Buffer Solutions

BUFFER SOLUTIONS A buffer solution is a solution which can resist changes in pH on addition of small quantities of acid or alkali or on dilution.

1. How buffers work

A buffer solution is a mixture of an acid and an alkali. The acid neutralises the alkali added and the alkali neutralises the acid added. The mixture cannot be a mixture of a strong acid and a strong alkali, or the two will react with each other (eg a mixture of NaOH and HCl would react with each other and thus not behave as an effective buffer). If the acid and alkali in the buffer are too weak, however, they will not react effectively with the acid or alkali that are added. A suitable mixture is one which contains a mixture of acid and alkali strong enough to react with H3O+ and OH-, but weak enough not to react with each other. An ideal mixture for this purpose is a mixture of a weak acid and its conjugate base. For example CH3COOH and CH3COONa, or NH4Cl and NH3. The acids and bases in these mixtures will react with OH- and H3O+ respectively but not with each other.

a) Resisting pH change on addition of acid and alkali These mixtures will react with acid and alkali as follows:

CH3COOH(aq) + OH-(aq) ->CH3COO-(aq) + H2O(l) CH3COO-(aq) + H3O+(aq) ->CH3COOH(aq) + H2O(l) NH4+(aq) + OH-(aq) ->NH3(aq) + H2O(l) NH3(aq) + H3O+(aq) ->NH4+(aq) + H2O(l)

b) Resisting pH change on dilution On dilution, both the weak acid and the weak base can dissociate more to compensate for the dilution: CH3COOH(aq) + H2O(l) ->CH3COO-(aq) + H3O+(aq) CH3COO-(aq) + H2O(l) ->CH3COOH(aq) + OH-(aq) NH4+(aq) + H2O(l) ->NH3(aq) + H3O+(aq) NH3(aq) + H2O(l) ->NH4+(aq) + OH-(aq)