Ternary HNO3-H2SO4-H2O Mixtures: A Simplified Approach for the Calculation of the Equilibrium Composition

Concentrated ternary mixture of nitric acid, sulfuric acid, and water are often used in the chemical industry for a wide range of applications. Among these, nitrations of organic substrates are the most common. Nevertheless, there is still a poor understanding of speciation of the acids in the mixtures, and different simplified or semiempirical approaches have been found in the literature to model nitration kinetics by mixed acids. Most of the found investigations are relevant in restricted ranges of experimental conditions, resulting in a fragmentary vision. In this work, most of the approaches found in the literature were reviewed in order to define their limits of validity, and new empirical equations for the estimation of nitronium ion concentration are proposed. Moreover, on the basis of original experimental data, a simplified model is proposed to calculate the dissociation of acids at varying experimental conditions, and the main assumptions reported in the literature were verified. Adopted simplifying assumptions were verified at sulfuric acid concentrations higher than 2.1 mol·L–1 and nitric acid concentrations ranging from 2.0 to 11 mol·L–1.