The Equation of State for Caspian Sea Waters

Abstract

The density ρ of Caspian Sea waters was measured as a function of temperature (273.15–343.15) K at conductivity salinities of 7.8 and 11.3 using the Anton-Paar Densitometer. Measurements were also made on one of the samples (S = 11.38) diluted with water as a function of temperature (T = 273.15–338.15 K) and salinity (2.5–11.3). These latter results have been used to develop an equation of state for the Caspian Sea (σ = ±0.007 kg m⁻³)

$$ \left( {\rho - \rho ^{0} } \right)\,{\text{kg m}}^{{-3}} = A + BS + CS^{2} $$

where ρ⁰ is the density of water and the parameters A, B and C are given by

$$\begin{aligned} A &= 0.0350 - 1.73{\text{E-}}03\left( {T - 273.15} \right) + 5.2{\text{E-}}05\left( {T - 273.15} \right)^{2}\\ &\quad- 4.947 {\text{E-}}07\left( {T - 275.15} \right)^{3}\end{aligned} $$

$$\begin{aligned} B &= 0.9877 - 3.66{\text{E-}}03\left( {T - 273.15} \right) + 4.903{\text{E-}}05\left( {T - 273.15} \right)^{2}\\ &\quad - 2.276{\text{E-}}07\left( {T - 273.15} \right)^{3}\end{aligned} $$

$$\begin{aligned} C = - 1.25{\text{E-}}04 \end{aligned}$$

Measurements of the density of artificial Caspian Sea water at 298.15 K agree to ± 0.012 kg m⁻³ with the real samples. These results indicate that the composition of Caspian Sea waters must be close to earlier measurements of the major components. Model calculations based on this composition yield densities that agree with the measured values to ± 0.012 kg m⁻³. The new density measurements are higher than earlier measurements. This may be related to a higher concentration of dissolved organic carbon found in the present samples (500 μM) which is much higher than the values in ocean waters (~65 μM).