The "Pure Water" Model, a New/Old Approach for the Medium and Ionic Strength Dependence of Formation Constants

Abstract

Several models and theories exist for the medium, ionic strength and temperature dependence of stability constants and activity coefficients, all with their pros and cons. Between the 1980s and 1990s, the group of Prof. Silvio Sammartano from the University of Messina proposed a model for the ionic strength (and temperature) dependence of formation constants, based on three assumptions: [H1] It is possible to express the dependence on ionic strength of formation constants by a simple equation, independently of the type of reactants and products, and only dependent on the type of reaction. [H2] All the deviations from the predicted behavior are ascribed to weak complex formation between components and/or species under study and the background ions (e.g., the ionic medium). This implies that "pure water" is considered as reference state, and some ions as non-interacting with the reactants and/or products involved in the studied equilibrium. [H3] Perchlorate does not interact with cationic species, tetraethylammonium cations (and higher tetraalkylammonium analogues) do not with O-donor ligands, and Na+ and K+ do not with N-donor ligands. Evidences collected during more than half a century demonstrated the validity of this "pure water model", showing its potential to model the speciation of several multicomponent complex systems in a simple way. This contribution describes the main features of the model through some examples, highlighting the theoretical and practical aspects of this approach in the speciation modeling of systems of different complexity, including real systems.