Existence of two forms of nitrate ion in dilute aqueous solutions. Thermodynamic parameters of interconversion

Abstract

Two well-defined isosbestic points (at 265 and 313 nm) were found in the UV absorption spectra of dilute (0.06 mol dm−3) aqueous solutions of NaNO3, KNO3 and (NMe4)NO3, measured at 13 temperatures in the (10–70) °C range. This finding, together with the apparent cation-independence of the spectra (confirmed by several statistical tests) indicates the existence of a chemical equilibrium between two species differing in the structure of solvation shells: α-NO3− ⇌ β-NO3−. Assuming the constancy of standard conversion enthalpy, ΔrH°, in the experimental temperature range, a simple nonlinear programming algorithm, maximizing the linearity of the ln K° vs. 1/T relationship, was devised in order to compute optimized estimates of K° (T), ΔrH° and ΔrS°, as well as the individual spectra of α-NO3− and β-NO3− species. The following results were obtained: K° ranges from ≈0.08 (10 °C) to ≈0.25 (70 °C), ΔrH° = 15 kJ mol−1, ΔrS° = 31 J K−1 mol−1, λmax(α-NO3−) = 301 nm, emax(α-NO3−) = 7.8 cm2 mmol−1, λmax(β-NO3−) = 317 nm, emax(β-NO3−) = 5.7 cm2 mmol−1. Since the ΔrH° value falls inside the range of the energies of hydrogen-bond breaking and, in addition, ΔrS° > 0, it is hypothesized that the α → β conversion might include the rupture of one hydrogen bond (in the hydration shell) per one α-NO3− entity.