Through the addition of common ions, the solubility of a compound generally decreases due to a shift in equilibrium.

Buffering solutions contain either an acid or base, accompanied by its conjugate counterpart.

Addition of more like conjugate ions will ultimately shift the pH of the solution.

For example, table salt (NaCl) placed in water eventually dissolves.

However, if more table salt is continuously added, the solution will reach a point at which no more can be dissolved; in other words, the solution is saturated, and the table salt has effectively reached its solubility limit.

This is because the rate of the forward (reactant to product) and reverse (product to reactant) reactions are equal.

The amount of NaCl that could dissolve to reach the saturation point would be lowered.

This phenomenon is the common ion effect and plays important roles in pharmaceutical and environmental areas.

For example, when calcium fluoride dissolves into calcium and fluoride ions, the solubility product expression is.

Therefore, if more latexCa2latex ions are placed in solution, the equilibrium will shift to the left, favoring the solid form and decreasing the solubility of the solid.

Addition of excess ions will alter the pH of the buffer solution.

Therefore, the common ion effect takes a role in pH regulation.

According to Le Chateliers principle, addition of more ions alters the equilibrium and shifts the reaction to favor the solid or deionized form.

In the case of an an acidic buffer, the hydrogen ion concentration decreases, and the resulting solution is less acidic than a solution containing the pure weak acid.

The percent dissociation of the hydrogen cyanide will decrease, therefore decreasing the H ions and increasing the pH of the solution.

Common Ion Effect On Solubility Pogil