270
Chemical Oceanography, 4th Edition
This indicates that at equilibrium the concentration of CO 2 is 670 times higher than H 2 CO 3 .
This has led workers to use the so- called hydration convention to define the first ionization
of carbonic acid (Equation 7.2). The thermodynamics of the carbonic acid system has been
reviewed (Millero, 1995). The stoichiometric association constant for the first ionization is
defined by
K 1 * = [H + ] T [HCO 3
– ] T /[CO 2 *]
(7.52)
where [CO 2 *] = [CO 2 ] + [H 2 CO 3 ], and the subscript T is used to denote total concentrations.
The concentration of dissolved CO 2 is related to the pressure by
[CO 2 *] = pCO 2 K 0
(7.53)
where K 0 is the Henry’s law constant, similar to the values described for other gases in
Chapter 6. The values of K 0 can be obtained from (Weiss, 1974)
ln K 0 = –60.2409 + 93.4517(100/T) + 23.3585 ln(T/100) + S[0.023517
– 0.023656(T/100) + 0.0047036(T/100)]
(7.54)
where T = t°C + 273.15. The solubility of CO 2 decreases with increasing temperature and
salinity as for other gases. The solubility of CO 2 is greater than that of O 2 or N 2 . The air
ratios are N 2 :O 2 :CO 2 = 240:630:1, and the solution ratios are 28:19:1. Henry’s law is not
obeyed at high pH because of the formation of HCO 3
– and CO 3
2– . The stoichiometric value
of K 1 * is related to the thermodynamic value by
K 1 = a H a HCO 3 /aCO 2 a H 2 O = K 1 *γ H γ HCO 3 /(γ CO 2 a H 2 O )
(7.55)
γ CO 2 = [CO 2 ] 0 /[CO 2 ], where the superscript zero denotes the solubility in pure water. The
activity coefficients given are the total values and include the effects of the formation of
ion pairs. The values of pK 1 * in seawater (K 1 * in moles per kilogram of seawater) can be
calculated from (S = 0 to 45 and t = 0 to 50°C; Millero et al., 2006)
ln K 1 * = ln K 1 + A S 0.5 + B S + C S 2
A = 12.10 – 489.634/T – 1.881 ln T
B = 0.022 – 2.635/T
(7.56)
C = 0.0000474
The thermodynamic values of pK 1 in water are given by
ln K 1 = 290.9097 – 14554.21/T – 45.0575 ln T
(7.57)
The stoichiometric association constant for the second ionization of carbonic acid is
defined by
K 2 * = [H+] T [CO 3
2– ] T /[HCO 3
– ] T
(7.58)
Chemical Oceanography, 4th Edition
This indicates that at equilibrium the concentration of CO 2 is 670 times higher than H 2 CO 3 .
This has led workers to use the so- called hydration convention to define the first ionization
of carbonic acid (Equation 7.2). The thermodynamics of the carbonic acid system has been
reviewed (Millero, 1995). The stoichiometric association constant for the first ionization is
defined by
K 1 * = [H + ] T [HCO 3
– ] T /[CO 2 *]
(7.52)
where [CO 2 *] = [CO 2 ] + [H 2 CO 3 ], and the subscript T is used to denote total concentrations.
The concentration of dissolved CO 2 is related to the pressure by
[CO 2 *] = pCO 2 K 0
(7.53)
where K 0 is the Henry’s law constant, similar to the values described for other gases in
Chapter 6. The values of K 0 can be obtained from (Weiss, 1974)
ln K 0 = –60.2409 + 93.4517(100/T) + 23.3585 ln(T/100) + S[0.023517
– 0.023656(T/100) + 0.0047036(T/100)]
(7.54)
where T = t°C + 273.15. The solubility of CO 2 decreases with increasing temperature and
salinity as for other gases. The solubility of CO 2 is greater than that of O 2 or N 2 . The air
ratios are N 2 :O 2 :CO 2 = 240:630:1, and the solution ratios are 28:19:1. Henry’s law is not
obeyed at high pH because of the formation of HCO 3
– and CO 3
2– . The stoichiometric value
of K 1 * is related to the thermodynamic value by
K 1 = a H a HCO 3 /aCO 2 a H 2 O = K 1 *γ H γ HCO 3 /(γ CO 2 a H 2 O )
(7.55)
γ CO 2 = [CO 2 ] 0 /[CO 2 ], where the superscript zero denotes the solubility in pure water. The
activity coefficients given are the total values and include the effects of the formation of
ion pairs. The values of pK 1 * in seawater (K 1 * in moles per kilogram of seawater) can be
calculated from (S = 0 to 45 and t = 0 to 50°C; Millero et al., 2006)
ln K 1 * = ln K 1 + A S 0.5 + B S + C S 2
A = 12.10 – 489.634/T – 1.881 ln T
B = 0.022 – 2.635/T
(7.56)
C = 0.0000474
The thermodynamic values of pK 1 in water are given by
ln K 1 = 290.9097 – 14554.21/T – 45.0575 ln T
(7.57)
The stoichiometric association constant for the second ionization of carbonic acid is
defined by
K 2 * = [H+] T [CO 3
2– ] T /[HCO 3
– ] T
(7.58)