The common ion effect
When a saturated solution of a sparingly soluble salt is produced by adding the solid to water, the relative concentrations of the ions in solution depend on the formula of the salt.
How are the concentrations of Pb2+ ions and SO42– ions related when lead(II) sulfate, PbSO4, is added to water?
The concentrations of Pb2+ and SO42– ions will be equal.
How are the concentrations of Ca2+ and F– ions related when calcium fluoride, CaF2, is added to water?
The concentration of F– ions will be twice the concentration of Ca2+ ions.
Sometimes, however, the presence of other compounds already dissolved in the water can significantly affect the solubility of a sparingly soluble salt.
The solubility, for example, of calcium hydroxide in solutions such as sodium hydroxide or calcium chloride is much less than in water. This is because the solutions already contain hydroxide ions or calcium ions. These are called common ions and the impact of these ions is known as the common ion effect.
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We can understand the common ion effect better by looking at the way that common ions affect the position of equilibrium of calcium hydroxide.
The solubility equation for calcium hydroxide is:
Ca(OH)2(s) Ca2+(aq) + 2OH–(aq)
If Ca2+ or OH– ions are added to the system, what happens to the position of equilibrium?
The position of equilibrium moves from products to reactants.
How will this affect the solubility of calcium hydroxide?
The calcium hydroxide will be less soluble in the presence of Ca2+ or OH– ions.
The effect of common ions on the solubility of calcium hydroxide can be quantified. Two saturated solutions can be created by dissolving solid calcium hydroxide in water and dissolving solid calcium hydroxide in a calcium chloride solution. The concentration of the hydroxide ions in each saturated solution can be determined and compared.
How might you determine the concentration of hydroxide ions in the saturated solutions?
The concentration of hydroxide ions can be determined by titration with dilute hydrochloric acid solution.
The common ion effect can be generalised to say 'in the presence of A+ and B– ions from a second source, the solubility of the salt AB is reduced'.