Modeling the Density and Isentropic Compressibility of Seawater

Abstract

Early studies have shown that the physical chemical properties of seawater can be estimated from the values for the major sea salts. These estimates have been made by assuming that the apparent molar properties of seawater, Φ(SW), are equal to the weighted sum of the major components of seawater ϕ(MX) $$ \Upphi \left( {\text{SW}} \right) \, = \sum_{\text{M}} \sum_{\text{X}} E_{\text{M}} E_{\text{X}} \phi \left( {\text{MX}} \right) $$ where E M and E X are the equivalent fractions of the cations (M) and anions (X) of each major sea salt (MX). This simple chemical model has been shown to predict the apparent molar volume, compressibility, and heat capacity of seawater from 0 to 40 °C and salinities as high as 40. In this paper, we have extended this model to 95 °C for the apparent molar volume and compressibility determined from density and sound speed measurements. The results indicate that reasonable estimates of the density and speed of sound of seawater and brines can be made using this approach from 5 to 90 °C and salinities of 40 g·kg−1. For high concentrations of seawater or brines, it is necessary to consider the volume of mixing of cations and anions of similar charge in the solution.