BUFFER SOLUTIONS AND THE HENDERSON- HASSELBALCH EQUATION
Buffer solutions are solutions that protect against large shifts in pH in the event of a shock due to the sudden addition of a strong acid or base. The pH of a buffer solution does, however, change slightly. Such a solution can be made with a combination of either
- • A weak acid and its salt (conjugate base) or
- • A weak base and its salt (conjugate acid).
Thus, acetic acid plus sodium acetate constitutes a buffer system, as does ammonium hydroxide plus ammonium chloride.
To determine the pH of a buffer solution, the Henderson- Hasselbalch equation may be used. In the case of a weak acid (e.g., CH3COOH) and its conjugate base salt (e.g., NaCH3COO):
The buffering capacity of a system refers to the amount of acid or base it can absorb before its pH changes. Buffering capacity is generally at a maximum when [weak acid] = [conjugate base], so that pH = pKa.
Amphoterism refers to the property of certain compounds to act as both acids and bases. For example, the compound Al(0H)3, which is categorically a base, may react with acid to form [Л1(И20)б]3+ and with excess base to form [Al(OHh]-1. Both of these species are termed complex ions and are water soluble. They are governed by complex-ion equilibrium constants, as explained in Section 2.5 of this chapter. Aluminum oxide, ALO3, is also amphoteric. The hydroxides as well as oxides of zinc(II) and chromium(III) are similarly amphoteric.