In the open ocean, the principal chemical constituents in
seawater have nearly the identical concentrations relative to one another and nearly constant concentrations relative to the total salinity.
Consequently by determining the concentration of one major chemical constituent of sea water, you
can calculate the total salinity of sea water and also the concentration of each
individual principal chemical element.
The concentration of salinity in sea water is important to
oceanographers because it can be used to:
- trace the origin of water masses and circulation patterns in
- understand the mixing of human-caused pollution with the
natural chemical constituents of seawater, and
- assess the influence of seawater composition on biochemical
reactions within marine organisms.
We have already mentioned that the proportion
of any one of the principal chemical elements in seawater is constant relative to the
other chemical elements in seawater. Therefore, if the salinity of seawater
increases, then the concentrations of all chemical elements increase (equally).
This constancy of chemical composition is caused by the
endless processes of circulation and mixing by ocean currents, wind action, water
temperature, chemical solubilities, and biochemical reactions of the organisms.
Because of seawater's constancy of chemical
composition, the total salinity can be determined from the concentration
of the chloride ion since the ratios of sodium to chloride to magnesium
to calcium and to any other major constituent remain constant.
We can construct a graph of the total salt content of sea
water in parts per thousand (o/oo), or salinity (S), versus concentration of the chloride
(Cl-), from which we can determine the relationship between salinity and
chlorinity (concentration of Cl- ion) for sea water