Studies of concentrated electrolyte solutions using the electrodynamic balance. 2. Water activities for mixed-electrolyte solutions

Abstract

An electrodynamic balance has been used to measure the water activity as a function of solute concentration at 20 °C for three aqueous electrolyte mixtures: NaCI-KCI, NaCl-KBr, and NaCl-(NH₄)₂SO₄. The measurements were performed by levitating single, charged particles of these electrolyte mixtures within the balance and measuring the mass of the particles as a function of the surrounding relative humidity. The deliquescence behavior of the mixed-electrolyte particles was also observed. For the alkali halide mixtures, the low-concentration data were consistent with earlier investigations. Data were obtained at higher concentrations than previously reported. The amount of water in the NaC1-(NH₄)₂SO₄ particles after drying was unknown. This, together with the lack of previously reported water activity data for this mixture, complicated the analysis of these experiments. Three models of mixed-electrolyte solutions-the Zdanovskii-Stokes-Robinson, Reilly-Wood-Robinson, and Pitzer methods-agreed well with the experimental data for the NaCl-KCl and NaCl-KBr systems over the range of concentrations that the models could be applied. The mixing rules' predictions were consistent with the experimental observations for the NaCl-(NH₄)₂SO₄ system assuming a small amount of water was retained in the particles after drying.

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