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salinity

Definition

This term, which describes the quantity of inorganic salts in a solution or water sample, is the subject of much confusion and extensive learned discourse. Traditionally, salt was defined as the weight in grams of the dissolved inorganic matter in one kg of water after all Br- and I- had been replaced by the equivalent quantity of Cl- and all HCO3 and CO3= converted to oxide. In this way, salinity scales were defined on a mass fraction basis (in theory, dimensionless in the SI system) and were expressed as "parts per thousand" (% or 103) indicating that the weight fraction was multiplied by a factor of 1000: standard sea water had a value of 35%.In recent years, a more robust definition of salinity has been sought. Absolute salinity (SA) is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water. In practice, this quantity cannot be measured directly but a Practical Salinity (below) has been defined for the reporting of scientific observations. Practical Salinity (S), of a sample of sea water, is defined in terms of the ratio K 15 of the electrical conductivity of the sea water sample ( at 150C and one standard atmospheric pressure) to that of a potassium chloride (KC1) solution in which the mass fraction of KC1 is 32.4356 x 10 3 at the same temperature and pressure. The K 15 value equal to 1 corresponds, by definition, to a value of 35 (= primary standard salinity) on the Practical Salinity Scale (PSS). In this definition, salinity is defined in terms of a conductivity ratio between a sample and standard sea water and is, therefore, a dimensionless value and not a strict unit/designator. On this basis, any oceanic water having a precisely known conductivity of near unity at 150C with the standard KC1 solution can be used as a secondary standard for the routine calibration of oceanographic equipment.

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