380 THE TRANSITION ELEMENTS
Addition of hydrogen peroxide to a solution of a dichromate
yields the blue colour of 'peroxochromic acid\ This is a test for
soluble chromates and dichromates.
Chromates and dichromates are used in industry as oxidising
agents, for example in the coal tar industry, in the leather industry
(chrome tanning), and in the dye industry as mordants. Some
chromates are used as pigments, for example those of zinc and lead,
Chromates and dichromates are poisonous.Oxidation state + 3
This is the most common and stable state of chromium in aqueous
solution. The Cr^3 + ion, with 3d^3 electrons, forms mainly octahedral
complexes [CrX 6 ], which are usually coloured, and are kinetically
inert, i.e. the rate of substitution of X by another Ugand is very slow;
consequently a large number of such complexes have been isolated
(see below, under chromium(III) chloride).
CHROMIUM(IH) CHLORIDE, CrCl 3
Chromium(III) chloride is prepared in the anhydrous form:
- By the reaction of chlorine with a heated mixture of chrom
ium(III) oxide and carbon:
Cr 2 O 3 + 3C1 2 + 3C -> 3COT + 2CrCl 3- By the reaction of sulphur dichloride oxide with the hydrated
chloride:
CrCl 3 .6H 2 O 4- 6SOC1 2 -» CrCl 3 + 6SO 2 t + 12HC1T
Anhydrous chromium(III) chloride is a peach-coloured solid,
which is insoluble in water unless a trace of reducing agent is present.
Solution then occurs readily to give a green solution from which the
green hydrated chloride, CrCl 3 ,6H 2 O, can be crystallised out. If this
substance is treated with silver nitrate, only one third of the chlorine
is precipitated; hence the formula is [Crm(H 2 O) 4 Cl 2 ]+Cr .2H 2 O.
with two chloride ions as ligands in the complex ion. Two other
forms of formula CrCl 3 .6H 2 O are known; one is (pale green)
LCr(H2O) 5 Cl]^2 + [Cr] 2 .H 2 Olrom which silver nitrate precipitates
two thirds of the chlorine; and the other is [Cr(H 2 O) 6 ]Cl 3 (grey-blue)
from which all the chlorine is precipitated by silver nitrate. These
three compounds are isomers, and the cations can be represented
thus: