432 THE ELEMENTS OF GROUPS IB AND MB
Gold(III) chloride dissolves in hydrochloric acid to form tetra-
chlorauric acid, HAuCl 4. Here again, the gold(III) is 4-co-ordinate
in the ion [AuQ 4 ]~. If alkali is added to this acid, successive
replacement of chlorine atoms by hydroxyl groups occurs, forming
finally the unstable tetrahydroxoaurate{III) ion, [Au(OH) 4 ] ~~:
[AuCl 4 ] ~ -+ [AuCl 3 OH] ~ -> [Au(OH) 4 ] "
This ion is very easily reduced to gold, and hence alkaline solu-
tions of chloraurates(III) (often wrongly called kgold chloride') are
used with a reducing agent to prepare colloidal gold.
Other than the fluoride, no compounds of gold(III) are known in
which gold acts as a metal ion, i.e. there are no gold(III) salts. There
are, however, numerous complexes of gold(III) which are 4-co-ordin-
ate, for example the compound diethyl gold(III) sulphate
[(C 2 H 5 ) 2 Au] 2 SO 4 .4H 2 O, which has the structure:AuA
H 5 C 2TESTS FOR GOLD COMPOUNDS
Gold compounds are all easily reduced in alkaline solution to
metallic gold which may occur in colloidal form and so be red,
blue or intermediate colours. Reduction to gold, followed by
weighing of the metal precipitated, may be used in quantitative
analysis.II (ZINC)f CADMIUM, MERCURYThese elements formed Group IIB of Mendeleef 's original periodic
table. As we have seen in Chapter 13, zinc does not show very
marked 'transition-metal' characteristics. The other two elements
in this group, cadmium and mercury, lie at the ends of the second
and third transition series (Y-Cd, La-Hg) and, although they
resemble zinc in some respects in showing a predominantly 4- 2
oxidation state, they also show rather more transition-metal
characteristics. Additionally, mercury has characteristics, some of
which relate it quite closely to its immediate predecessors in the
third transition series, platinum and gold, and some of which are
decidedly peculiar to mercury.