Equilibria in Solutions of Weak Acids and Bases
This is the identical value obtained using the quadratic equation. Notice also that the
between the two approximations grew smaller. The second approximation
gave a smaller correction than the first. Each succeeding approximation differs from
the preceding one by smaller and smaller amounts. We stop the calculation when the
difference between two approximations is insignificant. Try this approach by reworking
Practice Exercise 26 and solving for [OH
] in the 0.0010
solution using the
method of successive approximations.
UFFERS ENABLE THE CONTROL OF
Many chemical and biological systems are quite sensitive to pH. For example, if the pH of
your blood were to change from what it should be, within the range of 7.35 to 7.42, either to
7.00 or to 8.00, you would die. Thus, a change in pH can produce unwanted effects, and sys-
tems that are sensitive to pH must be protected from the H
that might be formed
or consumed by some reaction.
are mixtures of solutes that accomplish this. The solu-
tion containing this mix of solutes is said to be
or it is described as a
Buffers contain a weak acid and a weak base
contains solutes that enable it to resist large changes in pH when small amounts of
either strong acid or strong base are added to it. Ordinarily, the buffer consists of two solutes,
one providing a weak Brønsted acid and the other a weak Brønsted base. Usually, the acid and
base represent a conjugate pair. If the acid is molecular, then the conjugate base is
a soluble salt of the acid.
For example, a common buffer system consists of acetic acid plus
sodium acetate, with the salt’s acetate ion serving as the Brønsted base. In your blood, carbonic
, a weak diprotic acid) and the bicarbonate ion (HCO
, its conjugate base)
serve as one of the buffer systems used to maintain a remarkably constant pH in the face of
the body’s production of organic acids by metabolism. Another common buffer consists of the
weakly acidic cation, NH
, supplied by a salt like NH
Cl, and its conjugate base, NH
One important point about buffers is the distinction between keeping a solution at a
particular pH and keeping it neutral—at a pH of 7. Although it is certainly possible to
prepare a buffer to work at pH 7, buffers can be made that will work around any pH value
throughout the pH scale.
A buffer works by neutralizing small additions
of strong acid or base
To work, a buffer must be able to neutralize either a strong acid or strong base that is added
to it. This is precisely what the weak base and weak acid components of the buffer do.
Consider, for example, a buffer composed of acetic acid, HC
, and acetate ion,
, supplied by a salt such as NaC
. If we add extra H
to the buffer (from a
strong acid) the acetate ion (the weak conjugate base) can react with it as follows.