Articles | Volume 14, issue 1
Research article
04 Jan 2018
Research article |  | 04 Jan 2018

The density–salinity relation of standard seawater

Hannes Schmidt, Steffen Seitz, Egon Hassel, and Henning Wolf

Abstract. The determination of salinity by means of electrical conductivity relies on stable salt proportions in the North Atlantic Ocean, because standard seawater, which is required for salinometer calibration, is produced from water of the North Atlantic. To verify the long-term stability of the standard seawater composition, it was proposed to perform measurements of the standard seawater density. Since the density is sensitive to all salt components, a density measurement can detect any change in the composition. A conversion of the density values to salinity can be performed by means of a density–salinity relation. To use such a relation with a target uncertainty in salinity comparable to that in salinity obtained from conductivity measurements, a density measurement with an uncertainty of 2 g m−3 is mandatory. We present a new density–salinity relation based on such accurate density measurements. The substitution measurement method used is described and density corrections for uniform isotopic and chemical compositions are reported. The comparison of densities calculated using the new relation with those calculated using the present reference equations of state TEOS-10 suggests that the density accuracy of TEOS-10 (as well as that of EOS-80) has been overestimated, as the accuracy of some of its underlying density measurements had been overestimated. The new density–salinity relation may be used to verify the stable composition of standard seawater by means of routine density measurements.

Short summary
A new density–salinity relation of standard seawater was developed based on highly accurate density measurements. The comparison of densities calculated using the new relation with those using the reference equation of state TEOS-10 showed significant deviations that are likely caused by accuracy overestimations of some density data underlying TEOS-10 and EOS-80. The new relation enables the determination of seawater density from salinity and vice versa with unprecedented accuracy.