Salinity

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 [Page 880]samples and the fixed relationship between salinity (5) and chlorinity:

This was known as Knudsen's equation, after Martin Hans Christian Knudsen, a Danish physicist (1871–1949), and was redefined in 1969 as

Defining salinity in terms of chlorinity alleviates the practical difficulties of measuring salinity through evaporating water samples to dryness. For calibration purposes, artificial water with salinity almost equal to 35‰, known as Copenhagen water, is manufactured to serve as a reference. Copenhagen water has a chlorinity of 19.381‰. This approach requires the chemical titration of water samples usually obtained by Nansen bottles, named after Fridtjof Bedel-Jarlsberg Nansen, a Norwegian explorer and scientist (1861–1930), which are self-closing containers that collect water from different depths.

Pure water is a poor electrical conductor. However, the presence of dissolved salts greatly increases its conductivity, which, in fact, is a function of pressure, temperature, and the degree of ionization of the dissolved salts. In the second half of the 20th century, technical improvements in the measurement of the electrical conductivity of seawater led to the development of the so-called salinometers. Conductive sali-nometers measure the ratio between the conductivity of the sample against that of a reference sample of known salinity. Researchers using conductivity salinometers in the beginning were giving a salinity value of 35‰ to any sample having the same conductivity as the Copenhagen water, even though the mass of salt per kilogram of water was not guaranteed to be the same in both cases. This was because conductivity depends on the degree of ionization of the dissolved salts and not on the absolute mass of salt. In 1978, the salinity scale was redefined in terms of the conductivity ratio, K15, between any given sample and the reference

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