Influence of copper(II) salts on the solubility of caffeine in water: complex formation and the salting-out effect

Abstract

Saturation curves of caffeine monohydrate cf·H₂O and of several complex compounds in the systems H₂O–Cu[NO₃]₂–cf (I) and H₂O–CuCl₂–cf (II) have been determined at various temperatures. Solubility curves of cf·H₂O in solutions of Cu[ClO₄]₂(III) and Cu[SO₄](IV) have also been examined. A regular variation in behaviour of the four systems is observed. A model is proposed and empirical expressions for (I) and (II) are calculated which take into account two different factors affecting the solublization of caffeine by dilute solutions of copper(II) salts, i.e. complex-formation equilibria and the salting-out effect. A qualitatively very good agreement between the model and the experimental data is found. Thermodynamic quantities are also calculated.