Page 78 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION
complex ion is formed by the union of a simple ion with either other ions of
opposite charge or with neutral molecules as shown by the following examples.
When potassium cyanide solution is added to a solution of silver nitrate, a
white precipitate of silver cyanide is first formed because the solubility product
of silver cyanide:
1
[Ag ' x CCN - 1 = K~(A~cN) (27)
is exceeded. The reaction is expressed:
CN - + Ag ' = AgCN
The precipitate dissolves upon the addition of excess of potassium cyanide, the
complex ion [Ag(CN),] - being produced:
AgCN(so1id) + CN-(excess) = [Ag(CN),] -*
(or AgCN + KCN = K [Ag(CN),] - a soluble complex salt)

This complex ion dissociates to give silver ions, since the addition of
sulphide ions yields a precipitate of silver sulphide (solubility product
1.6 x mol3 L-3), and also silver is deposited from the complex cyanide
solution upon electrolysis. The complex ion thus dissociates in accordance with
the equation:
[Ag(CN),] - = Ag' + 2CN-
Applying the Law of Mass Action, we obtain the dissociation constant of the
complex ion:
[Ag'] x [CN-l2
C{Ag(CN)2) -1 = Kdiss
which has a value of 1.0 x mol2 L-2 at the ordinary temperature. By
inspection of this expression, and bearing in mind that excess of cyanide ion is
present, it is evident that the silver ion concentration must be very small, so
small in fact that the solubility product of silver cyanide is not exceeded.
The inverse of equation (28) gives us the stability constant or formation
constant of the complex ion:

K= [{Ag(CN)2} -1 = 1021 ~2
[Ag'] x [CN-l2
Consider now a somewhat different type of complex ion formation, viz. the
production of a complex ion with constituents other than the common ion
present in the solution. This is exemplified by the solubility of silver chloride
in ammonia solution. The reaction is:

AgCl + 2NH = [Ag(NH 3)2] ' + Cl -
Here again, electrolysis, or treatment with hydrogen sulphide, shows that silver

*Square brackets are commonly used for two purposes: to denote concentrations and also to
include the whole of a complex ion; for the latter purpose curly brackets (braces) are sometimes
used. With careful scrutiny there should be no confusion regarding the sense in which the square
brackets are used: with complexes there will be no charge signs inside the brackets.