182
C. ALBERS
which apparently has its maximum, if [H’] = K and dB/dpH = 0.575
C,. This formula was derived by van Slyke, who called dB/dpH the
buffer capacity of a solution. Obviously the buffer capacity depends on
(1) the concentration Cp of the buffer substance, (2) the dissociation
constant K of the buffer substance, and (3) the concentration of hydrogen ions. Buffer systems where Cp = [A-] + [AH] is constant are called
“homogeneous systems.” Biological examples are tissues and blood
passing through tissues. The buffer substances in the blood are the
plasma proteins and especially hemoglobin in the red cells. Because
of their ampholytic dissociation, plasma proteins and hemoglobin can
be considered to be much weaker acids than carbonic acid. They are
able therefore to buffer the hydrogen ions arising from the carbonic
acid. In the tissues, proteins are less important for buffering which is
effected chiefly by anorganic and organic phosphate compounds. A
homogeneous buffer system is completely described by Eq. (14). If a
narrow range of [H’] is considered, dBldpH is nearly constant and the
relationship between B and pH can be approximated by a straight line.
This fact is used for the determination of the so-called buffer lines of
true plasma (see below).
Quite another type of buffering is realized in systems where [AH]
is kept constant rather than C,,. In the case of a bicarbonate buffer [AH]
is kept constant if the partial pressure of CO,, and hence the product
Spco,, remains unchanged. Since addition of an acid to a bicarbonate
buffer would primarily increase the concentration of carbonic acid
[Eq. ( 9 b ) J and of CO, [Eq. (ga)], there must be another systcm
linked to the buffer which takes up the excess of CO,. An example
of such a buffer is seawater which is in equilibrium with the pro, of the
atmosphere. If an acid is added to seawater, CO, escapes into the atmosphere; conversely, if a base is added, CO, is taken up from the atmosphere until pco, is restored to its initial value. Since we have two systcms
linked to each othcr, such buffering is said to occur in a heterogeneous
system. The quantitative behavior of such a heterogeneous system is
obtained from Eq. (12c) where the denominator is kept constant while
the numerator is changed. Two curvcs corresponding to Spco, = 0.01
and 0.1 mmole/liter and pK’ = 6.4 are shown in Fig. 5. In contrast to
the sigmoid-shaped curves of Figs. 1 and 2, we obtain curves with a
slope and thus with a buffer capacity increasing continuously with the
pH. For Spro2 = const the buffer capacity is found by differentiation
of Eq. (12c) to be dB/dpH = 2.3 [HCO,-1. The absolute value of pH
depends on pco2, a tenfold change in pcn2 results in a change of pH by
C. ALBERS
which apparently has its maximum, if [H’] = K and dB/dpH = 0.575
C,. This formula was derived by van Slyke, who called dB/dpH the
buffer capacity of a solution. Obviously the buffer capacity depends on
(1) the concentration Cp of the buffer substance, (2) the dissociation
constant K of the buffer substance, and (3) the concentration of hydrogen ions. Buffer systems where Cp = [A-] + [AH] is constant are called
“homogeneous systems.” Biological examples are tissues and blood
passing through tissues. The buffer substances in the blood are the
plasma proteins and especially hemoglobin in the red cells. Because
of their ampholytic dissociation, plasma proteins and hemoglobin can
be considered to be much weaker acids than carbonic acid. They are
able therefore to buffer the hydrogen ions arising from the carbonic
acid. In the tissues, proteins are less important for buffering which is
effected chiefly by anorganic and organic phosphate compounds. A
homogeneous buffer system is completely described by Eq. (14). If a
narrow range of [H’] is considered, dBldpH is nearly constant and the
relationship between B and pH can be approximated by a straight line.
This fact is used for the determination of the so-called buffer lines of
true plasma (see below).
Quite another type of buffering is realized in systems where [AH]
is kept constant rather than C,,. In the case of a bicarbonate buffer [AH]
is kept constant if the partial pressure of CO,, and hence the product
Spco,, remains unchanged. Since addition of an acid to a bicarbonate
buffer would primarily increase the concentration of carbonic acid
[Eq. ( 9 b ) J and of CO, [Eq. (ga)], there must be another systcm
linked to the buffer which takes up the excess of CO,. An example
of such a buffer is seawater which is in equilibrium with the pro, of the
atmosphere. If an acid is added to seawater, CO, escapes into the atmosphere; conversely, if a base is added, CO, is taken up from the atmosphere until pco, is restored to its initial value. Since we have two systcms
linked to each othcr, such buffering is said to occur in a heterogeneous
system. The quantitative behavior of such a heterogeneous system is
obtained from Eq. (12c) where the denominator is kept constant while
the numerator is changed. Two curvcs corresponding to Spco, = 0.01
and 0.1 mmole/liter and pK’ = 6.4 are shown in Fig. 5. In contrast to
the sigmoid-shaped curves of Figs. 1 and 2, we obtain curves with a
slope and thus with a buffer capacity increasing continuously with the
pH. For Spro2 = const the buffer capacity is found by differentiation
of Eq. (12c) to be dB/dpH = 2.3 [HCO,-1. The absolute value of pH
depends on pco2, a tenfold change in pcn2 results in a change of pH by