CHAPTER 17
common-ion effect:
Whenever a weak electrolyte and a strong electrolyte containing a common ion are
together in solution, the weak electrolyte ionizes less than it would if it were alone in
solution.
Solutions that contain high concentrations (10
-3
M or more) of a weak conjugate acid–
base pair and that resist drastic changes in pH when small amounts of strong acid or
strong base are added to them are called buffered solutions (or merely buffers).
A buffer resists changes in pH because it contains both an acid to neutralize added OH-
ions and a base to neutralize added H+ ions.
There are two ways to make a buffer:
1. Mix a weak acid or a weak base with a salt of that acid or base.
2. Make the conjugate acid or base from a solution of weak base or acid by the addition
of strong acid or base.
The key is to have roughly equal concentrations of both the weak acid and its conjugate
base.
Principle: As long as the amounts of HA and A- in the buffer are
large relative to the amount of OH added, the ratio [HA]>[A-]
does not change much and, thus, the change in pH is small.
Two important characteristics of a buffer are its capacity and its effective pH range.
Buffer capacity is the amount of acid or base the buffer can neutralize before the pH
begins to change to an appreciable degree. The buffer capacity depends on the amount of
acid and base used to prepare the buffer.
The pH range of any buffer is the pH range over which the buffer acts effectively. Buffers
most effectively resist a change in pH in either direction when the concentrations of weak
acid and conjugate base are about the same. Because log 10 = 1, buffers usually have a usable
range within
±
1 pH unit of pKa (that is, a range of pH = pKa
±
1).
There are two main reasons to do titrations:
(1) you want to know the concentration of one of the reactants
(2) you want to know the equilibrium constant for the reaction.
