Page 57 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

EFACT OF AClDS ON THE SOLUBlLlTY OF A PRECIPITATE 2.9

commences to precipitate, silver ions will be in equilibrium with both salts, and
equations (1) and (2) will be simultaneously satisfied, or

[I-] - Ks(Agl) 1.7 x 10-l6
and --- - -- - = 1.4 x
[Cl-] KSogc,) 1.2 x 10"
Hence when the concentration of the iodide ion is about one-millionth part of
the chloride ion concentration, silver chloride will be precipitated. If the initial
concentration of both chloride and iodide ions is 0.1 M, then silver chloride will
be precipitated when

Thus an almost complete separation is theoretically possible. The separation is
feasible in practice if the point at which the iodide precipitation is complete can
be detected. This may be done: (a) by the use of an adsorption indicator (see
Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see
Chapter 15).
For a mixture of bromide and iodide:

Precipitation of silver bromide will occur when the concentration of the bromide
ion in the solution is 2.0 x 103 times the iodide concentration. The separation
is therefore not so complete as in the case of chloride and iodide, but can
nevertheless be effected with fair accuracy with the aid of adsorption indicators
(Section 10.75(c)).

2.9 EFFECT OF AClDS ON THE SOLUBlLlTY OF A PRECIPITATE
For sparingly soluble salts of a strong acid the effect of the addition of an acid
will be similar to that of any other indifferent electrolyte but if the sparingly
soluble Salt MA is the Salt of a weak acid HA, then acids will, in general, have
a solvent effect upon it. If hydrochloric acid is added to an aqueous suspension
of such a salt, the following equilibrium will be established:

If the dissociation constant of the acid HA is very small, the anion A- will be
removed from the solution to form the undissociated acid HA. Consequently
more of the Salt will pass into solution to replace the anions removed in this
way, and this process will continue until equilibrium is established (i.e. until
[M +] x [A -1 has become equal to the solubility product of MA) or, if sufficient
hydrochloric acid is present, until the sparingly soluble Salt has dissolved
completely. Similar reasoning may be applied to salts of acids, such as
phosphoric(V) acid (KI = 7.5 x mol L-'; K, = 6.2 x 10-8mol L-';
K3=5 x 10-13molL-'), oxalic acid (KI =5.9 x 10-2molL-'; K2=6.4 x
10-'mol L-'), and arsenic(V) acid. Thus the solubility of, Say, silver
phosphate(V) in dilute nitric acid is due to the removal of the PO:- ion as