Modeling Speciation in Nitric Acid Solutions Using the Binding Mean Spherical Approximation

A model was developed[1] to describe the thermodynamic properties (osmotic and activity coefficients) and degree of association of electrolytes. It is assumed that the cation and the anion can form two different kinds of pairs: a solvent separated ion pair and a chemical pair (undissociated molecule). The model is an extension of the binding or associative mean spherical approximation (BiMSA or AMSA), 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 equilibrium 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 the solutions of nitric acid for which the 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), allowed us to determine[1] the speciation of nitric acid, i.e the concentrations of chemical and solvent separated ion pairs, which is in good agreement with experimental results.

[1] S. Hlushak, J. P. Simonin, S. De Sio, O. Bernard, A. Ruas, P. Pochon, S. Jan and P. Moisy, “Speciation in aqueous solutions of nitric acid” Dalton Transactions, 42(8), 2853-2860 (2013)