Apparent molar isentropic compressions of electrolytes as a function of the solution molality

Abstract

A semi-empirical equation is derived to express apparent molar isentropic compressions of electrolytes as a function of the solution molality. This is achieved by combining a recent, more amenable equation for the difference between apparent molar isentropic and isothermal compressions, with Pitzer equations for the apparent molar quantities isothermal compression, isobaric expansion and isobaric thermal capacity. In addition to Pitzer's parameters, the new equation contains only pure-solvent quantities. Second and higher virial coefficients for the apparent molar isentropic compression embody contributions from lower-order coefficients. Contrary to conventional wisdom, the Debye–Huckel limiting law (DHLL) for apparent molar isentropic compressions of salt solutions cannot be translated into a linear dependence on a function of the solution molality. The equation herein introduced is tested with extensive literature data for aqueous solutions of sodium chloride up to saturation concentration at 278, 298 and 318 K. Evidence is found for quaternary ion interactions in apparent molar compressions of concentrated salt solutions below 318 K. The new equation and its up to five-parameter empirical versions are capable of extrapolation to infinite dilution. For the first time, experimental and theoretical DHLL slopes for the apparent molar isentropic compression are found to be in fair agreement. Finally, revised values are given for some thermodynamic properties of aqueous sodium chloride.