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Chemical Oceanography, 4th Edition
7.2
Acid–Base Equilibria in Seawater
The equilibrium between acids and bases in seawater is important in controlling the carbonate system in the oceans. Before we discuss the equilibria of CO 2 in seawater, however,
it is important to understand the concept of pH and how it is measured in seawater. Water
is a weak electrolyte and dissociates according to
H 2 O = H + + OH –
(7.6)
This dissociation is defined by the equilibrium constant:
K W = a H a OH / a H 2 O = [H + ][OH – ]γ H γ OH / a H 2 O
(7.7)
where a is activity, and γ is the activity coefficient. For dilute solutions, a H 2 O = 1.0, γ H = 1.0,
γ OH = 0, and K W = 1 × 10 –14 = [H + ][OH – ] at 25°C. The pH is defined by
pH = –log [H + ] = 7.0
(7.8)
The thermodynamic dissociation constant varies as a function of temperature:
ln K W = 149.9802 – 13847.26/T – 23.6521 ln T
(7.9)
K W is also a function of pressure:
ln(K 2 P W / K W 0) = –(ΔV°/RT) P + (0.5 ΔK°/RT) P 2
(7.10)
Table 7.2
Fate of 1095 Molecules of CO 2 Added
to the Atmosphere after 1000 Years
Components
Totals
Inorganic Pool
Atmosphere
15
15
Ocean
CO 2
5
HCO 3
–
875
CO 3
2–
105
985
985
1000
Organic Pool
Terrestrial
54
Oceanic
41
95
95
Total
1095