Buffer Capac ity
Introduction
Acid/base chemistry, essentially the exchange of a pro ton, affects much of the chemistry w e study.
Therefore, an understanding the chemistry of acids and bases is essential. That is why the distinction
between strong and weak acids and b ases and their reactions was a signif icant portion of the general
chemistry curriculum. One topic discussed in general chemistry was that of buffers. A buffer is defined as
a solution that res ists changes in pH upon the addition of an acid or base. The behavior of a buffer depends
upon the equilibrium of the given ac id/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 w eak acid and its conjugate base or a weak base and its
conjugate acid. Th erefore, 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 b e close to the pOH .)
The second criter ia brings us to the topic of this experi ment: buffer cap acity. Roughly sp eaking, buffer
capacity is the ab ility of a buffer solu tion to resist a change in pH.
Buffer capacity is measured as the amount of acid/bas e it takes the change th e 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-).
HA +H2O!H3O++A!
(1)
The equilibrium constant (Ka) for this process can be written in the form of the Henderson-Hass elbalch
equation.
pH =pKa+log [A!]
[HA]
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(2)
The buffer capacity (β) is derived from this same equilibrium. We will cover this derivation in detail
during the practicum.
