A new model to describe the solubility of amino acids and beta-lactam compounds as a function of pH is proposed. The description of the solid liquid equilibrium entails simultaneous calculation of phase and chemical equilibria. The liquid-phase non-ideality was accounted for through an extended Pitzer model for polyelectrolyte solutions. A statistical thermodynamic interpretation of Pitzer's binary interaction parameters is also presented. The model was successfully applied to different systems containing amino-acids, as well as glycine and its oligopeptides and beta-lactam compounds such as ampicillin and 6-aminopenicillanic acid. (C) 2013 Elsevier B.V. All rights reserved.