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Thermal and apparent molar properties of seawater at high temperatures and pressures


S. Berndt1, J. Safarov1,2, F. Millero3, R. Feistel4, A. Heintz1 and E. Hassel1

1University of Rostock, Germany
2Azerbaijan Technical University, Azerbaijan
3University of Miami, USA
4Leibniz-Institut für Ostseeforschung, Germany

Keywords: seawater, vibration tube densimeter, equation of state, apparent molar volume
property: density
material: standard seawater

The majority of water on earth is seawater, a solution of salts of nearly constant composition dissolved in water. The most recent standard formulation for seawater is restricted in its range of applicability due to a lack of density measurements at higher temperatures and elevated pressures. It is importance to have a reliable base of thermodynamic data for scientific investigations and design of many natural and technical processes. The equation of seawater, which is currently in use as the international standard for oceanography, is the 1980 International Equation of State (EOS-80). The state parameters interval of EOS-80 [temperature T=(271.15 to 313.15) K, pressures up to 100 MPa and salinities up to S=42] are not large. It is strongly necessary to improve EOS-80, using new density data of pure water (IAPWS-95) and the more accurate recent international standards of temperature (ITS-90). These should cover a broad range of pressure, temperature and salinity. After the publication and international agreement of EOS-80, there were no new experimental (p,ρ,T) measurements available. Also, no other EOS has been developed in the wide range of state parameters using the highly accurate IAPWS 1995 values of pure water. The authors of EOS-80 showed, that it can be modified, if more reliable data for pure water will become available in the future. The present work therefore fills an essential data gap. The main objective of our experimental investigations is the construction of a comprehensive and accurate thermodynamic equation of state over the ranges of interest in oceanographic research, underwater technology and land-based industrial plants running on seawater. For this purpose, the (p,ρ,T) behaviour of seawater at T=(273.15 to 468.15) K, pressures up to p=140 MPa and salinities up to 55.529 g/kg is measured. During the experiments, the DMA HPM vibrating tube densimeter was used. The various thermal properties, such as isothermal compressibility κT, isobaric thermal expansibility Δp, the difference between specific heat capacities cp-cv, thermal pressure coefficient γ and internal pressure, pint were calculated using the (p,ρ,T) data of seawater. Using the (p,ρ,T) data of pure water, the apparent molar volumes VΦ and compressibilities κΦ of sea salt in water are determined, which are fitted to the Pitzer equation for the analysis of the Debye-Hückel limiting slopes of the sea salts for evaluation of the infinite-dilution apparent molar volume of sea salt. The results obtained within from this work regarding the seawater thermodynamic data will be very helpful and useful for the oceanography research, like for ocean modelling, deep-sea research etc. and other oceanographic research.


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