Encyclopedia of Environmental Science and Engineering, Volume I and II

1258 WATER CHEMISTRY

equilibrium with pco , despite any variation of pH by the addi-

tion of strong base or strong acid. This simple model has its

counterpart in nature when CO 2 reacts with bases of rocks,

for example with clays and silicates.

Figure 4 shows the distribution of the solute species of

such a model. A partial pressure of CO 2 equivalent to that

in the atmosphere and equilibrium constants valid at 25C

have been assumed. The equilibrium concentrations of the

individual carbonate species can be expressed as a function

of and [H^ ^ ] 2. From Henry’s Law,

[*] 23 KpH CO 2 ,HCO (14)

and Eqs. (5) to (9), one obtains

CKpTH

1

0

a CO^2 (15)

[]

[]

HCO

3 CO H CO

1

0

1

22





a

a

Kp

K

HHKp (16)

FIGURE 2 Closed system capacity diagram: pH contours for alkalin-

ity versus CT (total carbonate carbon). The point defining the solution

composition moves as a vector in the diagram as a result of the addition

(or removal) of CO 2 , NaHCO 3 , and CaCO 3 (Na 2 CO 3 ) or CB (strong base)

and C (strong acid). (After K.S. Deffeyes, Limnol., Oceanog., 10 , 412,

1965.) Figure from Stumm, W. and J. Morgan, Aquatic Chemistry, Wiley-

Interscience, New York, 1970, p. 133.

CB

CA

CO 2 NaHCO 3

CaCO 3

Na 2 CO 3

1 Dilution

1

1

(^12)
2
0
–0.5
0
1
2
3
11.5
11.4
11.3
11.211.1
11.010.9
10.810.7
10.610.5
10.410.3
10.210.1
10.0
9.9
9.8
9.6
9.7
9.5
9.0
8.5
8.0
7.5
7.0
6.9
6.8
6.7
6.6
6.5
6.4
6.3
6.2
6.1
6.0
5.9
5.8
5.7 5.6
5.5 5.4
5.3 5.2
5.1 5.0
4.5 4.0
3.9 3.8
3.7 3.6
3.5
3.4
Alkalinity (milliequivalents/liter)
CT(Total Carbonate carbon; millimoles/liter)
C023_002_r03.indd 1258C023_002_r03.indd 1258 11/18/2005 1:32:07 PM11/18/2005 1:32:07 PM