267
The Carbonate System
C = 2698/T
D = –1776/T
which can be used to convert from one scale to the other. The pH is also determined on the
seawater scale, which is defined by
[H + ] SWS = [H + ] F + [HSO 4
– ] + [HF] = [H + ] F (1 + β HSO 4 [SO 4
2– ] + β HF [F – ])
(7.32)
where
log β HF = 1590.2/T – 12.641 + 1.525 I 0.5
(7.33)
The seawater scale is related to the total and free scales by
pH SWS = pH T – log {(1 + β HSO 4 [SO 4
2– ] + β HF [F – ])/(1 + β HSO 4 [SO 4
2– ])}
(7.34)
pH SWS = pH F – log {(1 + β HSO 4 [SO 4
2– ] + β HF [F – ])}
(7.35)
At 25°C and S = 35, pH SWS = 8.06, pH T = 8.05, pH F = 8.19, and pHa = 8.32.
Since variations in the liquid junction potentials of various reference electrodes are different, it is better to use pH SWS , pH T , or pH F scales. Seawater buffers are available that can
be used to calibrate electrodes on these scales at a given temperature and salinity.
The buffers are made up in a seawater media without carbonate and borate (see Table 7.3).
Equal amounts of the acid and base form of the buffer are added to this seawater media.
The pH of the buffers can be determined from equations derived by Dickson (1993) from
the EMF data of Bates and Calais (1981) and Bates and Erickson (1986).
Although EMF measurements are normally used to measure pH, it is also possible to use
indicators that absorb light to measure pH. An indicator is an acid or base whose ionized
and un- ionized species absorb light differently. If the stoichiometric constant is known for
the indicator, the pH is given by
pH = pK HA * + log C A / C HA
(7.36)
where C A and C HA are the concentrations of the acid anion and un- ionized acid. The values
of C A and C HA are directly proportional to the absorbance of light at a given wavelength.
This method of measuring pH may prove useful for monitoring changes over long periods
Table 7.3
Composition of Synthetic Seawater
Used to Study Acid–Base Equilibria
Salt
Molality
NaCl
0.42764
Na 2 SO 4
0.02927
KCl
0.01058
MgCl 2
0.05474
CaCl 2
0.01075
NaF
0.00007
The Carbonate System
C = 2698/T
D = –1776/T
which can be used to convert from one scale to the other. The pH is also determined on the
seawater scale, which is defined by
[H + ] SWS = [H + ] F + [HSO 4
– ] + [HF] = [H + ] F (1 + β HSO 4 [SO 4
2– ] + β HF [F – ])
(7.32)
where
log β HF = 1590.2/T – 12.641 + 1.525 I 0.5
(7.33)
The seawater scale is related to the total and free scales by
pH SWS = pH T – log {(1 + β HSO 4 [SO 4
2– ] + β HF [F – ])/(1 + β HSO 4 [SO 4
2– ])}
(7.34)
pH SWS = pH F – log {(1 + β HSO 4 [SO 4
2– ] + β HF [F – ])}
(7.35)
At 25°C and S = 35, pH SWS = 8.06, pH T = 8.05, pH F = 8.19, and pHa = 8.32.
Since variations in the liquid junction potentials of various reference electrodes are different, it is better to use pH SWS , pH T , or pH F scales. Seawater buffers are available that can
be used to calibrate electrodes on these scales at a given temperature and salinity.
The buffers are made up in a seawater media without carbonate and borate (see Table 7.3).
Equal amounts of the acid and base form of the buffer are added to this seawater media.
The pH of the buffers can be determined from equations derived by Dickson (1993) from
the EMF data of Bates and Calais (1981) and Bates and Erickson (1986).
Although EMF measurements are normally used to measure pH, it is also possible to use
indicators that absorb light to measure pH. An indicator is an acid or base whose ionized
and un- ionized species absorb light differently. If the stoichiometric constant is known for
the indicator, the pH is given by
pH = pK HA * + log C A / C HA
(7.36)
where C A and C HA are the concentrations of the acid anion and un- ionized acid. The values
of C A and C HA are directly proportional to the absorbance of light at a given wavelength.
This method of measuring pH may prove useful for monitoring changes over long periods
Table 7.3
Composition of Synthetic Seawater
Used to Study Acid–Base Equilibria
Salt
Molality
NaCl
0.42764
Na 2 SO 4
0.02927
KCl
0.01058
MgCl 2
0.05474
CaCl 2
0.01075
NaF
0.00007