116
Exercise 8
DISSOLVED INORGANIC CARBON
The dissolved inorganic carbon (DIC) of water is the sum of all carbon present as COz,
H z C0 3 , HC0 3 -, and CO/ -. This quantity is of particular interest in evaluating the
amount of inorganic carbon available for photosynthesis (see Chapter 14). In
controlled light-dark experiments, estimates of photosynthesis and respiration rates
can be done from changes in DIe.
Hypolimnetic accumulations of DIC also have been used to obtain whole-lake
estimates of respiration (see Chapter 27). DIC is measured by acidifying a sample in an
airtight syringe, equilibrating the COz gas that evolves from the sample in a headspace
of helium gas, and analyzing with either a gas chromatograph equipped with a suitable
column (Porapak Q works well) and thermoconductivity detector (Schindler et ai.,
1972,1973; Stainton 1973) or with an infrared gas analyzer (Menzel and Vaccaro, 1964).
The results are compared to known standards of solutions of NaHC0 3 and expressed
as pM/1 (see also Exercise 9).
Total inorganic carbon in fresh waters can be estimated from measurements of
alkalinity and pH from:
DIC = [[Alk] - [OH-] + [H+]]/[O:l + 20: z ]
Below pH = 8.3 the equation may be simplified to:
DIC = [[Alk] - [OH-] + [H+]] [1;:.3 + 1 ]
(9)
(10)
Any noncarbonate alkalinity (e.g., borates, organic alkalinity) should be subtracted
from the alkalinity before using these equations. Further details can be found in Stumm
and Morgan (1981). A diagram of the DIC, pH, and alkalinity relationship is shown in
Fig. 8.5.
The total DIC offresh water can be estimated from measurements of total alkalinity,
pH, and temperature:
C available = (total alkalinity) x (pH factor from Table 8.2)
or
= (total alkalinity - phenolphthalein alkalinity, when
present) x (0.240)
= mgC/l
The pH factors, based on known dissociation constants of the carbonate complex at a
given concentration of hydrogen ions and temperature, have been conveniently
Figure 8.5. Alkalinity versus. DIe at various pH levels. Data are from
Eq. (9) (see text).
_I
~
U
Q)
F
2'
C
U
~
<{
i U
08
06
0.4
07
00
07
-
t -
00
() 5
ole
G2
60
56
5 ,
52
50
4 ~)
4()
- 1 -
1 - -
1 0
1 S
LO
moljl )
Exercise 8
DISSOLVED INORGANIC CARBON
The dissolved inorganic carbon (DIC) of water is the sum of all carbon present as COz,
H z C0 3 , HC0 3 -, and CO/ -. This quantity is of particular interest in evaluating the
amount of inorganic carbon available for photosynthesis (see Chapter 14). In
controlled light-dark experiments, estimates of photosynthesis and respiration rates
can be done from changes in DIe.
Hypolimnetic accumulations of DIC also have been used to obtain whole-lake
estimates of respiration (see Chapter 27). DIC is measured by acidifying a sample in an
airtight syringe, equilibrating the COz gas that evolves from the sample in a headspace
of helium gas, and analyzing with either a gas chromatograph equipped with a suitable
column (Porapak Q works well) and thermoconductivity detector (Schindler et ai.,
1972,1973; Stainton 1973) or with an infrared gas analyzer (Menzel and Vaccaro, 1964).
The results are compared to known standards of solutions of NaHC0 3 and expressed
as pM/1 (see also Exercise 9).
Total inorganic carbon in fresh waters can be estimated from measurements of
alkalinity and pH from:
DIC = [[Alk] - [OH-] + [H+]]/[O:l + 20: z ]
Below pH = 8.3 the equation may be simplified to:
DIC = [[Alk] - [OH-] + [H+]] [1;:.3 + 1 ]
(9)
(10)
Any noncarbonate alkalinity (e.g., borates, organic alkalinity) should be subtracted
from the alkalinity before using these equations. Further details can be found in Stumm
and Morgan (1981). A diagram of the DIC, pH, and alkalinity relationship is shown in
Fig. 8.5.
The total DIC offresh water can be estimated from measurements of total alkalinity,
pH, and temperature:
C available = (total alkalinity) x (pH factor from Table 8.2)
or
= (total alkalinity - phenolphthalein alkalinity, when
present) x (0.240)
= mgC/l
The pH factors, based on known dissociation constants of the carbonate complex at a
given concentration of hydrogen ions and temperature, have been conveniently
Figure 8.5. Alkalinity versus. DIe at various pH levels. Data are from
Eq. (9) (see text).
_I
~
U
Q)
F
2'
C
U
~
<{
i U
08
06
0.4
07
00
07
-
t -
00
() 5
ole
G2
60
56
5 ,
52
50
4 ~)
4()
- 1 -
1 - -
1 0
1 S
LO
moljl )