SALINITY- A CONSERVATIVE PROPERTY OF SEAWATER

Salinity is the amount of dissolved salts or ions in seawater. The unit of measurement is grams of salt per 1000 grams of seawater or simply parts per thousand (written as ‰). The approximate salinity of seawater is 35 ‰ (equal to 3.5 %). As salinity includes only the dissolved salts, other constituents of seawater, such as dissolved gasses and solid particles do not contribute to salinity. Six ions (substances with positive or negative charge resulting from the loss or gain of electrons) account for about 99% of the salts in seawater. In order of abundance, from most to least, the major constituents are: Chloride (Cl^-), Sodium (Na⁺), sulfate (SO₄^-2), magnesium (Mg⁺²), calcium (Ca⁺²) and potassium (K⁺). Next in abundance, with concentrations between 1 and 100 parts per million (ppm), are minor constituents. Those substances present at very low concentrations (less than 1 ppm) are called trace elements. Virtually every one of the approximately 100 known elements is found in seawater, but with only a dozen or so of them present as major and minor substances, most of these constituents are present as trace elements. Salinity varies throughout the oceans largely due to climatic effects. Addition or removal of water, not addition or removal of salts account for changes in salinity. The processes of evaporation and precipitation (rain or snow) remove and add water to seawater and thereby change the amount of the salt in water or its salinity. This is illustrated in the accompanying diagrams. For simplicity, dissolved salts are shown concentrated in the bottom of the container; in reality they are evenly distributed throughout the water. The left diagram illustrates water of normal salinity (35 ‰). The middle sketch shows seawater that has been diluted by fresh water, as might occur near a river; this water is brackish (20 ‰ salinity). The last diagram represents seawater that has been evaporated to about one-half of its original volume; the salinity is doubled (70‰) and this condition is called hypersaline. Note that the total salt content of these waters does not change, only the amount of water. As there has been no addition or removal of salt, the dissolved constituents have not changed in concentration with respect to each other; that is, they are present in constant proportions to one another. This is an example of a conservative property of seawater. The property of constant proportion applies only to major and some minor constituents. Trace elements, whose concentrations are very low, usually vary in concentration throughout the ocean because of a number of factors (solubility, reactivity, used by organisms, etc.). Thus, most trace elements are nonconservative. The property of constant proportions has a practical application. It is very difficult and tedious to determine salinity by measuring the concentrations of every ion in seawater. Instead, because of constant proportions, it is necessary to measure the concentration of only one element or ion. From this value salinity can easily be calculated. The most easily measured ion in seawater is chloride (Cl^-). Salinity is therefore determined indirectly by measuring Cl^- and multiplying its concentration by a known factor (or proportion). Salinity measurements have become even more automated, however, by using the property of conductance. Ions in water conduct an electrical charge (your car battery uses this principle), and the conductance is directly proportional to the concentration of ions. Salinity is now determined by measuring the conductance of seawater.