Density of Standard Seawater from 298.15 K Down to the Supercooled Liquid Region and up to 110 MPa

Relative density measurements of International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater (P161, Sp = 34.995) were carried out with the recently developed dual-capillary method. Isobaric measurements were performed from 298.15 K down to the supercooled liquid region, ∼8–13 K below the equilibrium freezing line, at pressures from atmospheric up to 110 MPa. Densities relative to the density at the reference temperature of 293.15 K are presented together with derived thermal expansion coefficients for individual isobars. Expanded uncertainties (k = 2) of the relative densities range from 0.000 020 to 0.000 031. A particular advantage of the present approach is that the thermal expansion coefficients close to the equilibrium freezing line are based on measurements from both of its sides—in the stable and metastable regions. Besides the dual-capillary data, results of complementary density measurements with IAPSO Standard Seawater using a vibrating tube densimeter (VTD) from 275.15 to 343.15 K at ambient pressure are presented. The dual-capillary and VTD results are compared with the Thermodynamic Equation of Seawater 2010 (TEOS-10). The new data agree with TEOS-10 within its expanded uncertainties from 273.15 to 313.15 K and they deviate significantly outside this temperature range. A simple correlation based on scaling the equation of state for supercooled ordinary water was developed to represent standard seawater densities in the studied region. The correlation represents measured densities within 10 ppm up to 70 MPa and within 25 ppm up to 110 MPa.