Experimental and theoretical studies on water activity: aqueous solutions of amino acids with electrolytes

In the recent past a considerable body of work has been published on the measurement of activity coefficients and solubility data in aqueous amino acid solutions containing electrolytes [1]. However, no information on water activity in this type of system has yet been published [2]. In this work water activity (aw) in DL-alanine, glycine or L-serine aqueous systems with ammonium sulfate, at 298.15 K, was measured. The LabMASTER-aw water activity instrument (Novasina, Switzerland) was used to perform the measurements. The instrument, with a controlled chamber temperature (± 0.2 K), shows high precision (± 0.001 aw units). The new experimental data was used to test different theoretical schemes such as Zdanovskii-Stokes-Robinson (ZSR) model [3] and its extension, or the Clegg-Seinfeld-Brimblecombe (CSB) approach [4], in the calculation of amino acid and electrolyte activity coefficients in those solutions. Comparisons between the different approaches are given.

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