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, Δ_rH°, 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), Δ_rH° and Δ_rS°, 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), Δ_rH° = 15 kJ mol⁻¹, Δ_rS° = 31 J K⁻¹ mol⁻¹, λ_max(α-NO₃⁻) = 301 nm, ε_max(α-NO₃⁻) = 7.8 cm² mmol⁻¹, λ_max(β-NO₃⁻) = 317 nm, ε_max(β-NO₃⁻) = 5.7 cm² mmol⁻¹. Since the Δ_rH° value falls inside the range of the energies of hydrogen-bond breaking and, in addition, Δ_rS° > 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.