MEDIUM AND IONIC STRENGTH DEPENDENCE OF FORMATION CONSTANTS. THE “PURE WATER” MODEL

Several models exist to take into account the dependence of activity coefficients (and, in turn, of stability constants) on system conditions, with particular reference to ionic strength and temperature (since most reactions occur at ambient pressure, dependence on the latter assumes relevance in just particular cases like, e.g., in geochemical studies). However, the most common theories and approaches (e.g., Davies, Bromley, Pitzer, Specific ion Interaction Theory – SIT) can be considered as evolutions of the (Extended) Debye-Hückel equation. All of them have pros and cons, so that none can be considered better than others1. Between the ’80s and ’90s, the group of Prof. Silvio Sammartano from the University of Messina proposed, with the precious support of colleagues from the Italian Universities of Torino, Catania and Palermo, a model2 (with the relative equation), for the ionic strength (and temperature) dependence of formation constants, based on three simple 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 dependent on the type of reaction only.[H2]: All the deviations from the predicted behaviour 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 in those universities demonstrated the validity of this “pure water model”, and showed the potential of this approach to model the speciation of several multicomponent complex systems in a very simple way. This contribution describes the main features of the pure water model through some examples, highlighting the theoretical and practical aspects of this approach in the speciation modelling of systems of different complexity, including real systems. References: 1. Castellino, L., ... & Milea, D., Chemometr. Intell. Lab. Syst. 239 (2023) 104860, and refs therein. 2. Daniele, P.G., ... & Sammartano S., in Miscellany of scientific papers in honour of Enric Casassas, professor of Analytical Chemistry, S. Alegret, J.J. Arias, D. Barcelò, J. Casal, G. Router, Eds., Universitat Autonoma de Barcelona, Bellaterra, Spain, 1991, pp. 121-126 Acknowledgements: This contribution is based upon work from COST Action CA18202, NECTAR − Network for Equilibria and Chemical Thermodynamics Advanced Research, supported by COST (European Cooperation in Science and Technology) and is dedicated to the memory of Prof. Silvio Sammartano, for the unquestionable impact he had in developing research in this and related fields.