High pressure Raman spectroscopy of concentrated aqueous calcium nitrate (Ca(NO3)2) solutions

Abstract

A new high pressure Raman cell has been constructed and subsequently used to study the spectrum of the NO3 ion in concentrated aqueous Ca(NO3)2 solutions as a function of concentration, temperature and pressure. The concentration dependence of the spectra agrees with that of earlier studies. The use of a computer programme to resolve spectral contours into their Gaussian component bands, and the use of Normal Probability graphs have shown each of the 700 cm-1 and 1000 cm-1 regions of the spectrum to contain only two component bands. The temperature and pressure dependence of the ratios of the areas of these component bands in the 700 cm-1 region was undertaken for a Ca(NO3)2, 3.24H2O solution. For the temperature study a decrease in the ratio of the area of the high frequency band to the area of the low frequency band as temperature increases was observed, in agreement with the trend observed in an earlier study. The pressure study on this region, however, has inferred that as pressure increases the opposite trend in the ratio of the two areas occurs. Assuming the low and high frequency bands to represent the 'free' and 'bound' nitrate species respectively it may thus be concluded that increasing temperature favours the former whilst increasing pressure favours the latter species. A similar pressure study of the 1000 cm-1 region for the same solution has also inferred the above trend with increasing pressure.