Buffers

A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid; such mixtures are effective in resisting change in pH when strong base or acid is added to a solution. By definition a weak acid or a weak base is one that only slightly dissociates when dissolved in water. Acetic acid is a weak acid whose behavior can be described by the following equilibrium expression and acid dissociation constant.

If one mole of acetic acid is added to water with a final volume of 1 Liter, then only about 0.4% of the acid is dissociated. If a strong base is added to a weak acid, the acid dissociates, releasing a proton (H+) to neutralize the base. In this way, the weak acid decreases the change in pH that results from adding the base.

If 0.010 moles of NaOH (a strong base) are added to pure water (final volume 1 Liter), the pH of the solution will be 12. On the other hand, if 0.010 moles of NaOH are added to a solution of 0.010 M acetic acid (final volume 1 Liter), the pH increases from 3.2 to 4.76, according to the Henderson-Hasselbalch equation.

To prepare a 0.010M sodium acetate buffer, pH 4.76, you need to add NaOH to a concentration of 0.010 M to a 0.010 M acetic acid solution. This is normally done by adding 0.010 moles of acetic acid to 900 mL of water, then adding 0.010 moles of NaOH gradually while checking the pH with a pH meter. The solution is then brought to a final volume of 1 Liter in a volumetric flask. It is important to check the pH with a calibrated meter because pKA values for weak acids and bases vary with temperature and salt concentration, so the values predicted by the Henderson-Hasselbalch equation are only approximate. Another way to prepare a 0.010 M sodium acetate buffer, pH 4.76, is to mix equal volumes of 0.010 M acetic acid and 0.010 M sodium acetate.

A plot of pH vs. equivalents of NaOH added to a solution of acetic acid shows that it is most effective in buffering when the pH is close to the pKA.

Although this application offers a choice of only three buffers, other buffers with a wide range of pKA values are available to allow the preparation of effective buffers at almost any pH value. In selecting a buffer, it is important to consider sensitivity to change in temperature, concentration and ionic strength, as these all may affect the pKA for the buffer.

Related topics