5. ACID-BASE BALANCE
185
be expressed as the ionic strength p, which is half the sum of the
products cizi2, where C i is the concentration of the ion species i and zi
is the corresponding valency. The ionic strength of human plasma is
0.167 at 38°C. Table I shows the ionic composition of plasma of various
freshwater and marine fish and the ionic strength calculated from these
data. Some freshwater fish such as Cyprinus carpio and Lampetra
fluoiatilis have about the same ionic strength as mammals. In many
species the ionic strength is higher, reaching about 0.3 for Raia and 0.7
for hfyxine. In solutions having such an ionic strength the law of mass
action can be applied only in a much more sophisticated form. Since the
quantitative approach to the ionic interaction is beyond the scope of
this chapter, the reader is referred to any good textbook on physical
chemistry for details. Briefly summarized, an increase in ionic strength
has two consequences: ( 1 ) it decreases the solubility of gases and ( 2 )
it increases the equilibrium constants for most reactions. The latter becomes evident if we recall from Eq. ( 4 ) that the operational constant
K' is related to the thermodynamic constant K by the activity coefficient
K'= K / f . Since f is lowered by an increase in ionic strength, K' must
increase (or pK' decrease) together with p. Examples are seen in Table
I1 where K,' of boric acid and of carbonic acid as well as K2' of carbonic
acid are listed as a function of temperature and chlorinity of seawater.
Values for the solubility S of CO, are also given. Obviously at any given
Table I1
Effect of Temperature and Chlorinity on the Dissociation Constants of
Boric Acid and Carbonic Acid and t,he Solubility of C o p
Carbonic acid
sco,
("C)
(%a)
K' X loo
K'i X 10'
K'z X 10'
atm)
0
0
0.40
0.26
0.023
77.0
15
1.10
0.58
0.43
67.4
20
1.29
0.63
0.57
64.0
10
0
0.50
0.34
0.032
53.6
15
1.35
0 . 7 4
0.60
47.2
20
1.58
0.80
0.80
45.2
20
0
0.60
0.40
0.042
39.4
15
1.62
0.89
0.76
35.1
20
1.95
0.97
1.02
33.7
30
0
0.72
0.45
0.051
29.9
15
1.95
1.01
0.93
27.0
20
2.29
1.10
1.26
26.0
t
Chlorinity
Boric acid
(mmole/liter
a From Harvey (1966). Reprinted by permission of Cambridge University Press.
185
be expressed as the ionic strength p, which is half the sum of the
products cizi2, where C i is the concentration of the ion species i and zi
is the corresponding valency. The ionic strength of human plasma is
0.167 at 38°C. Table I shows the ionic composition of plasma of various
freshwater and marine fish and the ionic strength calculated from these
data. Some freshwater fish such as Cyprinus carpio and Lampetra
fluoiatilis have about the same ionic strength as mammals. In many
species the ionic strength is higher, reaching about 0.3 for Raia and 0.7
for hfyxine. In solutions having such an ionic strength the law of mass
action can be applied only in a much more sophisticated form. Since the
quantitative approach to the ionic interaction is beyond the scope of
this chapter, the reader is referred to any good textbook on physical
chemistry for details. Briefly summarized, an increase in ionic strength
has two consequences: ( 1 ) it decreases the solubility of gases and ( 2 )
it increases the equilibrium constants for most reactions. The latter becomes evident if we recall from Eq. ( 4 ) that the operational constant
K' is related to the thermodynamic constant K by the activity coefficient
K'= K / f . Since f is lowered by an increase in ionic strength, K' must
increase (or pK' decrease) together with p. Examples are seen in Table
I1 where K,' of boric acid and of carbonic acid as well as K2' of carbonic
acid are listed as a function of temperature and chlorinity of seawater.
Values for the solubility S of CO, are also given. Obviously at any given
Table I1
Effect of Temperature and Chlorinity on the Dissociation Constants of
Boric Acid and Carbonic Acid and t,he Solubility of C o p
Carbonic acid
sco,
("C)
(%a)
K' X loo
K'i X 10'
K'z X 10'
atm)
0
0
0.40
0.26
0.023
77.0
15
1.10
0.58
0.43
67.4
20
1.29
0.63
0.57
64.0
10
0
0.50
0.34
0.032
53.6
15
1.35
0 . 7 4
0.60
47.2
20
1.58
0.80
0.80
45.2
20
0
0.60
0.40
0.042
39.4
15
1.62
0.89
0.76
35.1
20
1.95
0.97
1.02
33.7
30
0
0.72
0.45
0.051
29.9
15
1.95
1.01
0.93
27.0
20
2.29
1.10
1.26
26.0
t
Chlorinity
Boric acid
(mmole/liter
a From Harvey (1966). Reprinted by permission of Cambridge University Press.