78
Chemical Oceanography, 4th Edition
2.6.2 evaporation in isolated basins
The formations of evaporites in hot arid regions (Stassfurt deposits) were first studied by
Van Hoff in 1912 (Borchert, 1965). During the slow evaporation of sea water, the precipitation of salts changes the composition of the solution. We have studied the changes in the
composition of the major components of sea water in Mexican lagoons during evaporation
to a salinity of close to 200 or Cl equal to 100. Our results are shown in Figure 2.18 and
Figure 2.19. At a Cl of 40 (S = 72), there is a loss of Ca 2+ and SO 4
2– , probably as CaSO 4 • 2H 2 O
(gypsum). The loss of HCO 3
– indicates that CaCO 3 , as aragonite, may also be precipitated.
The small decreases in Mg 2+ and K + could be due to coprecipitation. Thermodynamic calculations indicate that the precipitation of CaSO 4 in the lagoon waters (Figure 2.20) occurs
close to the predicted chlorinity. If sea water is evaporated further, other salts will precipitate (see Table 2.9).
Artificial Estuary
Cl
0
5
10
15
20
S
T - S
COND or S
DENS
0.00
0.02
0.04
0.06
0.08
S COND
S DENS
Figure 2.17
Comparison of the salinity determined from conductivity and density for estuarine waters.
Table 2.7
Composition of 1 Liter of Average World River Water
Species
10 6 g i
10 3
ni
10 3 ei
10 3 Ii
Na +
6.5
0.283
0.283
0.283
Mg 2+
4.1
0.169
0.337
0.674
Ca 2+
15.0
0.374
0.749
1.496
K +
2.3
0.059
0.059
0.059
Cl –
7.8
0.220
0.220
0.220
SO 4
2–
11.2
0.117
0.233
0.466
HCO 3
–
58.4
0.950
0.950
0.950
CO 3
2–
—
0.002
0.004
0.008
NO 3
–
1.0
0.016
0.016
0.016
Si(OH) 3 O –
—
0.005
0.005
0.005
1/2∑ =
1.086
1.428
2.089
Si(OH) 4
21.5
0.213
0.213
—
g T = 126.8
n T = 1.299
e T = 1.641
I T = 2.089
Note: To convert to molar units multiply by the density. To
convert to molal units, divide by X H 2 O = 0.96483.
Chemical Oceanography, 4th Edition
2.6.2 evaporation in isolated basins
The formations of evaporites in hot arid regions (Stassfurt deposits) were first studied by
Van Hoff in 1912 (Borchert, 1965). During the slow evaporation of sea water, the precipitation of salts changes the composition of the solution. We have studied the changes in the
composition of the major components of sea water in Mexican lagoons during evaporation
to a salinity of close to 200 or Cl equal to 100. Our results are shown in Figure 2.18 and
Figure 2.19. At a Cl of 40 (S = 72), there is a loss of Ca 2+ and SO 4
2– , probably as CaSO 4 • 2H 2 O
(gypsum). The loss of HCO 3
– indicates that CaCO 3 , as aragonite, may also be precipitated.
The small decreases in Mg 2+ and K + could be due to coprecipitation. Thermodynamic calculations indicate that the precipitation of CaSO 4 in the lagoon waters (Figure 2.20) occurs
close to the predicted chlorinity. If sea water is evaporated further, other salts will precipitate (see Table 2.9).
Artificial Estuary
Cl
0
5
10
15
20
S
T - S
COND or S
DENS
0.00
0.02
0.04
0.06
0.08
S COND
S DENS
Figure 2.17
Comparison of the salinity determined from conductivity and density for estuarine waters.
Table 2.7
Composition of 1 Liter of Average World River Water
Species
10 6 g i
10 3
ni
10 3 ei
10 3 Ii
Na +
6.5
0.283
0.283
0.283
Mg 2+
4.1
0.169
0.337
0.674
Ca 2+
15.0
0.374
0.749
1.496
K +
2.3
0.059
0.059
0.059
Cl –
7.8
0.220
0.220
0.220
SO 4
2–
11.2
0.117
0.233
0.466
HCO 3
–
58.4
0.950
0.950
0.950
CO 3
2–
—
0.002
0.004
0.008
NO 3
–
1.0
0.016
0.016
0.016
Si(OH) 3 O –
—
0.005
0.005
0.005
1/2∑ =
1.086
1.428
2.089
Si(OH) 4
21.5
0.213
0.213
—
g T = 126.8
n T = 1.299
e T = 1.641
I T = 2.089
Note: To convert to molar units multiply by the density. To
convert to molal units, divide by X H 2 O = 0.96483.