Studies of concentrated electrolyte solutions using the electrodynamic balance. 1. Water activities for single-electrolyte solutions

Abstract

An electrodynamic balance apparatus has been used to suspend single, 20-µm-diameter, charged droplets of aqueous salt solutions in a humid environment. From measurements of the humidity surrounding the droplet and the relative mass of the particle in its wet and dry states, the dependence of water activity on solute concentration was determined for aqueous solutions of the following salts at 20^oC: NaCl, NaBr, KCl, KBr, NH_4Cl, Na_2SO_4, (NH_4)_2SO4, CaC1_2, MnC1_2, and FeC1_3. This technique allowed the thermodynamics of highly supersaturated solutions to be studied, and for most of the salts, the measurements were made to higher concentrations than have previously been reported at comparable temperatures. The dependence of the solute activity coefficient on concentration was calculated for each of the salts, and the features of this dependence are discussed in relationship to ionic hydration and association. Several semiempirical electrolyte solution models were tested against the data, and it was found that salt-specific model parameters estimated from low-concentration data could not be reliably used to predict the solution behavior at high concentrations. However, with estimated parameters based on the full range of the data, the models were able to represent the experimental data to within the uncertainty in the measurements.