POTASSIUM CHROMATE AND DICHROMATE 321In Group VIII, each position instead of being filled by a
single element is occupied by a group of three elements. Thus
there appear in triads: iron, cobalt, and nickel; ruthenium,
rhodium, and palladium; and osmium, iridium, and platinum.
In this group there is no subdivision into families, but all the
members are heavy metals.
Of the heavy metals discussed above, the ones that are of
most frequent occurrence and that are to receive detailed treat-
ment in this chapter are chromium, manganese, and iron.PREPARATION 61
POTASSIUM CHROMATE AND DICHROMATE, K 2 Cr04 AND K 2 Cr 207
The most important source of chromium is the mineral chromite,
FeO-Cr 2 O 3 or Fe(CrO 2 ) 2. This substance, as indicated by the
formula, may be regarded as a compound of ferrous oxide and
chromic oxide, in which ferrous oxide is the basic constituent and
chromic oxide the acidic. Chromite is a difficult material to
decompose, and the ordinary method by which this is done is
treatment at a high temperature with an alkali and an oxidizing
agent. The iron is thereby converted to the ferric condition
(Fe 2 O 3 ), and the chromium is oxidized to the sexivalent con-
dition (CrO 3 ), at the same time combining with the alkali to form
a chromate.
In the commercial method for manufacturing chromates, atmos-
pheric oxygen is the oxidizing agent. The chromite is mixed
with sodium carbonate and calcium carbonate, the latter to give
porosity, and then heated for a considerable time in a furnace
with free access of air. Treatment of the cooled furnace product
with water causes a metathesis between the CaCrO* and Na2CO 3 ,
and Na^CrC^ is obtained in the aqueous extract. The exact
amount of H 2 SO 4 is added: 2Na2CrO 4 + H 2 SO 4 -> Na 2 Cr 207 +
Na^SC^ + H 2 O. By a carefully regulated procedure the
is crystallized from the solution of the very soluble
and lastly the latter salt itself separates on cooling. Chromates
and dichromates bear the same relation to each other as sulphates
and acid sulphates, Na 2 SO 4 and NaHSO 4 ; acid chromate and acid
sulphate differ by the ease with which the former loses water,
2NaHCrO 4 ?± Na 2 Cr 207 + H 2 O