Figure 3.1
Distribution diagram of α against pH for
acid AH and its conjugate base A–.concept developed above is a versatile and widely used method of pH control. A buffer may be
prepared to meet a particular pH requirement by the use of standard pKa values in conjunction with
equation (3.21). This equation gives a good approximation provided that the solution contains at least
0.1 mol dm–^3 of the buffer and that the required pH lies close to the value of pKa. For example, if a
solution needs to be buffered to pH = 9, the ammonium ion (pKa = 9.62) can be used. Substitution in
equation (3.20) enables the necessary solution composition to be calculated
whence the ratio. A typical buffer solution might thus be prepared containing 1 mol
dm–^3 of NH 3 and 1.82 mol dm–^3 of ions. The pH obtained will be independent of the buffer
concentration provided that the ratio of acid to conjugate base is maintained constant. However if the
buffer solution is to stablize the pH when large amounts of H+ ions are involved it must be reasonably
concentrated (i.e. greater than 0.1 mol dm–^3 ).
In practice, a buffer solution (see Table 3.4) is prepared by the partial neutralization of the selected
weak acid or base with a suitable strong acid or base, or by the addition of the calculated amount of the
corresponding salt. The assumption is made that the salt is completely dissociated in solution, e.g. an
NH 3 + buffer may be produced by the partial neutralization of an ammonia solution with
hydrochloric acid or by the addition of the appropriate quantity of ammonium chloride to an initial
ammonia