred; potassium chloride is added and the less soluble
potassium dichromate obtained.
The dichromate ion is a useful oxidising agent in acid solution,
and is used in volumetric analysis: