pKb = pOH(at the half-eq point) = 14 – pH(at the half-eq point)For the titration curve we just showed, the unknown weak acid has a pKa = 4.5.
Acid-Base Indicators
An indicator is just the conjugate pair of a weak acid or base, where each
conjugate is a different color. Here’s an example:
Note that the full chemical formula of the indicator compound is not important,
hence the abbreviation as “H-Indicator” above. All this means is that the
hydrogen ion is part of the indicator itself, and when the indicator dissociates, it
dissociates into a hydrogen cation and the indicator anion. Since only a trace
amount of an indicator is used in a titration, the acid-base dissociation doesn’t
impact the solution’s overall pH. Instead, the indicator’s dissociation equilibrium
is shifted one way or another depending upon the solution’s pH, according to Le
Chatelier’s principle. If the indicator above were used in a titration, then in
acidic solutions, the indicator would be driven to the conjugate acid form (red),
and in basic solutions, it would be driven to the conjugate base form (yellow).
All you really need to know for this test is that a chemical acid-base indicator is
a substance that changes color in a pH range ±1 of its pKa.
For example, thymol blue, which has a pKa = 2, undergoes a red-to-blue color
change in the pH range 1 to 3. Also, litmus paper changes color at about pH 7.
At pH’s lower than 7, the paper is red, while at pH’s greater than 7, the paper is
blue. Keep these things in mind when selecting an appropriate chemical
indicator.