For over 80 years it has been widely accepted that greater uniformity would be achieved in salinity determinations if all laboratories used the same suitably calibrated standard. The standard adopted was IAPSO Standard Seawater, which is a filtered, natural seawater to which only distilled water has been added. The main standard has a salinity of ca 35 (P-series) and is intended for single-point calibration of bench salinometers. It is certified in conductivity ratio (K₁₅) relative to a defined potassium chloride solution and in chlorinity. Two low-salinity standards (L-series) of salinity ca 10 and 30, similarly calibrated, are also available. These are suitable for checking the offset of salinometer comparator bridges at other points on the salinity scale as well as for use in low-salinity areas, such as the Baltic Sea. These salinity standards can be obtained, at cost, from the IAPSO Standard Seawater Service, which is operated by Ocean Scientific International Limited (OSIL) whose "Seawater Division" produces a range of natural seawater standards for the calibration of marine instruments. These include, in addition to the IAPSO Standard Seawater for salinity, Nutrient Standard Solutions and Low Nutrient Seawater. The seawater products are produced and calibrated to the highest standards, are approved by all the major international oceanographic bodies and are certified to ISO9002. OSIL seawater standards are used by scientists and technicians in over 80 countries worldwide who depend on them for data quality and comparability. Until 1975 the Service was based in Copenhagen.

Standard Seawater has always been certified in chlorinity and its widespread use helped to eliminate one source of discrepancy between determinations carried out by different laboratories in the period when the chlorinity titration was routinely used for determining of salinity. In the past twenty to thirty years, however, the chlorinity titration has been replaced by the measurement of electrical conductivity for salinity determination and this has led to some major developments, including two revisions of the definition of salinity, which have been reviewed by Lewis (1980); see also Unesco (1981c). In particular, JPOTS instigated and coordinated work in several laboratories to establish relationships between salinity, conductivity ratio, temperature, pressure and density, which resulted in the adoption of the Practical Salinity Scale, 1978, and the International Equation of State of Seawater, 1980 (Unesco, 1981a).

In the Practical Salinity Scale, practical salinity is defined in terms of the ratio K₁₅ of the electrical conductivity of the seawater sample, at a temperature of 15° C and a pressure of one standard atmosphere, to that of a potassium chloride (KCl) solution, in which the mass fraction of KCl is 32.4356 x 10^-3 at the same temperature and pressure. This means that all conductivity ratios are referred to a defined, reproducible conductivity standard. It would not be possible to make this KCl standard solution for distribution, however, nor would it be desirable for each laboratory to prepare its own. Instead, JPOTS recommended that IAPSO Standard Seawater should in future be calibrated directly against the defined KCl solution, thus giving a means by which bench salinometers can be calibrated indirectly in accordance with the definition of Practical Salinity. IAPSO Standard Seawater is still certified in chlorinity also, but it should be stressed that this is to be regarded as a separate independent variable relationship to salinity.

Lewis, E.L. 1980. The Practical Salinity Scale 1978 and its antecedents. IEEE J. Ocean. Eng., OE-5(1): 3-8.

Mantyla, A.W. 1987. Standard Seawater Comparisons updated. J. Phys. Ocean., 17: 543-548.

Unesco. 1981a. The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980. Techn. Pap. mar. sci, 36: 25 pp.

Unesco. 1981c. Background papers and supporting data on the Practical Salinity Scale 1978. Tech. Pap. mar. Sci, 37: 144 pp.

Unesco. 1985. The International System of Units (SI) in Oceanography. Techn. Pap. Mar. Sci, 45: 124 pp.