The pH of salt solutions

The pH of a salt depends on whether the acid and base from which it is formed are strong or weak.

In general, the salt of a strong acid and weak base will form an acidic solution when dissolved in water.

Example: ammonium nitrate.

When ammonium nitrate dissolves in water, the following two equations are involved in producing the ions present in the solution:

NH₄NO₃(s) + H₂O(l)→NH₄⁺(aq) + NO₃^-(aq)

H₂O(l)OH^-(aq) + H⁺(aq)

The NH₄⁺(aq) and OH^-(aq) ions produced are involved in an additional equilibrium for a weak base. The equilibrium position lies towards the formation of NH₃(aq):

NH₄⁺(aq) + OH^-(aq)NH₃(aq) + H₂O(l)

This additional equilibrium removes hydroxide ions, but not hydrogen ions. The water equilibrium shifts to the right, to replace these lost hydroxide ions and, in so doing, also produces more hydrogen ions. There are now more hydrogen ions than hydroxide ions present, leading to a pH of less than 7.

The salt of a weak acid and strong base will form an alkaline solution when dissolved in water.

Example: sodium ethanoate.

When sodium ethanoate dissolves in water, the following two equations are involved in producing the ions present in the solution:

CH₃COONa(s) + H₂O(l)→Na⁺(aq) + CH₃COO^-(aq)

H₂O(l)OH (aq)^- + H⁺(aq)

The H⁺(aq) and CH₃COO^-(aq) ions produced are involved in an additional equilibrium for a weak acid. The equilibrium position lies towards the formation of CH₃COOH(aq):

CH₃COO^-(aq) + H⁺(aq)CH₃COOH(aq)

This additional equilibrium removes hydrogen ions, but not hydroxide ions. The water equilibrium shifts to the right, to replace these lost hydrogen ions and, in so doing, also produces more hydroxide ions. There are now more hydroxide ions than hydrogen ions present, leading to a pH of more than 7.

Soaps are also salts of weak acids (such as stearic and oleic acid) and strong bases (such as sodium and potassium hydroxide). As a result, soaps are usually alkaline in nature.

The salt of a strong acid and strong base will form a neutral solution when dissolved in water

Example: sodium chloride.

When sodium chloride dissolves in water, the following two equations are involved in producing the ions present in the solution:

NaCl(s) + H₂O(l)→Na⁺(aq) + Cl^-(aq)

H₂O(l)OH^-(aq) + H⁺(aq)

The ions present are not involved in an equilibrium associated with a weak acid or base.

The equal proportions of hydrogen ions and hydroxide ions in the solution are not disturbed by additional equilibria, and so the pH of the solution is equal to 7.