Interactions in aqueous alkali metal nitrate solutions

Raman (R) and infrared (I) spectra of aqueous alkali metal nitrate solutions have been recorded at 25 °C for a wide range of concentrations. In dilute solution the nitrate ion is perturbed by solvent water; the effects attributed to cations are nonspecific. The solvated nitrate ion generates the following spectrum: 1404 cm−1 (I, R), 1348 cm−1 (I, R), 1049 cm−1 (R), 825 cm−1 (I), 719 cm−1 (I, R). In more concentrated solutions band positions, intensities, and especially half-widths show a marked concentration and cation dependence. Changes in band half-width with concentration have been related to hydrated radii of the cations and are discussed in terms of interaction of solvated ions. For solutions of lithium and sodium nitrate more concentrated than 7 M a splitting of the ν4(E′) mode of nitrate ion into components at 720 and 740 cm−1 has been detected. Contact between ions is believed to account for the spectral changes observed for these high concentrations although water is shown to still markedly influence the interaction.