Practical salinity scale

UNESCO (1981). Background papers and supporting data on the Practical Salinity Scale 1978. UNESCO Technical Papers in Marine Science No. 37. UNESCO. 

This standard seawater has since proven problematic because it is based on real seawater, whose conductivity is influenced by concentration variations in the nonconservative ions and subtle fluctuations in the ratios of the major ions. To eliminate these issues, a practical salinity scale (PSS-78) was adopted by international agreement in 1978. As a... 

This new relationship was termed the practical salinity scale and is based on a background paper by Lewis (1978). The practical salinity of a water sample is defined in terms of the conductivity ratio, F15, which is defined as follows ... 

Salinity was first rigorously defined by Knudsen (1902, p. 28) as the weight in grams of the dissolved inorganic matter in one kilogram of seawater after all bromide and iodide have been replaced by the equivalent amount of chloride and all carbonate converted to oxide. In 1978, the JPOTS decided that a new definition was needed for salinity that was based more on a salinity/conductivity ratio and was termed the practical salinity scale. 

Salinity is presently determined by measuring the conductance of seawater by using a salinometer. The modem definition of salinity uses the practical salinity scale, which replaces the chlorinity-salinity relationship with a definition based on a conductivity ratio (Millero, 1996). A seawater sample of salinity S= 35 has a conductivity equal to that of a KCl solution containing a mass of 32.435 6g KCl in 1 kg of solution at 15 °C and 1 atm pressure. No units are necessary on the practical salinity scale however, in practice, one often sees parts per thousand, ppt, or the abbreviation psu. New salinometers using this method are capable of extremely high precision so that the salinity ratio can be determined to 1 part in 40 000. At a typical salinity near 35 this procedure enables salinities to be determined to an accuracy of 35.000 0.001. This is much better than most chemical titrations, which, at best, achieve routine accuracy of 0.5 parts per thousand. 

Constants are based on the total pH scale, pHj (Dickson, 1984,1993). Values are first presented at 1 atm pressure and then equations are given for calculating the pressure effect onK T is temperature in either degrees Kelvin (T), or degrees centigrade (Tc). Salinities are on the practical salinity scale. Equilibrium constants for the equilibria other than Kh, K, K2, Kg, and Kw given in Appendix 4Al.l(c) can be found in DoE (1994) and in Zeebe and Wolf-Gladrow (2000). 

For the simplicity of calculations, we have discussed salinity in units of g/kg. This Absolute Salinity differs from the commonly measured Practical Salinity (scale PSS-78) by about 0.5% (Millero andLeung, 1976 Feistel, 2003 Jackett etal., 2006 Millero et al., 2008). For the estimates in this section, this difference is quantitatively irrelevant. The density anomaly of the Baltic Sea is even less than this, up to about 0.01 %, and is ignored here as well (Millero and Kremling, 1976). 

In Table 20.4, the abbreviation psu, short for practical salinity unit, indicates salinity expressed in the Practical Salinity Scale of 1978 (PSS-78) as a dimensionless quantity. The term psu is not an official unit (Unesco, 1985 Siedler, 1998) but is in widespread use and is particularly helpful to distinguish, say, a given salinity value from absolute salinity in g/kg. Before 1978, salinity was computed from chlorinity, CL by the Cox scale, 5= 1,80655 xCZ (Mamayev et al, 1991), The recommended numerical conversion factor between the PSS-78 salinity and the Cox salinity is 1, Cox salinity is usually expressed in parts per thousand, ppt, %o, or g/kg. None the less, it is lower by about 0.5% than the absolute salinity of seawater in grams of dissolved substance per kilogram of seawater, which in turn is not exactly known but can be estimated sufficiently well (Millero et al., 2008), see Section 20.2.1. 

Practical salinity scale A dimensionless scale for salinity. 

The Practical Salinity Scale 1978and the International Equation of State of Seawater1980, Unesco Technical Papers in Marine Science No. 36, Unesco, Paris, 1981 sections No. 37, 38,39, and 40 in this series give background papers and detailed tables. 

In this chapter, a brief overview of the early definitions of salinity based on the chlorinity concept is given. It is followed by a description of the definition of the Practical Salinity Scale 1978 (PSS78), which is based on the measurement of electrical conductivity. Finally, methods are described that are used to derive salinity (and thus density) with modern instrumentation, both from bottle samples on a bench and in situ. 

The 1969 definition and the available corrections for temperatures higher than 10 °C were suitable for use with bench salinometers. However, for use with in situ measuring systems, it was insufficient because it did not cover the whole oceanic range of parameters. It was this severe lack of validity and the demand to define salinity solely from physical parameters that led to the definition of the Practical Salinity Scale of 1978. 

To determine salinity from bottle samples, bench salinometers have now replaced the chlorinity titration method almost completely, initially for practical reasons. Subsequently, as the 1978 Practical Salinity Scale has been adopted, consistent application of this scale requires the use of salinometers. 

The definitions of 1902 and 1969 give identical results at a salinity of 35 %o 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. The Practical Salinity Scale defined in 1978 is a complex function related to the ratio (K) of the electrical conductivity of a seawater sample to that of a potassium chloride (KCl) solution with a mass fraction in KCl of 0.0324356, at the same temperature and pressure. 

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