Oceanography (2+1)

Constancy of composition

Salinity determinations from the world’s oceans have revealed an important, findings that although salinity varies quite a bit because of differences in the total amount of dissolved salts, the relative proportions of the major constituents are constant. In other words the ratio of any two major constituents dissolved in seawater, such as Na⁺/K⁺ or Cl^-/SO₄^2-, is a fixed value, whether the salinity is 25, 30, 35%, or whatever. To put it in more familiar terms, let’s imagine that the ratio of females to males in a populations is 1:4 (1 female for every 4 males) and that this ratio never changes regardless of population size. This means that the total number of people in the population can vary but the relative proportion of females to males does not change. In other words, the ratio of females to males is constant and is independent of population size. Just so, the ratio of any two major salt constituents in ocean water is constant and is independent of salinity. This is called constancy of composition.

This important discovery, made during the Glomar challenger expedition is termed the principle of constant proportion or constant composition, and was a major breakthrough in determining salinity of sea water in a rapid, accurate, and economical manner. In theory, all that need be done to quantify salinity is to measure the amount of only a single major ion dissolved in a sample of sea water, because all the other major constituents occur in fixed amounts relative to that ion. Chemists chose to measure Cl^- for determining the salinity of sea water, because it is the most abundant solute in sea water and its concentration is easily determined. Actually, elements in the halogen family, which include chlorine, bromine, iodine, and fluorine, are difficult to distinguish analytically from one another. Therefore, chemists determine, not merely the Cl^- content, but the chlorinity, tha is, the total quantity of halogens dissolved in water, expressed as g/kg (%). It is then simple matter to convert chlorinity to salinity by the given formula

Salinity (%)= 1.80655 x chlorinity (%)

Today, oceanographers rely on a variety of methods, including the electrical conductivity of sea-water, to make routine determinations of salinity.