pH and Buffers

Abstract

Many biomolecules possess weakly acidic or weakly basic groups whose ionization state can affect both biological function and stability, and many biological processes, such as oxidation–reduction reactions, produce or consume hydrogen ions. The extents of the reactions and the rates at which they proceed depend on the acidity of the reaction environment. Buffers are used to maintain a constant hydrogen ion activity.

Keywords: weak acid; weak base; pH; buffer; temperature; ionic strength

Figure 1.

Titration of 0.1 mol L−1 HCl (a strong acid) with NaOH. The pH at the start is about 1 because HCl is nearly 100% dissociated ([H+] = 10−1 mol L−1). The pH at any point along the curve is calculated from the concentration of HCl remaining at that point. At the equivalence point, the pH is 7 because the solution contains only NaCl, which, being a salt of a strong acid and a strong base, does not act as an acid or a base. Beyond the equivalence point, the pH depends on the concentration of excess OH present.

Figure 2.

Titration of 0.1 mol L−1 HA (a weak acid, pKa = 6) with NaOH. The starting pH is calculated from eqn or . The pH at any point along the titration curve is calculated from eqn . If dilution is negligible (because concentrated or solid NaOH is used), the pH at the equivalence point is that of a 0.1 mol L−1 solution of A, a weak base (eqns or and ).

Figure 3.

Titration curve of 0.1 mol L−1 diprotic weak acid (pKa1 = 4; pKa2 = 7). The starting pH depends almost exclusively on the dissociation of H2A and can be calculated from eqns or using pKa1. When half of the original H2A has been converted to HA, pH = pKa1. The predominant species at the first equivalence point is HA (∼94% of the total), where the pH is the average of the two pKa values. When half of the HA has been converted to A2−, pH = pKa2. If the volume does not change, the pH at the endpoint is that of a 0.1 mol L−1 solution of the base A2− (eqns or and ).

Figure 4.

Titration of 0.05 mol L−1 Tris (pKa = 8.1) with 1 mol L−1 HCl. The volume at the start is assumed to be 1.0 litre. Note that the pH 7.5 buffer has a rather poor practical buffer capacity in the acid direction.

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Further Reading

Blanchard JS (1984) Buffers for enzymes. In: Jakoby WB (ed.) Methods in Enzymology, vol. 104C, pp. 404–414. New York: Academic Press.

Good NE and Isawa S (1972) Hydrogen ion buffers. In: San Pietro A (ed.) Methods in Enzymology, vol. 24B, pp. 53–68. New York: Academic Press.

Good NE, Winget GD, Winter W, Connolly TN, Izawa S and Singh RMM (1966) Hydrogen ion buffers for biological research. Biochemistry 5: 467–477.

Segel IH (1976) Biochemical Calculations, 2nd edn. New York: Wiley.

Sigma‐Aldrich (1999) Biological buffers. In: Biochemicals and Reagents for Life Science Research (Catalog), p. 1910. Poole, UK: Sigma‐Aldrich Company Ltd.

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Segel, Irwin H, and Segel, Leigh D(Aug 2001) pH and Buffers. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0003117]