engleski

Two forms of nitrate ion in dilute aqueous solutions

The temperature dependence of UV absorption spectra of dilute (c = 0.06 mol dm-3) aqueous solutions of NaNO3, KNO3 and (NMe4)NO3 was studied in the (10&#8230 ; 70) °C range. The temperature change from 10 °C to 70 °C, caused the .300 nm peak (usually assigned to đ*&#8249 ; n transition of nitrate ion) to slightly shift from 301.5 to 303.0 nm, while the absorption minimum shifted from 262.0 to 264.5 nm. These spectral changes were found to be reversible which is, along with appearance of two well-defined isosbestic points (at 265 and 313 nm), almost direct evidence of the equilibrium between two spectrally distinct forms od nitrate ion. The existence of two distinguishable species was additionally confirmed by principal-components analysis (PCA). The effects of the temperature and the cations on the characteristic points of the spectra (i.e. the peak wavelength, ëmax, and the position of spectral minimum, ëmin) were examined by performing both parametric and non-parametric (Friedman&#8217 ; s) two-way factorial analyses of variance. It was shown that spectra were cation-independent while temperature was proved to be highly significant factor influencing the shape and position of the UV bands of nitrate ion. The spectral changes observed could be explained by temperature dependent breaking of hydrogen bonds in the hydration shell of the nitrate ion.

nitrate ion ; aqueous solutions ; absorption

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