Chapter 3 - Aqueous solubility of volatile nonelectrolytes

Abstract

The solubilities of gases in water and their distribution between coexisting liquid and steam are of great interest for hydrothermal processes because gaseous solutes are usually present. For many geochemical and industrial processes in aqueous media, it is important to have a suitable formulation to describe the solubilities of gases over a wide range of temperatures and pressures. This chapter focuses on aqueous Henry's constants kH at high temperatures, as well as on the alternative description by means of the distribution equilibrium constant KD. These two quantities are easily determined from the experimental solubilities of gaseous solutes when the solvent is far from its critical point. The thermodynamic states along the liquid–vapor coexistence curve are particularly relevant to describe the solubility of slightly soluble gases in water. Hydrothermal systems often operate at total pressures above 5 MPa, while for many practical cases the partial pressure of the solute gas may be smaller than 0.1 MPa. Thus, the clear difference between the two fluid phases at ambient conditions is blurred to some extent in hydrothermal processes, and more so as the temperature approaches Tc1. A thorough thermodynamic analysis produces values of Henry's constant and of KD that depend only on temperature. The use of these quantities to answer questions about the concentration of gases in water or in steam at high temperature is simpler. The two experimental methods used to determine kH for volatile solutes are analytic and synthetic method.