The chemistry of natural waters - Geochemistry: An Introduction - page 145

Alkalinity =

650

−

2 −

HCO 3 + 2CO 3

600

2Ca 2 +

2SO 2 −

550

2Mg 2 +

500

K +

450

100

Na + Cl −

50

0

Cations Anions

Figure 7.6 The composition of seawater in number of charges (equivalent) per unit weight. Alkalinity (2 meq

kg − 1 in seawater) is the deficit of charge carried by totally dissociated (therefore chemically
inert) anions compared with the charge carried by totally dissociated cations. In a solution, this
deficit, which is essentially balanced by carbonates, is constant. It is a measure of its capacity to
neutralize strong acids (after Broecker, 1994 ).

the physical variables of temperature and, to a lesser extent, pressure. These variables are
not independent and we seek to work with conservative variables, which exclude pH and
carbonate concentrations. Combining the expressions for Alk and

CO 2 gives:

CO 2 3

= Alk − CO 2

HCO 3 =

2 CO 2 − Alk (7.35)

In seawater ( Fig. 7.6 ) , alkalinity varies from 2.0 to 2.4 meq kg − 1 (the equivalent is
a number of charges expressed in molar units). Some 90% of dissolved carbonate is in
the form [HCO 3 ]. Alkalinity is much lower, or even negative, in rain water and river
water. By combining these expressions with those describing the solubility of CO 2 and
the dissociation of carbonic acid, we obtain the very important equation:

2

k CO 2 K 1 (

K 2 2 CO 2 − Alk )

P CO 2 = (7.36)

( Alk − CO 2 )

This expression is plotted for the ocean-atmosphere system in Fig. 7.7 , where CO 2
pressures are expressed in ppmv (parts per million volume) and it is taken that at 25 ◦ C,
k CO 2

is approximately equal to 0.029 moles per kg and per atm. The pH of seawater can