The Use of Calculated Species Distribution Diagrams to Analyze Thermodynamic Selectivity

Abstract

This paper presents a simple method for the analysis of thermodynamic selectivity in systems characterized by concomitant equilibria in solution. In this context selectivity is understood as the preferential binding of one chemical species over others under equivalent experimental conditions. The method is based on the calculation, through experimental equilibrium constants, of the concentration of all species formed at equilibrium as a function of environmental variables (pH, reagent concentrations). The proposed didactic itinerary is organized so as to give shape to the concept of thermodynamic selectivity through the analysis of three examples of increasing complexity.In simple systems the conventional species-percentage-pH diagrams, representing individual species, are used, while in more complex systems in which many species are formed, individual species curves are replaced by cumulative concentration curves accounting for the binding of each substrate in all its forms to each receptor in all its forms. The case of selectivity patterns depending on two variables (two reagent concentrations) is illustrated through three-dimensional distribution diagrams. All calculations can be performed by means of common computer programs.