The chemistry of natural waters - Geochemistry: An Introduction - page 144

7.7 The carbonate system

By using methods from solution chemistry, it is possible to calculate the speciation of
carbonates in water, for example the α HCO 3

fraction of carbonate dissolved as bicarbonate

ions. This parameter is defined as:

HCO 3

α HCO 3 = CO 2 3 (7.33)

+ HCO 3 +

[H 2 CO 3 ]

Dividing this equation by [H 2 CO 3 ], utilizing the two dissociation constants of (7.1 and
7.2) , and multiplying by [H + ] 2 gives:

K 1 H +

α HCO 3 = (7.34)

K 1 H + + H + 2

K 1 K 2 +

with similar expressions for α H 2 CO 3 . Carbonate speciation therefore only

depends on [H + ]. If these expressions are plotted against pH, three dominance zones can
be identified, separated by the p K 1 (6.4) and p K 2 (10.3) of carbonic acid ( Fig. 7.5 ). At
low pH values, the surplus of protons leaves all the carbonates as carbonic acid H 2 CO 3 ,
while at high pH the deficit of protons favors CO 2 3 ;HCO 3 is the dominant ion in the
intermediate pH zone. Natural water is weakly acidic or weakly basic. Seawater, with a pH
of 7.6-8.0, is largely dominated by HCO 3 ions.
The carbonate system in a solution in contact with the atmosphere is controlled by the
chemical variables P CO 2 , pH, alkalinity Alk

α CO 2 3 and

2 CO 2 3

, the sum of available

≈ HCO 3 +

CO 2 3 (omitting H 2 CO 3 and atmospheric CO 2 ), and

CO 2 ≈ HCO 3 +

carbonates

Acidic Basic

1.0

CO 2 − H 2 CO 3

HCO −

0.8

0.6

0.4

0.2

p K 1 p K 2

0.0

0 2 4 6 8 10 12 14

pH

Figure 7.5 Speciation of dissolved carbonates as a function of the pH of the solution (see (7.34 ) ). Notice that

the predominance of each carbonate species changes when pH values equal to p K are crossed.
Much natural water has a pH close to neutral and is therefore dominated by the bicarbonate
ion HCO 3 .