1.4: pH Buffers
Controlling pH is critically important in the qualitative analysis of cations. Often, pH needs to be maintained in a narrow range.
A pH buffer is an aqueous solution consisting of a weak acid and its conjugate base or vice versa. It minimizes the pH change when a small amount of a strong acid or a strong base is added to it.
For example, adding 0.020 mol \(\ce{HCl}\) into 1 L of water changes pH from 7 to 1.7, i.e., about 80% change in pH. Similarly, adding 0.020 mol \(\ce{NaOH}\) to the same water changes pH from 7 to 12.3, i.e., about 80% change in pH. In contrast to pure water, 1 L of buffer solution containing 0.50 mol a week acid acetic acid (\(\ce{CH3COOH}\)) and 0.50 mol of its conjugate base \(\ce{CH3COO^-}\) changes pH from 4.74 to 4.70 by the addition of the same 0.020 mol \(\ce{HCl}\) and from 4.74 to 4.77 by the addition of 0.020 mol \(\ce{NaOH}\), i.e., about 1% change in pH, as illustrated in Fig. 1.7.1.
The buffer contains a weak acid and its conjugate base in equilibrium. For example, acetic acid/sodium acetate buffer has the following equilibrium:
\[\ce{CH3COOH + H2O <<=> H3O^{+} + CH3COO^{-}}\nonumber\]
The molar concentration of hydronium ions [\(\ce{H3O^+}\)] defines the pH of the solution, i.e., \(\mathrm{pH}=-\log \left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\). The conjugate base consumes any strong acid added to the mixture:
\[\ce{HA + CH3COO^{-} -> CH3COOH + A^{-}}\nonumber\]
, where \(\ce{HA}\) is any strong acid and \(\ce{A^-}\) is its conjugate base. The concentration of \(\ce{CH3COOH}\) increases and \(\ce{CH3COO^-}\) decrease, but pH decreases little because [\(\ce{H3O^+}\)] is almost not affected. Similarly, the weak acid consumes any strong base added.
\[\ce{MOH + CH3COOH -> CH3COO^{-} + M^{+} + H2O}\nonumber\]
, where \(\ce{M^+}\) is its conjugate acid. The concentration of \(\ce{CH3COOH}\) decreases and \(\ce{CH3COO^-}\) increases, but pH increases little because [\(\ce{H3O^+}\)] is almost not affected. Buffers are employed on several occasions during the qualitative analysis of cations.