Sample Buffer Calculation

Suppose a biochemist wishes to study a reaction at pH 4.00. To prevent the pH from drifting during the reaction, the experimenter should use a buffer solution consisting of a specific mixture of a weak acid and its conjugate base. From Table 2.6, one discovers the pKa of formic acid (3.75) is close to the pH value required. An acetic acid-acetate mixture would not be so satisfactory, because the pKa of acetic acid (4.76) is nearly 1 pH unit away. The ratio of formate ion to formic acid required can be calculated from the Henderson - Hasselbalch equation:

which can be rewritten as

This equation tells us then that one can make a formate buffer at pH 4.0 by using 0.1 M formic acid (HA = HCOOH) and 0.178 M sodium formate (A- = HCOO-). Alternatively, exactly the same solution could be prepared by titrating a 0.1 M solution of formic acid to pH 4.00 with sodium hydroxide.

See also: Henderson-Hasselbalch Equation, pKa