Page 120 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Factors affecting solubility of precipitate

1. Common-ion effect

+ -
AgCl  Ag + Cl

+ -
AgNO → Ag + NO
3 3
➢ A common ion is one of the respective ions of sparingly soluble salt in its saturated
solution.
➢ Adding AgNO (or NaCl) to a saturated solution of AgCl will cause an increase in
3
+ -
Ag (or Cl ) concentration which will make the ionic product more than K .
sp
+ -
Q = [Ag ] [Cl ] > K
sp
+ -
➢ Excess Ag (or Cl ) disturbs the equilibrium.
➢ To reestablish equilibrium, some AgCl will precipitate out of the solution, until the
ionic product is again equal to K .
sp
➢ So, The effect of adding a common ion, is a decrease in the solubility of AgCl and

increase its precipitation, however the equilibrium constant will remain the same.

+ -
[Ag ] [Cl ] = Ksp

2. Effect of pH

➢ The solubility of many substances is affected by changes in the pH of the solution.
Example:
2+ -
Mg(OH)  Mg + 2 OH
2
-
Excess OH

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