Figure 3.9 shows the variation of buffer capacity with pH for the acetic acid-acetate buffer used in the numerical examples above (c0 = 0.3754 mol dm 3) as calculated from equation (3.89). It should be noted that fi is at a maximum when pH = pKa (that is, at pH 4.76). When selecting a weak acid for the preparation of a buffer solution, therefore, the chosen acid should have a pKa as close as possible to the pH required. Substituting pH = pKa into equation (3.89) gives the useful result that the maximum buffer capacity is fi max = 0.576c 0, where c0 is the total buffer concentration.

Buffer solutions are widely used in pharmacy to adjust the pH of aqueous solutions to that required for maximum stability or that needed for optimum physiological effect. Solutions for application to delicate tissues, particularly the eye, should also be formulated at a pH not too far removed from that of the appropriate tissue fluid, as otherwise irritation may be caused on administration. The pH of tears lies between 7 and 8, with an average value of 7.4. Fortunately, the buffer capacity of tears is high and, provided that the solutions to be administered have a low buffer capacity, a reasonably wide range of pH may be tolerated, although there is a difference in the

Figure 3.9 Buffer capacity of acetic acid-acetate buffer (initial concentration = 0.3754 mol dm-3) as a function of pH.

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