Page 69 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

THE HYDROLYSIS OF SALTS 2.18

maintain the equilibrium, the large initial concentration of A- ions must be
reduced by combination with H+ ions to form undissociated HA:

The hydrogen ions required for this reaction can be obtained only from the
further dissociation of the water; this dissociation produces simultaneously an
equivalent quantity of hydroxyl ions. The hydrogen ions are utilised in the
formation of HA; consequently the hydroxide ion concentration of the solution
will increase and the solution will react alkaline.
It is usual in writing equations involving equilibria between completely
dissociated and slightly dissociated or sparingly soluble substances to employ
the ions of the former and the molecules of the latter. The reaction is therefore
written:

This equation can also be obtained by combining (i) and G), since both
equilibria must CO-exist. This interaction between the ion (or ions) of a Salt and
water is called 'hydrolysis'.
Consider now the Salt of a strong acid and a weak base {class (3)). Here
the initial high concentration of cations M+ will be reduced by combination
with the hydroxide ions of water to form the little-dissociated base MOH until
the equilibrium:

is attained. The hydrogen ion concentration of the solution will thus be
increased, and the solution will react acid. The hydrolysis is here represented by :

For salts of class (4), in which both the acid and the base are weak, two
reactions will occur simultaneously

The reaction of the solution wiil clearly depend upon the relative dissociation
constants of the acid and the base. If they are equal in strength, the solution
will be neutral; if Ka > K,, it will be acid, and if K, > Ka, it will be alkaline.
Having considered al1 the possible cases, we are now in a position to give a
more general definition of hydrolysis. Hydrolysis is the interaction between an
ion (or ions) of a Salt and water with the production of (a) a weak acid or a
weak base, or (b) of both a weak acid and a weak base.
The phenomenon of Salt hydrolysis may be regarded as a simple application
of the general Br~nsted-Lowry equation

Thus the equation for the hydrolysis of ammonium salts

is really identical with the expression used to define the strength of the
ammonium ion as a Br~nsted-Lowry acid (see Section 2.4) and the constant
Ka for NH; is in fact what is usually termed the hydrolysis constant of an
ammonium salt.