ν2 Band region of nitrate as an indicator for contact ion pairing in aqueous lithium and calcium nitrate solutions

Abstract

The FTIR spectra of 2 and 10 mol dm^–3 LiNO₃ solutions, and of 0.5, 3 and 5 mol dm^–3 Ca(NO₃)₂ solutions in D₂O are shown in order to demonstrate the usefulness of the ν₂ band region of nitrate as an indicator for contact-ion pairing in aqueous alkali-metal and alkaline-earth-metal nitrate solutions. In fourth-derivative curves of the original composite bands, the development of a second band and its increase with concentration is seen at low frequency of the band, arising from the ‘free’ nitrate. The band at low frequency is assigned to the contact ion pair, and the assignment is supported by a parallel study of the ν₄ band region of the same solutions. The separation of the two bands in the ν₂ band region is between 3.3 and 5.2 cm^–1, and for 10 mol dm^–3 LiNO₃ solution it is even less than half of the average half-bandwidths of the two component bands. Reliable curve-fitting of the composite bands is possible, despite their strong overlap, by comparison of the fourth derivatives of the original composite bands with those of the sum of the curve-fitted component bands. We suggest that this comparison of fourth derivatives can, for spectra of high signal-to-noise ratio, provide reliable curve fits even when the separation of the peak maxima is less than half of the average of the half-bandwidths of the component bands.