When an acidic substance is added to a buffer system it would immediately react with the basic component, as a basic substance would react with the acidic component. One therefore concludes from the table that the addition of either 0.1 M acid or 0.1 M base to a buffer system consisting of 0.5 M acetic acid and 0.5 M acetate ion would cause the pH to change by only 0.18 pH units. This is to be contrasted with the pH changes that would result from the addition of 0.1 M acid to water (i.e., 7.0 to 1.0, for a change of 6.0 pH units), or from the addition of 0.1 M base to water (i.e., 7.0 to 13.0, also for a change of 6.0 pH units).
A very useful expression for describing the properties of buffer system can be derived from consideration of ionization constant expressions. For an acidic substance, equation (22) can be rearranged as
Taking the negative of the base10 logarithms of the various quantities yields the relation known as the Henderson-Hasselbalch equation:
Equation (37) indicates that when the concentration of acid and its conjugate base are equal (i.e., [HA] = [A-]), then the pH of the solution will equal the pKA value. As a result, a buffer system is chosen so that the target pH is approximately equal to the pKA value.
A buffer system can be envisioned as consisting of a partially completed neutralization reaction:
where comparable amounts of HA and A- are present in the solution. The buffer region within a neutralization reaction is shown in Figure 2, where the horizontal region in the graph of anion concentration and observed pH reveals the buffer region of the system. For practical purposes, the buffer region would extend over [HA]/[A-] ratios of approximately 0.2 to 0.8.
It is clear from the preceding discussion that a buffer solution acts to reduce the change in pH when small amounts of either acid or are added to the solution. However, all buffer solutions will necessarily have a limited capacity to absorb amounts of acid or base without significant pH change, as can be illustrated by the example of a buffer solution that contains 1 mole each of a weak acid and its conjugate base (in the form of its salt). Since the buffer absorbs OH- through reaction with its weak acid component, the capacity of the buffer to respond will be exhausted once 1 mole of base is added. Alternatively, since the buffer absorbs H3O+ through reaction with its conjugate base component, the capacity of the buffer to respond will be exhausted once 1 mole of acid is added. In practice, the ability of the buffer to respond would have been seriously impaired well before the equivalent amount of acid or base had been added.
The term buffer capacity refers to the amount of base (or acid) that a given buffer solution can absorb without experiencing a significant change in pH. It is clear that since the amount of acid or base that can be absorbed by a buffer solution depends on the initial concentrations of components, the buffer capacity of a system will be strongly dependent on those initial concentrations. In particular, the more dilute a buffer solution is prepared to be, the less capacity it will have.
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