The speed of sound in mixtures of the major sea salts. A test of Young's rule for adiabatic PVT properties

The relative sound speed of mixtures of aqueous solutions of NaCl-MgSO₄ and MgCl₂-Na₂SO₄ at I=0.1 and 0.5m have been determined at 5, 15, and 25°C and pressures to 1000 bars. The resulting sound speeds, adiabatic and apparent molal compressibilities have been compared to results estimated from binary solutions using an additivity principle - Young's rule. The estimated sound speeds agree with the measured values for the NaCl-MgSO₄ system to ±0.15 m-sec^-1 and for the Na₂SO₄-MgCL₂ system to ±0.20 m-sec^-1. The deviations increase with increasing ionic strength (±0.08 m-sec^-1 at I=0.1 and ±0.25 m-sec^-1 at I=0.5 m). The sound speed of seawater have also been estimated from 0 to 40°C, 0.1 to 0.7 ionic strength and 0 to 1000 bars. The estimates were found to be in good agreement (±0.4 m-sec^-1) with the measured values. These results indicate that reasonable estimates of the adiabatic PVT properties of dilute mixtures of electrolyte solutions can be made using the additivity principle, without excess mixing terms.