Solubilities of helium, argon, and nitrogen in molten nitrates at pressures up to 1 kbar

Abstract

The following solubilities have been measured over a temperature range of 100 deg. and at pressures up to 1 kbar : He in LiNO₃ and NaNO₃; Ar in LiNO₃, NaNO₃, RbNO₃ and AgNO₃; N₂ in LiNO₃ and NaNO₃. Henry's law was obeyed approximately in all systems. For Ar in NaNO₃, which was studied most extensively, a plot of solubility against pressure showed curvature above 500 bar. This could be accounted for by allowing for gas imperfection and for the finite partial molar volume of the dissolved gas (estimated to be 34±8 ml/mol). The solubilities increased with rising temperature, with heats of solution in the range 13–20 kJ/mol. The standard entropies of solution were between –8 and –24 J K^–1 mol^–1, referring to states of equal concentration in the gaseous and liquid phases. These entropies are more negative than those for solutions of the same gases in organic solvents or in molten halides, but less negative than for aqueous solutions. For a given melt at a given temperature, the solubilities decreased with increasing size of the solute molecule, while for a given gas in a series of melts the solubilities were in the inverse sequence of the surface tensions. These trends are correctly predicted by a model in which the free energy of solution is equated to the work of formation of cavities in the melt to accommodate the gas molecules.