Practical Salinity

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

The saltiness of the ocean is defined in terms of salinity. In theory, this term is meant to represent the total number of grams of dissolved inorganic ions present in a kilogram of seawater. In practice, salinity is determined by measuring the conductivity of a sample and by calibration through empirical relationships to the International Association of Physical Sciences of the Ocean (IAPSO) Standard Sea Water. With this approach, salinity can be measured with a precision of at least 0.001 parts per thousand. This is fortunate, considering that 75% of all of the water in the ocean falls neatly between a salinity of 34 and 35. Obviously, these high-precision measurements are required to observe the small salinity variations in the ocean. 

Figure 1 The solubility of the principal atmospheric gases in seawater, as a function of temperature. Units are millilitres of gas contained in a litre of seawater of salinity 35 psu, assuming an overlying atmosphere purely of each gas. Note that salinity is defined in terms of a conductivity ratio of seawater to a standard KC1 solution and so is dimensionless. The term practical salinity unit , or psu, is often used to define salinity values, however. It is numerically practically identical to the old style unit of parts per thousand by weight...

PSU (practical salinity units) are a unit of measurement of salinity, i.e. of the total amount of dissolved salts in water. If sea water has a salinity of 5 psu, 5 g of salt are dissolved in 1000 g of water. 

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... 

Table 3.3 Equation Relating the Conductivity Ratio Measured by an Inductive Salinity to Practical Salinity, PSS 1978.

The PSS-78 is based on the measurement of a conductivity ratio and, hence, is technically unitless. Nevertheless, some oceanographers use a psu designation to represent a practical salinity unit and others report salinity in units of parts per thousand (%o). The latter convention has been adopted in this text. In any event, it is important to appreciate that the practical salinity is no longer directly traceable to the theoretical definition given in Eq. 3.1. 

This equation is valid for practical salinity from 0 to 42 and temperatures from -2 to 40°C. 

Salinity A measure of the salt content of seawater now expressed in practical salinity units related to measurement via conductivity. 

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 ... 

More specifically, practical salinity is related to the ratio F15 by the following equation ... 

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. 

Fig. 3 Vertical profiles of the water potential temperature (T , degrees Celsius), water salinity (S, practical salinity unit), and water specific potential density (or , kgm-3) a in upper layer of the Black Sea central area in August 1995 and b in deep layer (mean values based on high vertical resolution CTD measurements). 1 Upper mixed layer, 2 seasonal pycnocline (thermocline), 3 cool intermediate layer, 4 main pycnocline (halocline), 5 deep pycnocline, 6 bottom mixed layer...

Fig. 4 Vertical sections of (a,b) climatic water temperature (degrees Celsius) and (c,d) climatic water salinity (practical salinity units) in the Black Sea along 36° E in March (a,c) textbfa,c) and September (b,d)...

Table 1 Parameters of the climatic annual cycle of variability of water temperature (degrees Celsius) and water salinity (practical salinity units) at standard levels of main baro-clinic layer in the Black Sea central area (offshore end of standard section 3, see Fig. 1). Tmin. Tmax.Smin. and Smax annual extremes of climatic monthly mean temperature and salinity mon corresponding months of extremes DT and DS ranges of temperature and salinity annual cycles...

Fig. 7 a Climatic fields of the Black Sea upper mixed layer thickness (m) in area with sea depth >50 m. b Sea surface temperature (degrees Celsius) and c sea surface salinity (practical salinity units), both superimposed on the areas of water subduction with velocities 1 < 40 m month-1, 2 40-80 m month-1, 3 > 80 m month-1 in March... 

Fig. 8 Climatic monthly fields of the water salinity (practical salinity units) of the Black Sea at a depth of 100 m in a February, b May, and c August and the salinity difference between August and February (d)...

Fig. 10 Climatic monthly fields of the Black Sea water salinity anomaly (practical salinity units) at a depth of 100 m received by subtraction of the annual mean and first EOF fields from the climatic monthly salinity field in a January, b February, c March, d April, e May, and f June. Dashed lines negative anomaly...

Fig. 13 Multiannual variability and quadratic trends of a sea surface temperature (degrees Celsius), b minimal temperature, and c sea surface salinity (practical salinity units) in the Black Sea in August at offshore ends of standard sections 1 from Sebastopol southwest-ward (see line 1 in Fig. 1) and 2 from Gelendjic southwestward (see line 3 in Fig. 1)...

The third category of salinity methodologies was based on conductometry, as the conductivity of a solution is proportional to the total salt content. Standard Seawater, now also certified with respect to conductivity, provides the appropriate calibrant solution. The conductivity of a sample is measured relative to the standard and converted to salinity in practical salinity units (psu). Note that although psu has replaced the outmoded %o, usually units are ignored altogether in modern usage. These techniques continue to be the most widely used methods because conductivity measurements can provide salinity values with a precision of 0.001 psu. Highly precise determinations require temperature control of samples and standards to within 0.001 °C. Application of a non-specific technique like conductometry relies upon the assumption that the sea-salt... 

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). 

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