Page 56 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 RNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

For [CrOi-] = 0.01, [Ag+] = 41.7 x 10-"/1 x IO-'

This decrease in solubility by the common ion effect is of fundamental
importance in gravimetric analysis. By the addition of a suitable excess of a
precipitating agent, the solubility of a precipitate is usually decreased to so small
a value that the loss from solubility influences is negligible. Consider a specific
case - the determination of silver as silver chloride. Here the chloride solution
is added to the solution of the silver salt. If an exactly equivalent amount is
added, the resultant saturated solution of silver chloride will contain 0.0015 g per L
(Example 1). If 0.2 g of silver chloride is produced and the volume of the solution
and washings is 500 mL, the loss, owing to solubility, will be 0.00075 g or
0.38 percent of the weight of the salt; the analysis would then be 0.38 percent
too low. By using an excess of the precipitant, Say, to a concentration of 0.01 M,
the solubility of the silver chloride is reduced to 1.5 x IO-' g L-' (Example 4),
and the loss will be 1.5 x IO-' x 0.5 x 100/0.2 = 0.0038 percent. Silver chloride
is therefore very suitable for the quantitative determination of silver with high
accuracy.
It should, however, be noted that as the concentration of the excess of
precipitant increases, so too does the ionic strength of the solution. This leads
to a decrease in activity coefficient values with the result that to maintain the
value of K, more of the precipitate will dissolve. In other words there is a limit
to the amount of precipitant which can be safely added in excess. Also, addition
of excess precipitant may sometimes result in the formation of soluble complexes
causing some precipitate to dissolve.

In the previous section the solubility product principle has been used in
connection with the precipitation of one sparingly soluble salt. It is now necessary
to examine the case where two slightly soluble salts may be formed. For
simplicity, consider the situation which arises when a precipitating agent is
added to a solution containing two anions, both of which form slightly soluble
salts with the same cation, e.g. when silver nitrate solution is added to a solution
containing both chloride and iodide ions. The questions which arise are: which
Salt will be precipitated first, and how completely will the first Salt be precipitated
before the second ion begins to react with the reagent?
The solubility products of silver chloride and silver iodide are respectively
1.2 x 10-'0mo12 L-' and 1.7 x 10-'6mo12 L-'; i.e.
[Ag'] x [Cl-] = 1.2 x IO-''
[Ag'] x [I-] = 1.7 x 10-l6
It is evident that silver iodide, being less soluble, will be precipitated first since
its solubility product will be first exceeded. Silver chloride will be precipitated
when the Ag+ ion concentration is greater than

and then both salts will be precipitated simultaneously. When silver chloride