Molal volumes and compressibilities of salts in seawater

Changes in the partial molal volumes (Delta(V) over bar*) and compressibilities (Delta(kappa) over bar*) for chemical reactions in seawater Delta V* = Sigma(i)(V) over bar* (products) - Sigma(i)(V) over bar* (reactants) Delta kappa* = Sigma(i)(kappa) over bar* (products) - Sigma(i)(kappa) over bar* (reactants) can be used to estimate the effect of pressure on ionic equilibria constants (K-P/K-0) ln(K*(P)/K*(0)) = (Delta V*/RT)P - 0.5 (Delta kappa*/RT)P-2 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 degrees C. These results have been used to determine the conventional partial molal volumes and compressibilities ((V) over bar*(H+) and (kappa) over bar* (H+) = 0) for a number of ions (H+, Li+, Na+, K+, Rb+, Cs+, NH4+, Mg-2(+), Ca-2(+), Mn-2(+), Co-2(+), Ni-2(+), Zn2+, Cd2+, Cl-, Br-, I-, OH-, NO3-, HCO3-, SO22- and CO32-) in seawater at 25 degrees C. The values of the partial molal volume (V) over bar* for F-, CO32- and SO42+ are higher in seawater than 0.725 m NaCl at the same ionic strength (S-P = 35.00 and S-A - 35.165 g/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 (kappa) over bar* of salts in seawater are compared to the results in 0.725 m NaCl. Since the values of (kappa) over bar* are similar in the two media, one can use the values in 0.725 m NaCl for seawater calculations. Correlations of the values of (V) over bar* and (kappa) over bar* in seawater and 0.725 m 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. (C) 2012 Elsevier Ltd. All rights reserved.