Buffers

A buffer is a solution that resists change in pH following the addition of an acid or base. A buffer can be created by mixing a weak acid (HA) with its conjugate base (A⁻). If an acid such as HCl is added to a buffer, A− can neutralize it, being converted to HA in the process. If a base is added, HA can likewise neutralize it, being converted to A⁻ in the process. Maximum buffering capacity occurs at a pH equal to the pK_a, but a conjugate acidbase pair can still serve as an effective buffer when the pH of a solution is within approximately ±1 pH unit of the pK_a. If the amounts of HA and A− are equal, the pH is equal to the pKa. As shown in Figure 1., a solution containing acetic acid (HA = CH3 – COOH) and acetate (A− = CH3 –COO⁻) with a pKa of 4.8 resists a change in pH from pH 3.8 to 5.8, with maximum buffering at pH 4.8. At pH values less than the pK_a, the protonated acid form (CH3 – COOH) is the predominant species in solution.

At pH values greater than the pK_a, the deprotonated base form (CH3 –COO⁻) is the predominant species.

Figure 1. Titration curve of acetic acid.