14
F.J. Millero
1.2
Equilibria of Carbonate Species
When carbon dioxide is in contact with water, equilibria, as defined by Eq. 1.1 to 1.4,
will be established. Kinetics can affect the features of these reactions. Reaction 1.2 is
first order with respect to CO2, and has a first-order rate constant kl = 0.03 S-I or a
half time tl12 = In 21 kl = 23 s. The dehydration reaction, H2C03 ~ CO2 + H20, is first
order with respect to [H2C031 with rate constant k_1 = 20 S-I and tl/2 = 0.03 s. The values for the forward and backward reactions
kl
CO 2 + H 2 0 ~ H 2 C0 3
LI
can be used to determine the equilibrium ratio of K = kll k_1 = 0.03/20 = 11 670. This
indicates that at equilibrium, the concentration of CO2 is 670 times higher than H 2 C0 3 •
This has led workers to use the so-called hydration convention to define the first ionization of carbonic acid.
The thermodynamics of the carbonic acid system have recently been reviewed
(Millero 1995). The dissociation constants have been determined in natural waters
using four pH scales (Dickson 1984; Millero 1986):
1. The activity scale
2. The free (F) proton scale
3. The total (T) proton scale
4. The seawater proton scale
pHNBS = -log aH
pHp = -log [H+]p
pHT = -log [H+h
pHsws = -log [H+lsws
where aH is the activity of the proton and [H+jp is the concentration of the free proton. The total seawater concentrations are related to the free concentrations by
(1.10 )
(1.11)
where [HS041 and [HFl are the concentration of the proton complexing with sol- and
F- in the solution. The relationships between the various scales are given in the Appendix along with equations for the dissociation constants of HSO~ and HF in seawater.
Since variations in the liquid junction potentials of various reference electrodes are
different, it is better to use pHsws, pHT or pHp scales (Millero 1986). Seawater buffers
are available that can be used to calibrate electrodes on these scales at a given temperature and salinity (Dickson 1993; Millero et al.I993a).Although emf measurements
are normally used to measure pH, it is also possible to use indicators that absorb light
to measure pH. Clayton and Byrne (1993) have developed an indicator that can be used
to measure the pH of seawater solution to a precision of 0.0004 and an accuracy of 0.003.
The stoichiometric dissociation constant for the first ionization is defined by
(1.12)
F.J. Millero
1.2
Equilibria of Carbonate Species
When carbon dioxide is in contact with water, equilibria, as defined by Eq. 1.1 to 1.4,
will be established. Kinetics can affect the features of these reactions. Reaction 1.2 is
first order with respect to CO2, and has a first-order rate constant kl = 0.03 S-I or a
half time tl12 = In 21 kl = 23 s. The dehydration reaction, H2C03 ~ CO2 + H20, is first
order with respect to [H2C031 with rate constant k_1 = 20 S-I and tl/2 = 0.03 s. The values for the forward and backward reactions
kl
CO 2 + H 2 0 ~ H 2 C0 3
LI
can be used to determine the equilibrium ratio of K = kll k_1 = 0.03/20 = 11 670. This
indicates that at equilibrium, the concentration of CO2 is 670 times higher than H 2 C0 3 •
This has led workers to use the so-called hydration convention to define the first ionization of carbonic acid.
The thermodynamics of the carbonic acid system have recently been reviewed
(Millero 1995). The dissociation constants have been determined in natural waters
using four pH scales (Dickson 1984; Millero 1986):
1. The activity scale
2. The free (F) proton scale
3. The total (T) proton scale
4. The seawater proton scale
pHNBS = -log aH
pHp = -log [H+]p
pHT = -log [H+h
pHsws = -log [H+lsws
where aH is the activity of the proton and [H+jp is the concentration of the free proton. The total seawater concentrations are related to the free concentrations by
(1.10 )
(1.11)
where [HS041 and [HFl are the concentration of the proton complexing with sol- and
F- in the solution. The relationships between the various scales are given in the Appendix along with equations for the dissociation constants of HSO~ and HF in seawater.
Since variations in the liquid junction potentials of various reference electrodes are
different, it is better to use pHsws, pHT or pHp scales (Millero 1986). Seawater buffers
are available that can be used to calibrate electrodes on these scales at a given temperature and salinity (Dickson 1993; Millero et al.I993a).Although emf measurements
are normally used to measure pH, it is also possible to use indicators that absorb light
to measure pH. Clayton and Byrne (1993) have developed an indicator that can be used
to measure the pH of seawater solution to a precision of 0.0004 and an accuracy of 0.003.
The stoichiometric dissociation constant for the first ionization is defined by
(1.12)