<P> For practical reasons salinity is usually related to the sum of masses of a subset of these dissolved chemical constituents (so - called solution salinity), rather than to the unknown mass of salts that gave rise to this composition (an exception is when artificial seawater is created). For many purposes this sum can be limited to a set of eight major ions in natural waters, although for seawater at highest precision an additional seven minor ions are also included . The major ions dominate the inorganic composition of most (but by no means all) natural waters . Exceptions include some pit lakes and waters from some hydrothermal springs . </P> <P> The concentrations of dissolved gases like oxygen and nitrogen are not usually included in descriptions of salinity . However, carbon dioxide gas, which when dissolved is partially converted into carbonates and bicarbonates, is often included . Silicon in the form of silicic acid, which usually appears as a neutral molecule in the pH range of most natural waters, may also be included for some purposes (e.g., when salinity / density relationships are being investigated). </P> <P> The term' salinity' is, for oceanographers, usually associated with one of a set of specific measurement techniques . As the dominant techniques evolve, so do different descriptions of salinity . Salinities were largely measured using titration - based techniques before the 1980s . Titration with silver nitrate could be used to determine the concentration of halide ions (mainly chlorine and bromine) to give a chlorinity . The chlorinity was then multiplied by a factor to account for all other constituents . The resulting' Knudsen salinities' are expressed in units of parts per thousand (ppt or ‰). </P> <P> The use of electrical conductivity measurements to estimate the ionic content of seawater led to the development of the scale called the practical salinity scale 1978 (PSS - 78). Salinities measured using PSS - 78 do not have units . The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS - 78 measurement values . </P>

Who correlated colour of ocean water and its salinity