Molal volumes and compressibilities of salts in seawater

Abstract

Changes in the partial molal volumes (ΔV¯∗) and compressibilities (Δκ¯∗) for chemical reactions in seawater ΔV∗=∑iV¯∗(products)-∑iV¯∗(reactants)Δκ∗=∑iκ¯∗(products)-∑iκ¯∗(reactants) can be used to estimate the effect of pressure on ionic equilibria constants (KP/K0) ln(K∗P/K∗0)=(ΔV∗/RT)P-0.5(Δκ∗/RT)P2 where R and T have their normal meaning and P is the applied pressure (bar). The superscript * is used to denote values for an ionic media not water. In this paper, the partial molal volumes and compressibilities of a number of salts have been measured in seawater at 25°C. These results have been used to determine the conventional partial molal volumes and compressibilities (V¯∗(H+) and κ¯∗(H+)=0) for a number of ions (H+, Li+, Na+, K+, Rb+, Cs+, NH4+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Cl−, Br−, I−, OH−, NO3-, HCO3-, SO22- and CO32-) in seawater at 25°C. The values of the partial molal volume V¯∗ for F−, CO32- and SO42+ are higher in seawater than 0.725m NaCl at the same ionic strength (SP=35.00 and SA∼35.165g/kg). This is due to the formation of strong complexes with Mg2+ and Ca2+ and the release of water molecules upon the formation of the complexes. The first measurements of κ¯∗ of salts in seawater are compared to the results in 0.725m NaCl. Since the values of κ¯∗ are similar in the two media, one can use the values in 0.725m NaCl for seawater calculations. Correlations of the values of V¯∗ and κ¯∗ in seawater and 0.725m NaCl have been developed to estimate the values for unknown ions in seawater. These results can be used to estimate the effect of pressure on ionic equilibria in the deep ocean.