Salinity was defined, in 1902, as the total amount of solid material (g) contained in one kg of seawater when all halides had been replaced by the equivalent of chloride, when all the carbonate was converted to oxide, and when all organic matter was completely oxidized. The definition of 1902 was translated into the following equation:
S (o/oo) = 0.03 + 1.805 Cl (o/oo)
where the salinity (S) and chlorinity (Cl) are expressed in parts per thousands (o/oo)
The fact that the equation of 1902 gives a salinity of 0.03 o/oo for zero chlorinity was a cause for concern and a program, led by UNESCO, helped to determine a more precise relation between chlorinity and salinity. The definition of 1969 produced by that study is:
S (o/oo) = 1.80655 Cl (o/oo)
The definitions of 1902 and 1969 give identical results at a salinity of 35 o/oo and do not differ significantly for most applications. The definition of salinity was reviewed again when techniques to determine salinity from measurements of conductivity, temperature and pressure were developed. Since 1978, the "Practical Salinity Scale" defines salinity in terms of a conductivity ratio:
S = 0.0080 - 0.1692 K0.5 + 25.3853 K + 14.0941 K1.5 - 7.0261 K2 + 2.7081 K2.5
where the practical salinity, symbol S, of a sample of sea water, is defined in terms of the ratio K of the electrical conductivity of a sea water sample of 15°C and the pressure of one standard atmosphere, to that of a potassium chloride (KCl) solution, in which the mass fraction of KCl is 0.0324356, at the same temperature and pressure.