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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-' (I, R), 1348 cin-' (I, R), 1049 cm-I (R), 825 cm-I (I), 719 cm-I (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 v4(E1) mode of nitrate ion into components at 720 and 740 cm-' 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.Canadian Journal of Chemistry, 46. 943 (1968) Interactions between ions in concentrated of strong acids (7) and of complex ions (8). aqueous solutions of strong electrolytes, such as In addition to the number of modes of vibration, the alkali metal nitrates, which are responsible their activity and polarization properties, which for apparent deviations of observed properties together characterize a species, variations in from properties predicted from theories which vibrational frequency and the width of spectral assume complete dissociation are often de-lines provide information about environmental scribed in terms of "ion-pairs". Methods for the influences. evaluation of the dissociation constant of the Results of a detailed study of vibrational ion-pair have been summarized in two recent spectra of aqueous nitrate solutions of lithium, monographs (1, 2). For the alkali metal nitrates sodium, potassium, and cesium are described the values of these constants increase as the below. The Raman spectra of these systems have hydrated radius of the cation decreases (2). been described before. The fact that the intensity Prue has noted that factors outside the scope of of the v,(Alt) nitrate line (ca. 1050 cm-l) in the the simple electrostatic model must be important Raman spectra of alkali metal nitrates is proand suggests consideration of the molecular portional to concentration was considered to be nature of the solvent (3). The nature of these sufficient proof of complete dissociation (6). ion-pairs and their dependence on the solvent More recently small deviations from linearity is not well understood. The characterization of have been interpreted in terms of the influence of the solvent by its bulk properties and the neglect hydrated cations on the nitrate ion (9). of solvent structure seriously limit the classical Other bands in the Raman spectrum have theories (4). Ion-solvent interactions are not received less attention. Several authors have readily accommodated by the "sphere in con...

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Communications to the Editor 1315 vent in the vicinity of the moving ion is considered to be the same as in the vicinity of a moving uncharged sphere; thus it is assumed not to be affected by the electrostatic interaction of the moving ion with the solvent dipoles. This electrostatic interaction would increase the residence time of oriented solvent dipoles near the ion and, consequently, the friction due to the dielectric relaxation of the solvent would be reduced. The neglect of this electrostatic effect would be particularly important when the electrostatic force is large, i.e., for small and/or polyvalent ions.

The laser Raman and infrared spectra of Li, Na, K, Rb, Cs bicarbonates and carbonates in H2O and D2O have been determined at 25 °C. The relative integrated intensities of the Raman bands have been measured for each salt from 0.01 M up to saturation. Changes observed in the carbonate spectrum with concentration have been interpreted in terms of solvation and ion-pairing of the carbonate ion. Based on the vibrational spectrum, there was no evidence for ionic interactions in the bicarbonate solutions. The hydrogen–deuterium exchange on the bicarbonate ion in H2O–D2O mixtures has been followed using Raman spectroscopy and the concentration quotients for the pertinent reactions have been calculated.