64
Chemical Oceanography, 4th Edition
The new thermodynamic equation of state requires an input of the absolute salinity.
The use of the reference salinity is sufficient for all the properties except the density and
enthalpy. Recent studies in the Arctic Ocean indicated that changes in dissolved organic
carbon may also affect change in the density. Most of this material has a neutral charge
affecting the density but not the conductivity.
2.3 Methods of Determination
The methods used to determine the major components of natural waters are given in
detail elsewhere (Culkin, 1965; Kremling, 1976). Some of the popular methods that have
been used are briefly discussed next.
2.3.1 Chloride
The chloride or chlorinity of seawater is determined by adding AgNO 3 to precipitate AgCl.
Ag + + Seawater → AgCl(s) + AgBr(s)
(2.16)
Potassium chromate is added as an indicator. After the halides are removed, silver chromate is precipitated:
2Ag + + CrO 4
2– → Ag 2 CrO4 4 (red solid)
(2.17)
Some workers have used other indicators, and modern workers use an automated potentiometric end point (using an Ag, AgCl electrode). Since the heat of precipitation of AgCl
is quite high and exothermic (heat is given off), we have used a titration calorimeter to
determine the end point (Millero, Schrager, and Hansen, 1974). Since the heat of solution of
AgNO 3 is endothermic (heat is taken up), the end point is very sharp. The AgNO 3 is added
by using a constant- flow burette, and the time required to reach the end point is recorded.
The system can be calibrated using NaCl or standard seawater.
No matter what method is used to determine the end point, standard seawater of known
chlorinity is used to standardize the AgNO 3 solution. Since the chlorinity is a conservative
component of sea water, it is important to measure or determine its value when studying
the other major components of the waters. The approximate value of Cl(‰) for sea water
can be determined from conductivity or density measurements. The measurements of
Cox, Culkin, and Riley (1967) yielded the equation
Cl(‰) = –0.050 + 15.66367R 15 + 7.08943R 15
2 − 5.91110R 15
3 + 3.31363R 15
4 – 0.73240R 15
5
(2.18)
for various ocean waters, where R 15 is the conductivity ratio at 15°C of the sample relative to
standard sea water. More reliable values for the chlorinity of ocean waters can be obtained
from the density or the conductivity ratio of sea water as a function of the salinity determined using the practical salinity scale and the international equation of state for sea water.
Chemical Oceanography, 4th Edition
The new thermodynamic equation of state requires an input of the absolute salinity.
The use of the reference salinity is sufficient for all the properties except the density and
enthalpy. Recent studies in the Arctic Ocean indicated that changes in dissolved organic
carbon may also affect change in the density. Most of this material has a neutral charge
affecting the density but not the conductivity.
2.3 Methods of Determination
The methods used to determine the major components of natural waters are given in
detail elsewhere (Culkin, 1965; Kremling, 1976). Some of the popular methods that have
been used are briefly discussed next.
2.3.1 Chloride
The chloride or chlorinity of seawater is determined by adding AgNO 3 to precipitate AgCl.
Ag + + Seawater → AgCl(s) + AgBr(s)
(2.16)
Potassium chromate is added as an indicator. After the halides are removed, silver chromate is precipitated:
2Ag + + CrO 4
2– → Ag 2 CrO4 4 (red solid)
(2.17)
Some workers have used other indicators, and modern workers use an automated potentiometric end point (using an Ag, AgCl electrode). Since the heat of precipitation of AgCl
is quite high and exothermic (heat is given off), we have used a titration calorimeter to
determine the end point (Millero, Schrager, and Hansen, 1974). Since the heat of solution of
AgNO 3 is endothermic (heat is taken up), the end point is very sharp. The AgNO 3 is added
by using a constant- flow burette, and the time required to reach the end point is recorded.
The system can be calibrated using NaCl or standard seawater.
No matter what method is used to determine the end point, standard seawater of known
chlorinity is used to standardize the AgNO 3 solution. Since the chlorinity is a conservative
component of sea water, it is important to measure or determine its value when studying
the other major components of the waters. The approximate value of Cl(‰) for sea water
can be determined from conductivity or density measurements. The measurements of
Cox, Culkin, and Riley (1967) yielded the equation
Cl(‰) = –0.050 + 15.66367R 15 + 7.08943R 15
2 − 5.91110R 15
3 + 3.31363R 15
4 – 0.73240R 15
5
(2.18)
for various ocean waters, where R 15 is the conductivity ratio at 15°C of the sample relative to
standard sea water. More reliable values for the chlorinity of ocean waters can be obtained
from the density or the conductivity ratio of sea water as a function of the salinity determined using the practical salinity scale and the international equation of state for sea water.