Journal of Solution Chemistry, Vol. 29, No. 7, 2000 Concentrated Aqueous Electrolyte Solutions: Analytical Equations for Humidity— Concentration Dependence Serge I. Prokopiev 1, * and Yuri I. Aristov 2 Received June 23, 1999; Revised February 18, 2000 The Henry’s law, Freundlich, and Dubinin–Astakhov, Brunauer–Emmett–Teller equations, and a new modification of the BET equation are used for the thermody- namic analysis of the activity of water over wide ranges of temperature (260–600 K) and water content (mole fraction 0.3–0.8) in concentrated aqueous solutions of electrolytes. The solutes considered are H 2 SO 4 , CaCl 2 , LiBr, LiCl, LiI, MgCl 2 , and NaOH in bulk solutions, and CaCl 2 and LiBr dispersed in porous matrixes. The generalized Henry’s law equation is found to afford a correct extrapolation of bulk solution properties measured at low and moderate salt concentrations to the region of low water content. The suggested modification of the BET equation as well as the Dubinin–Astakhov equation give an accurate (relative error less than 1%) description of literature data on the humidity–concentration dependence and can be used for the reliable prediction of this dependence in the region of intermediate conditions. KEY WORDS: Electrolyte solutions; thermodynamics; water activity; humidity. 1. INTRODUCTION Concentrated aqueous electrolytes are used for the reversible heat sorp- tion in a number of energy transformation devices. (1) The higher the solute concentration, the greater is the sorptivity of the solution. Recently, it has been found (2–5) that aqueous CaCl 2 and LiBr solutions confined to micro- and mesopores of silica gels remain liquid at higher salt concentration than 1 Institute for Water and Environmental Problems, 630090 Novosibirsk, Russia. email: s.pk@catalysis.nsk.su. 2 Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia. email: aristov@catalysis.nsk.su. 633 0095-9782/00/0700-0633$18.00/0  2000 Plenum Publishing Corporation