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Buffer Review

Buffer Review

The Henderson-Hasselbach approximation is also useful in calculating the composition of buffer solutions. Remember that buffer solutions are composed of a weak acid and its conjugate base. Consider the equilibrium acetic acid, which is a weak acid, and its conjugate base, the acetate ion:

CH3CO2H + H2O ↔ H3O+ + CH3CO2-

If the buffer solution contains equal concentrations of acetic acid and acetate, the pH of the solution is:

\[pH = pK_a + log \dfrac {[A]}{[HA]} = 4.7 + log\ 1 = 4.7\]

A look at the titration curve for the carboxyl group of Gly (see above) shows that when the pH = pKa, the slope of the curve (i.e. the change in pH with addition of base or acid) is at a minimum. As a general rule of thumb, buffer solution can be made for a weak acid/base in the range of +/- 1 pH unit from the pKa of the weak acids. At the pH = pKa, the buffer solution best resists addition of either acid or base, and hence has its greatest buffering ability. The weak acid can react with added strong base to form the weak conjugate base, and the conjugate base can react with added strong acid to form the weak acid (as shown below) so pH changes on addition of strong acid and base are minimized.

  • addition of strong base produces weak conjugate base: CH3CO2H + OH-  → CH3CO2- + H2O
  • addition of strong acid produces weak acid: H3O+ + CH3CO2 → CH3CO2H + H2O

There are two simple ways to make a buffered solution. Consider an acetic acid/acetate buffer solution.

  1. Make equal molar solution of acetic acid and sodium acetate, and mix them, monitoring pH with a pH meter, until the desired pH is reached (+/- 1 unit from the pKa).
  2. Take a solution of acetic acid and add NaOH at substoichiometric amounts until the desired pH is reached (+/- 1 unit from the pKa). In this method you are forming the conjugate base, acetate, on addition of the weak base:

CH3CO2H + OH- → CH3CO2- + H2O