CHAPTER 1 . Sea Water as an Electrolyte
29
1.4
Estimating Transport Properties
All of the properties discussed above were concerned with thermodynamic properties. It has been shown some time ago that the additivity principles discussed above
can also be used to estimate the transport properties of mixed electrolyte solutions.
For example, the viscosity of sea water has been estimated using these methods and
the results are in good agreement with the experimental measurements (Millero 1974).
More recently I have examined additivity methods to estimate the conductivity of
natural waters (Millero 2000a). This is now necessary since the salinity of lakes and
seas are frequently estimated from conductivity measurements made with the CTD
(conductivity, temperature and depth) systems used in the oceans (Mcmanus et al.
1992). Since it has been shown above and elsewhere that the properties of lakes and
seas are the same as sea water at the same salinity, it would be useful to be able to use
these conductivity measurements to determine the total salinity of the lakes, etc. This
salinity could be used to determine the physical chemical properties of the lake from
equations for sea water. It is thus appropriate to briefly discuss the effect of composition on the conductivity of lakes and other natural waters.
The conductivity, unlike the density, responds only to the ionic components of the
solution. The conductivity salinity is thus a strong function of the composition of the
waters. This has been demonstrated (Millero 1984, 2000a) by using the equivalent conductances of the major components of sea water and river waters of different composition. The conductance of sea water in dilute solutions is larger than the values for
"World" and St. Lawrence River waters. This is because the conductances of Na + and
cr are larger than Mg2+, Ca 2 + and RCO; : the major components of river water.
Since the effect of temperature and pressure on the physical chemical properties
are not strong functions of the composition, the relationship between the conductivity and the density salinity can be approximated by measurements or calculations at
25°C. The easiest way to determine this relationship is to make direct conductivity
and density measurements on samples from the lake. Recently this has been done
(Jellison et al.1999) on samples from Mono Lake. This requires measurements of both
conductivity and density as a function of temperature and composition of the lake
waters of variable salinity. This is quite time consuming and essentially defines an
equation of state for the lake independent of sea water.
A more useful approach is to make density and conductivity measurements at 25°C
on the waters collected from the lake and sea water of known salinity. This will allow
one to use a semi-empirical correlation to relate the sea water salinity determined by
conductivity to the lake conductivity:
S(Lakekond = S(Sea waterkond + llCond
(1.50 )
where llCond is an empirical function of the differences between the calculated salinity using the Practical Salinity Scale for the lake and sea water. If density measurements are also made, one can develop a relationship between the conductance salinity and total salinity derived from density measurements using the equation of state
of sea water (Millero and Poisson 1981)
29
1.4
Estimating Transport Properties
All of the properties discussed above were concerned with thermodynamic properties. It has been shown some time ago that the additivity principles discussed above
can also be used to estimate the transport properties of mixed electrolyte solutions.
For example, the viscosity of sea water has been estimated using these methods and
the results are in good agreement with the experimental measurements (Millero 1974).
More recently I have examined additivity methods to estimate the conductivity of
natural waters (Millero 2000a). This is now necessary since the salinity of lakes and
seas are frequently estimated from conductivity measurements made with the CTD
(conductivity, temperature and depth) systems used in the oceans (Mcmanus et al.
1992). Since it has been shown above and elsewhere that the properties of lakes and
seas are the same as sea water at the same salinity, it would be useful to be able to use
these conductivity measurements to determine the total salinity of the lakes, etc. This
salinity could be used to determine the physical chemical properties of the lake from
equations for sea water. It is thus appropriate to briefly discuss the effect of composition on the conductivity of lakes and other natural waters.
The conductivity, unlike the density, responds only to the ionic components of the
solution. The conductivity salinity is thus a strong function of the composition of the
waters. This has been demonstrated (Millero 1984, 2000a) by using the equivalent conductances of the major components of sea water and river waters of different composition. The conductance of sea water in dilute solutions is larger than the values for
"World" and St. Lawrence River waters. This is because the conductances of Na + and
cr are larger than Mg2+, Ca 2 + and RCO; : the major components of river water.
Since the effect of temperature and pressure on the physical chemical properties
are not strong functions of the composition, the relationship between the conductivity and the density salinity can be approximated by measurements or calculations at
25°C. The easiest way to determine this relationship is to make direct conductivity
and density measurements on samples from the lake. Recently this has been done
(Jellison et al.1999) on samples from Mono Lake. This requires measurements of both
conductivity and density as a function of temperature and composition of the lake
waters of variable salinity. This is quite time consuming and essentially defines an
equation of state for the lake independent of sea water.
A more useful approach is to make density and conductivity measurements at 25°C
on the waters collected from the lake and sea water of known salinity. This will allow
one to use a semi-empirical correlation to relate the sea water salinity determined by
conductivity to the lake conductivity:
S(Lakekond = S(Sea waterkond + llCond
(1.50 )
where llCond is an empirical function of the differences between the calculated salinity using the Practical Salinity Scale for the lake and sea water. If density measurements are also made, one can develop a relationship between the conductance salinity and total salinity derived from density measurements using the equation of state
of sea water (Millero and Poisson 1981)