Buffer Capacity

Buffer Capacity
Introduction
Acid/base chemistry, essentially the exchange of a proton, affects much of the chemistry we study.
Therefore, an understanding the chemistry of acids and bases is essential. That is why the distinction
between strong and weak acids and bases and their reactions was a significant portion of the general
chemistry curriculum. One topic discussed in general chemistry was that of buffers. A buffer is defined as
a solution that resists changes in pH upon the addition of an acid or base. The behavior of a buffer depends
upon the equilibrium of the given acid/base system. In order for a system to behave as a buffer it has to
fulfill two criteria:
• A buffer system is comprised of a weak acid and its conjugate base or a weak base and its
conjugate acid. Therefore, acetic acid and sodium acetate make a good buffer; but, nitric acid and
sodium hydroxide do not.
• The pKa of the weak acid should be close to the desired pH of the solution. (Or, the pKb of the
weak base should be close to the pOH.)
The second criteria brings us to the topic of this experiment: buffer capacity. Roughly speaking, buffer
capacity is the ability of a buffer solution to resist a change in pH.
Buffer capacity is measured as the amount of acid/base it takes the change the solution by one pH
unit. Though applicable to basic buffers, the examples used in this handout will all refer to weak acid
systems. A buffer works because the equilibrium that exists between the weak acid (HA) and its conjugate
base (A-).