Modelling speciation in nitric acid solutions using the Associative Mean Spherical Approximation
S. Hlushaka,b, O. Bernardb and J.P. Simoninb
aInstitute for Condensed Matter Physics, Svientsitskoho 1, 79011, Lviv, Ukraine, E–mail: stepan.hlushak@gmail.com
bLaboratoire PECSA (UMR CNRS 7195), Universit´e P.M. Curie, Paris, France
A model is developed to describe the thermodynamic properties (os- motic and activity coefficients) and the degree of association of associ- ating electrolytes. It is assumed that the cation and the anion can form two different kinds of pair: a solvent separated ion pair and a chemical pair (undissociated molecule). The model is an extension of the asso- ciative (binding) mean spherical approximation (AMSA or BiMSA), in which the size and charge of the ions in the chemical pair may differ from those of the free ions. Corrections arising from the description of the chemical equilibria conditions at Lewis-Randall (constants pressure) state, instead of McMillan-Mayer (constant solvent chemical potential) state, are taken into account in the model. The model is then applied to solutions of nitric acid for which experiment suggests the formation of two ion pairs. A fit of the osmotic coefficient and the proportion of free ions (obtained from Raman spectroscopy experiments), leads to a determination of the speciation in nitric acid solutions.
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