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Salinity is an essential quantity to calculate many of physical properties of oceans, but it is also a quantity hardly definable considering the complexity of this material in its bio-geochemical composition and the imperfections of the existing measurement techniques. The TEOS-10 gives several definitions to the notion of absolute salinity, usable in function of the properties to study, but they are based on the concept of a constant elemental composition of seawater, so that, if its major inorganic components are well known, its real composition varies in time and space and its determination is still a challenge.Most of salinity calculations are based on conductivity measurements. This publication reviews other techniques which are used or could be used to assess the absolute salinity of seawater, and question about the measurand of these techniques and the possibility to redefine the concept of salinity from physical properties.

A novel kind of pycnometer was designed for liquid density measurements suitable to be used over wide ranges of temperature and pressure. Two similar pycnometers have been manufactured and tested with n-nonane 99.7 mol% in the temperature range from 233 K to 313 K at pressure lower than 10 MPa. To validate the procedure, the experimental values of density of an n-nonane sample have been compared with the density values given in the literature by means of the En number. The density uncertainty of n-nonane is found to be 0.16 kg m−3 (0.02%) in the temperature range from 253 K to 313 K and lower than 0.07% between 233 K and 243 K over the whole pressure range tested.