Common ion effect, hydrolysis and buffers

Common ion effect: if you add an ion that is already present in an equilibrium, the equilibrium shifts back (Le Chatelier), so the weak acid or base ionises less. Adding sodium acetate (which supplies acetate ions) to acetic acid suppresses the acid's ionisation. In a solution with equal concentrations of acetic acid and acetate, [H⁺] = Ka, so pH = pKa = 4.74.

Salt hydrolysis: a salt's ions may react with water, so salt solutions are not always neutral. The rule: the weak partner decides.

• Salt of a strong acid and a strong base (NaCl): neutral.
• Salt of a weak acid and a strong base (CH₃COONa, NaCN, KF): basic, because the anion takes protons from water, leaving OH⁻.
• Salt of a strong acid and a weak base (NH₄Cl): acidic, because NH₄⁺ gives protons to water.
• Salt of a weak acid and a weak base: pH = 7 + ½(pKa – pKb), close to 7 when the two are of similar strength.

The hydrolysis constant for the anion of a weak acid is Kh = Kw/Ka, and the degree of hydrolysis is h = √(Kh/c).

A buffer solution resists changes in pH when a little acid or base is added. It contains a weak acid with its salt (acetic acid + sodium acetate) or a weak base with its salt (ammonia + ammonium chloride). The weak acid mops up added OH⁻; its anion mops up added H⁺. Our blood is buffered in this way.

The pH of an acid buffer is given by the Henderson equation: pH = pKa + log([salt]/[acid]). When salt and acid are equal, pH = pKa. Similarly, for half-neutralised ammonia, pOH = pKb, giving pH = 9.25.