5. ACID-BASEBALANCX
4.0
3.0
I .o
4.0
6.0
8 .O
10.0
12.0
PH
Fig. 2. Titration curve of 4 ml of 0.1 N DMO at 19.6% (Albers, 1966).
Therefore, if [H'] is very large with respect to K, h approaches zero.
If [ H'] is very small, h approaches unity. If [ H' ] = K, h is 0.5, that means
exactly one-half of the weak acid is dissociated when pH equals pK'.
A plot of h vs. pH according to Eq. (7b) is shown in Fig. 1 for lactic
acid (pK' = 3.9 at 25OC) and for boric acid (pK' = 9.2 at 25OC).
Exactly the same curve is obtained if we titrate a weak acid with a
strong base and plot the fractional extent of neutralization vs. pH. We
may call this curve the standardized titration curve, whereas the ordinary
titration curve is obtained by plotting the amount of base consumed
(e.g., in milliliter of 0.1 N NaOH) versus pH. This results in a sigmoidshaped curve too. It is customary to determine the pK' value of an acid
by reading the pH of half neutralization from a titration curve. Figure 2
shows the titration of the weak acid DMO (dimethyloxazoledinedione),
which is used for the indirect determination of the intracellular pH (see
below). From Fig. 2 we read for half neutralization pH = 6.48; pK' of
DMO is therefore 6.48 at that temperature.
C. Carbonic Acid
The amount of gaseous CO, physically dissolved in water or electrolyte solutions is proportional to the partial pressure of CO, according to
Henry's law
[COZI = S P C O ,
(8)
4.0
3.0
I .o
4.0
6.0
8 .O
10.0
12.0
PH
Fig. 2. Titration curve of 4 ml of 0.1 N DMO at 19.6% (Albers, 1966).
Therefore, if [H'] is very large with respect to K, h approaches zero.
If [ H'] is very small, h approaches unity. If [ H' ] = K, h is 0.5, that means
exactly one-half of the weak acid is dissociated when pH equals pK'.
A plot of h vs. pH according to Eq. (7b) is shown in Fig. 1 for lactic
acid (pK' = 3.9 at 25OC) and for boric acid (pK' = 9.2 at 25OC).
Exactly the same curve is obtained if we titrate a weak acid with a
strong base and plot the fractional extent of neutralization vs. pH. We
may call this curve the standardized titration curve, whereas the ordinary
titration curve is obtained by plotting the amount of base consumed
(e.g., in milliliter of 0.1 N NaOH) versus pH. This results in a sigmoidshaped curve too. It is customary to determine the pK' value of an acid
by reading the pH of half neutralization from a titration curve. Figure 2
shows the titration of the weak acid DMO (dimethyloxazoledinedione),
which is used for the indirect determination of the intracellular pH (see
below). From Fig. 2 we read for half neutralization pH = 6.48; pK' of
DMO is therefore 6.48 at that temperature.
C. Carbonic Acid
The amount of gaseous CO, physically dissolved in water or electrolyte solutions is proportional to the partial pressure of CO, according to
Henry's law
[COZI = S P C O ,
(8)