Two attributes of the oceans, temperature and salinity, determine the density of seawater, and the differences in density between the water masses in the world's oceans causes the water to flow in thermohaline circulation, thereby producing the greatest oceanic current on the planet.

Salinity is the distinct taste of seawater and is the result of the presence of dissolved salts (more than 85 dissolved constituents), among which chloride (Cl) and sodium (Na), the elements of common table salt, are the most abundant. The term salinity refers to the content of these dissolved salts and has been defined as grams of dissolved salts per kilogram of seawater. Salinity has been expressed as parts per thousand (% or ppt) and, more recently, by practical salinity units (psu).

On average, a kilogram of seawater has 35 grams of dissolved salts, so its salinity content is 35%, or 35 psu. The accuracy of most laboratory salinometers (see below) is about 0.001 psu. Thus, only those components with a concentration over 0.001% will contribute to such salinity estimates. Only 15 of the dissolved salts have concentrations above that limit.

A key observational result (known as the principle of Dittmar, after William Dittmar, a Scottish professor of chemistry; the principle of Maury, after Matthew Fontaine Maury, an American astronomer, oceanographer, and geologist; or the hypothesis of Forchhammer, after Johan Georg Forchhammer, a Danish mineralogist and geologist) is that the relative concentration between some of these most abundant salts is virtually constant over much of the World Ocean. This finding indicates that the physical characterization of seawater is given by its temperature, pressure, and a single number reflecting the concentration of the most abundant components. Salinity is that number.

measuring salinity

In 1902, an international commission defined salinity as the total amount of solid material, in grams, contained in 1 kg. of seawater when all the carbonate has been converted to oxide, the bromide and iodine replaced by chlorine, and all organic matter completely oxidized. With this definition in hand, the commission estimated the salinity of several seawater

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