Engineering Chemistry S Datta

TRANSITION METAL CHEMISTRY 293
Double Salts and Complex Salts
A double salt is formed generally when molar proportions of a monovalent cation
sulphate solution in water is mixed with trivalent cation sulphate solution in water and
evaporated to remove excess water, and the salt crystallises as X 2
SO 4
Y 2
(SO 4
) 3
.24H 2
O which is
the general formula of a common alum, where ‘X’ is monovalent cation, like NH 4+
, K + , Na + , and
‘Y’ is trivalent cation as Al +3 , Cr +3 , Fe +3 etc. Another important double salt is Mohr’s salt,
(NH 4
) 2
SO 4
.FeSO 4
.6H 2
O. A double salt exists as a distinct entity only in the solid crystalline
state, it loses its entity in solution. The double salt is characterised by complete dissociation in
solution and gives the reactions of all the ions formed by the salts in solutions, viz.
K 2
SO 4
.Al 2
(SO 4
) 3
.24H 2
O in solution gives the reactions of K + , Al +3 and SO
–2
4
ions.
A complex salt does not give all the ions of its constituents in solution. A complex
salt K 4
[Fe(CN) 6
] is prepared by adding KCN solution of FeSO 4
solution and boiling.
FeSO 4
+ 2KCN = Fe(CN) 2
+ K 2
SO 4
Fe(CN) 2
+ 4KCN = K 4
[Fe(CN) 6
]
The solution of K 4
[Fe(CN) 6
] does not respond to the tests for Fe +2 or CN – ions, but responds
to a test for a new ion [Fe(CN) 6
] –4 (ferrocyanide ion). This ion remains intact even in solution
K 4
[Fe(CN) 6
] 4K + + [Fe(CN) 6
] –4
and gives its distinctive reactions. So, it can be said, the complexions, like ferrocyanide ion, are
quite stable in the solid state as well as in solution. A complex salt is different from a double
salt in respect of its distinctive character even in solution.
Examples of complex salts
[Ag(NH 3
) 2
]Cl [Ag(NH 3
) 2
] ++ + Cl –
[Cu(NH 3
) 4
]SO 4
[Cu(NH 3
) 4
] ++ + SO 4
K[Ag(CN) 2
] K + + [Ag(CN) 2
] –
Instability Constants of Complex Salts
The dissociation of a complex ion, like ionisation of weak electrolyte, follows the law of
mass action. The dissociation constant is called instability constant (K ins
) and is a measure of
stability of a complex, the lower the value of K ins
, the higher is the stability of the complex and
vice versa.
K ins
=
+ −
[M ] [L ]
[ML]
Complex ions
Instability constant
[Ag(NH 3
) 2
] + Ag + + 2NH 3
6.8 × 10 –8
[Ag(CN) 2
] – Ag + + 2CN – 1.0 × 10 –21
[Cu(CN) 4
] –3 Cu + + 4CN – 5.0 × 10 –28
[Cd(CN) 4
] –2 Cd +2 + 4CN – 1.4 × 10 –17
Werner’s Theory for Formation of Complex Compounds
Werner put forward a theory for the formation of complex compounds. The theory is known
also as co-ordination theory. The postulates of his theory may be summarised as follows:
• Metals possess two types of valencies: (i) Primary or principal or ionisable valencies
(ii) Secondary or subsidiary or non-ionisable valencies.