Changes in the Raman spectrum of sulphuric acid on dilution

Abstract

The results of previous workers upon the Raman effect of sulphuric acid show a considerable lack of agreement, due no doubt in part to the intensity of the accompanying continuous background and the apparent diffuseness of certain of the lines. Thus both Nisi and Woodward found that change of the concentration of the add greatly modified the appearance of the spectrum, influencing both the positions and the relative intensities of the lines. Wood-ward gave photometer carves of the spectra for concentrations ranging from 100% to 25% by volume, and attributed the observed intensity changes to tho successive stages of ionization of the acid. The lines Δ v = 910 and 1140 cm. -1 (approximate values), which were strong in the 100% acid, became weaker and shifted somewhat as the concentration decreased; while a new line Δ v = 1046 cm. -1 made its appearance and increased correspondingly in intensity. At low concentrations this last line became double, a companion appearing at 982 cm. -1 . The frequencies 910 and 1110 were regarded as characteristic of the H 2 SO 4 molecule, 1016 of the HSO' 4 ion, and 982 of the SO" 4 ion. Other diffuse lines also showed frequency shifts. Bell and Fredrickson. however, whose measurements were made not only with a prism spectrograph but also with a grating, failed to find any appreciable shifts of the line with dilution. Moreover, they stated that on their plates the 1016 line gave no indication of becoming double at low concentrations, but merely grew more diffuse. The findings of other workers are referred to in the discussion of our results (see below). In view of the conflicting nature of the above evidence regarding the two stages of ionisation of the acid, it was thought, desirable to repeat tho investigations using a higher dispersion than that employed by Woodward ( loc. cit .), the object of the work being to obtain more conclusive photometer curves of the spectra. The curves we have actually obtained not only give a clear picture of the intensity changes involved, but have also been used in a rational method of wave-length determination, the adoption of which has made it possible for us to propose a somewhat more detailed interpretation of the spectra than has hitherto been attempted.