65
Composition of the Major Components of Seawater
2.3.2 Sulfate
The most widely used method to determine SO 4
2– in sea water is the addition of BaCl 2
and subsequent precipitation of BaSO 4 , which is weighed. Since other salts coprecipitate
(e.g., Ca 2+ ), errors can result from using this technique. Measurements relative to standard
sea water of known concentration can be very precise. The end point of the titration of
sea water with BaCl 2 can be determined using calorimetry (Millero, Schrager, and Hansen,
1974), potentiometry, or conductivity.
2.3.3 bromine
Bromine is normally determined after being coprecipitated with Cl – by the determination
of the weight loss after the Br 2 is liberated. This requires large samples since the concentration of Br – is quite low. The Br 2 is liberated by adding chromic acid or potassium permanganate. The liberated Br 2 can be determined colorimetrically or by titration.
2.3.4 Fluorine
Fluorine is analyzed in natural waters by using colorimetric methods or using a specific
ion electrode. The details are given elsewhere (Kremling, 1976).
2.3.5 bicarbonate and Carbonate
The concentrations of HCO 3
– and CO 3
2– are determined by measuring at least two parameters of the carbonate system (pH, the total alkalinity, TA; the total carbon dioxide, TCO 2 ; or
the partial pressure of CO 2 , pCO 2 ). In most routine work, pH and TA are used to characterize the carbonate system. More details of these measurements are discussed in Chapter 7.
2.3.6 boric acid and borate
Boron exists mainly as boric acid in sea water. At a pH of 8, the borate ion is 25% of the total
boron. The total boron is determined as boric acid by forming a complex with mannitol
and glycol. It is then converted to a strong acid and titrated with a base. It is first necessary
to destroy the organic boron compounds by oxidation with permanganate. Colorimetric
techniques using a colored indicator such as the boric acid–curcumin complex are much
quicker (Uppström, 1974). Recently, Lee and coworkers (2010) have improved on the earlier
methods and determined more reliable concentrations for boric acid in sea water.
2.3.7 Magnesium
The classical method for determining Mg 2+ is a gravimetric determination of the precipitate
formed following the addition of ammonium phosphate (after the Ca 2+ is removed). The magnesium ammonium phosphate is converted to magnesium pyrophosphate and weighed. In
recent years, volumetric methods have been used. EDTA (ethylenediamine N,N,N,N′ tetraacetic acid) has been used to titrate Mg 2+ after Ca 2+ has been removed. The total equivalents
of divalent ions can be determined by using an ion exchange resin or an EDTA titration.
The Mg 2+ is determined by difference after the subtraction of Ca 2+ and Sr 2+ . Problems in the
determination of the end point were discussed by Carpenter and Manella (1973).
Composition of the Major Components of Seawater
2.3.2 Sulfate
The most widely used method to determine SO 4
2– in sea water is the addition of BaCl 2
and subsequent precipitation of BaSO 4 , which is weighed. Since other salts coprecipitate
(e.g., Ca 2+ ), errors can result from using this technique. Measurements relative to standard
sea water of known concentration can be very precise. The end point of the titration of
sea water with BaCl 2 can be determined using calorimetry (Millero, Schrager, and Hansen,
1974), potentiometry, or conductivity.
2.3.3 bromine
Bromine is normally determined after being coprecipitated with Cl – by the determination
of the weight loss after the Br 2 is liberated. This requires large samples since the concentration of Br – is quite low. The Br 2 is liberated by adding chromic acid or potassium permanganate. The liberated Br 2 can be determined colorimetrically or by titration.
2.3.4 Fluorine
Fluorine is analyzed in natural waters by using colorimetric methods or using a specific
ion electrode. The details are given elsewhere (Kremling, 1976).
2.3.5 bicarbonate and Carbonate
The concentrations of HCO 3
– and CO 3
2– are determined by measuring at least two parameters of the carbonate system (pH, the total alkalinity, TA; the total carbon dioxide, TCO 2 ; or
the partial pressure of CO 2 , pCO 2 ). In most routine work, pH and TA are used to characterize the carbonate system. More details of these measurements are discussed in Chapter 7.
2.3.6 boric acid and borate
Boron exists mainly as boric acid in sea water. At a pH of 8, the borate ion is 25% of the total
boron. The total boron is determined as boric acid by forming a complex with mannitol
and glycol. It is then converted to a strong acid and titrated with a base. It is first necessary
to destroy the organic boron compounds by oxidation with permanganate. Colorimetric
techniques using a colored indicator such as the boric acid–curcumin complex are much
quicker (Uppström, 1974). Recently, Lee and coworkers (2010) have improved on the earlier
methods and determined more reliable concentrations for boric acid in sea water.
2.3.7 Magnesium
The classical method for determining Mg 2+ is a gravimetric determination of the precipitate
formed following the addition of ammonium phosphate (after the Ca 2+ is removed). The magnesium ammonium phosphate is converted to magnesium pyrophosphate and weighed. In
recent years, volumetric methods have been used. EDTA (ethylenediamine N,N,N,N′ tetraacetic acid) has been used to titrate Mg 2+ after Ca 2+ has been removed. The total equivalents
of divalent ions can be determined by using an ion exchange resin or an EDTA titration.
The Mg 2+ is determined by difference after the subtraction of Ca 2+ and Sr 2+ . Problems in the
determination of the end point were discussed by Carpenter and Manella (1973).