Background

A buffer, a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid, is a compound in a solution that resists changes in pH even if hydronium and hydroxide ions are being added to the solution. The buffer accomplishes this by reacting with and neutralizing any strong acids or bases that have been added and creating water and either the weak acid or base’s conjugate, thus effectively controlling the pH. The following reactions occur when hydronium or hydroxide ions are added to a buffer:

The buffer can regulate the pH as long as both the weak acid and conjugate base/weak base and conjugate acid are present in the solution; once either component is completely consumed, the buffer will no longer be effective.

Buffer solutions function based on the common ion effect. A buffer is only effective when the concentrations of weak acid and conjugate base are high relative to the concentrations of hydronium and hydroxide ions being added to the solution because when hydronium or hydroxide ions are added, the system will react completely with the hydronium or hydroxide ions to either produce more conjugate base (A-) or weak acid (HA) in the reactions given above. If the concentrations of conjugate base and weak acid are significantly higher than the added concentrations of hydronium or hydroxide ions, then the amount of conjugate base or weak acid formed by this disruption will be insignificant and will not shift the dissociation equilibrium of the buffer solution either direction:

The equivalence point of a titration is where the moles of titrant are equal to the moles of the analyte. The pKa, in an acid/base titration, is equal to the pH of the analyte solution halfway to the equivalence point, where the concentration of the acid is equal to the concentration of its conjugate base. The pKb is similar to the pKa in all ways except for that it is found only in base/acid titrations, when the analyte is a weak base.

In the introduction activity, we titrated 0.02 M citric acid with 0.1 M NaOH in order to learn about the buffer regions of titration curves. The pH range for the most effective buffering action of any weak acid is pKa +/- 1with a weak acid to conjugate base ratio as close to 1 as possible, as proven by the Henderson-Hasselbach equation:

Because the pH of a buffer system is equal to the pKa of the weak acid when the weak acid:conjugate base ratio is 1, the buffer system will maintain a pH around pKa +/-1, and so a buffer achieves the desired effect of inhibiting pH changes only if the desired pH is close to the pKa of the weak acid.

With this information, we determined the buffer region of this citric acid curve to be 3.00 – 5.00, since the pKa was 4.00 (see calculations). Even though citric acid is polyprotic, having three hydrogen ions attached to the citrate ion (H3C6H8O7), the titration curve does not have three equivalence points that physically show this; this might be due to the fact that the acids left behind after the first proton is removed are so weak that the addition of more base does not cause the other protons to strip away.