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

irritability of the various ionic species that are commonly used as buffer components. The pH of blood is maintained at about 7.4 by primary buffer components in the plasma (carbonic acid-carbonate and the acid-sodium salts of phosphoric acid) and secondary buffer components (oxyhaemoglobin-haemoglobin and acid-potassium salts of phosphoric acid) in the erythrocytes. Values of 0.025 and 0.039 mol dm 3 per pH unit have been quoted for the buffer capacity of whole blood. Parenteral solutions are not normally buffered, or alternatively are buffered at a very low capacity, since the buffers of the blood are usually capable of bringing them within a tolerable pH range.

Universal buffers

We have seen from Fig. 3.9 that the buffer capacity is at a maximum at a pH equal to the pKa of the weak acid used in the formulation of the buffer system and decreases appreciably as the pH extends more than one unit either side of this value. If, instead of a single weak monobasic acid, a suitable mixture of polybasic and monobasic acids is used, it is possible to produce a buffer which is effective over a wide pH range. Such solutions are referred to as universal buffers. A typical example is a mixture of citric acid (pKa1 = 3.06, pKa2 = 4.78, pKa3 = 5.40), Na2HPO4 (pKa of conjugate acid H2PO4 = 7.2), diethylbarbituric acid (pKa1 = 7.43) and boric acid (pKa1 = 9.24). Because of the wide range of pKa values involved, each associated with a maximum buffer capacity, this buffer is effective over a correspondingly wide pH range (pH 2.4-12).

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