66
Chemical Oceanography, 4th Edition
2.3.8 Calcium
The classical gravimetric method used to determine Ca 2+ is precipitation as the oxalate.
This can be used as a method to separate Ca 2+ and Mg 2+ . Calcium oxalate is weighed as is
or ignited to CaCO 3 or CaO. As with all precipitation techniques, coprecipitation problems
are encountered, and various steps have to be taken to obtain a pure precipitate. Most modern measurements of Ca 2+ have been made using volumetric titrations. The complexing
agent EGTA [ethylene glycol- bis (2-aminoethyl)-N,N,N′,N′ tetraacetic acid) is used. The end
point is detected potentiometrically or colorimetrically using metallochromic indicators.
2.3.9 Potassium
Early measurements of potassium were made after removal of divalent ions. The total
cations can be determined by ion exchange methods (titrating the released protons with
base). The divalent ions are removed and determined by EDTA titration. Potassium can
be determined by precipitation with chloroplatinate, K 2 PtCl 26 , which is sparingly soluble
in 80% ethanol (the Na + salt is very soluble). Since potassium tetraphenylboron is not very
soluble, it can be weighed after precipitation by adding sodium tetraphenylboron.
2.3.10 Sodium
Since the concentration of sodium is quite high, it cannot be accurately determined by
direct methods. The classical direct determination is made by precipitation with zinc
uranyl acetate, which is sparingly soluble. The most reliable method to determine Na + is by
difference. This can be done on individual samples by determining the total cations (equal
to the total equivalents of the anions) and subtracting the equivalent concentrations of
K + , Mg 2+ , Ca 2+ , and Sr 2+ . Since sea water should have the same equivalent concentration of
cations and anions, the preferred method is to determine the Na + after the other major
components are determined (i.e., by difference).
2.4
Composition and Stoichiometry of Average Seawater
To examine the physical chemical properties of sea water, it is necessary to select a reasonably reliable composition for average sea water (salinity [S] = 35, pH = 8.1, and temperature
[t] = 25°C). The major component measurements made by Cox, Culkin, Riley, and coworkers measured the major cations Na + , K + , Mg 2+ , Ca 2+ , and Sr 2+ in sea water. These results are
in reasonable agreement with the earlier results of Dittmar as recalculated by Lyman and
Fleming (1940). With the exception of Ca 2+ , all the studies demonstrated little or no depth
dependence for the major cations. The value of gCa/ Cl(‰) increased by 0.3 to 0.5% for
deep waters because of the dissolution of CaCO 3 (s). More is said in Chapter 7 about the dissolution of CaCO 3 (s). In the major oceans, the values of g i / Cl(‰) are reasonably constant.
Small variations do occur in some areas because of unique features, such as gCa/ Cl(‰)
in the Red Sea. The values of g i / Cl(‰) for average sea water are given in Table 2.3. As
discussed further in the chapter, it was the estimated reference composition for sea water
used when the physical properties were made (1970 to 1980). The values for Ca 2+ , K + , and
Sr 2+ were taken from the work of Culkin and Cox (1966) and Riley and Tongudai (1967);
Chemical Oceanography, 4th Edition
2.3.8 Calcium
The classical gravimetric method used to determine Ca 2+ is precipitation as the oxalate.
This can be used as a method to separate Ca 2+ and Mg 2+ . Calcium oxalate is weighed as is
or ignited to CaCO 3 or CaO. As with all precipitation techniques, coprecipitation problems
are encountered, and various steps have to be taken to obtain a pure precipitate. Most modern measurements of Ca 2+ have been made using volumetric titrations. The complexing
agent EGTA [ethylene glycol- bis (2-aminoethyl)-N,N,N′,N′ tetraacetic acid) is used. The end
point is detected potentiometrically or colorimetrically using metallochromic indicators.
2.3.9 Potassium
Early measurements of potassium were made after removal of divalent ions. The total
cations can be determined by ion exchange methods (titrating the released protons with
base). The divalent ions are removed and determined by EDTA titration. Potassium can
be determined by precipitation with chloroplatinate, K 2 PtCl 26 , which is sparingly soluble
in 80% ethanol (the Na + salt is very soluble). Since potassium tetraphenylboron is not very
soluble, it can be weighed after precipitation by adding sodium tetraphenylboron.
2.3.10 Sodium
Since the concentration of sodium is quite high, it cannot be accurately determined by
direct methods. The classical direct determination is made by precipitation with zinc
uranyl acetate, which is sparingly soluble. The most reliable method to determine Na + is by
difference. This can be done on individual samples by determining the total cations (equal
to the total equivalents of the anions) and subtracting the equivalent concentrations of
K + , Mg 2+ , Ca 2+ , and Sr 2+ . Since sea water should have the same equivalent concentration of
cations and anions, the preferred method is to determine the Na + after the other major
components are determined (i.e., by difference).
2.4
Composition and Stoichiometry of Average Seawater
To examine the physical chemical properties of sea water, it is necessary to select a reasonably reliable composition for average sea water (salinity [S] = 35, pH = 8.1, and temperature
[t] = 25°C). The major component measurements made by Cox, Culkin, Riley, and coworkers measured the major cations Na + , K + , Mg 2+ , Ca 2+ , and Sr 2+ in sea water. These results are
in reasonable agreement with the earlier results of Dittmar as recalculated by Lyman and
Fleming (1940). With the exception of Ca 2+ , all the studies demonstrated little or no depth
dependence for the major cations. The value of gCa/ Cl(‰) increased by 0.3 to 0.5% for
deep waters because of the dissolution of CaCO 3 (s). More is said in Chapter 7 about the dissolution of CaCO 3 (s). In the major oceans, the values of g i / Cl(‰) are reasonably constant.
Small variations do occur in some areas because of unique features, such as gCa/ Cl(‰)
in the Red Sea. The values of g i / Cl(‰) for average sea water are given in Table 2.3. As
discussed further in the chapter, it was the estimated reference composition for sea water
used when the physical properties were made (1970 to 1980). The values for Ca 2+ , K + , and
Sr 2+ were taken from the work of Culkin and Cox (1966) and Riley and Tongudai (1967);