A guide for the preparation and use of buffers in biological systems

interest. Sometimes the term pK b
is used. pK b
(“b” refers to base) is used when the
system is being considered as a base and the hydroxide ion concentration or pOH
is of greater interest.
Determination of pK a
pKa values are generally determined by titration. A carefully calibrated,
automated, recording titrator is used, the free acid of the material to be measured
is titrated with a suitable base, and the titration curve is recorded. The pH of the
solution is monitored as increasing quantities of base are added to the solution.
Figure 2 shows the titration curve for acetic acid. The point of inflection
indicates the pK a
value. Frequently, automatic titrators record the first derivative
of the titration curve, giving more accurate pK a
values.
Polybasic buffer systems can have more than one useful pK a
value. Figure 3
shows the titration curve for phosphoric acid, a tribasic acid. Note that the curve
has five points of inflection. Three indicate pK a1
, pK a2
and pK a3
, and two
additional points indicate where H 2
PO 4 – and HPO 4 – exist as the sole species.
8
6
pK a = 4.76
pH
4
2
0
NaOH
Figure 2: Titration Curve for Acetic Acid
12
pK a3 = 12.32
10
8
pH
6
pK a2 = 7.21
4
2
pK a1 = 2.12
NaOH
Figure 3: Titration Curve for Phosphoric Acid
6