The Raman spectra of selenious acid and its sodium salts

Summary

The Raman spectra of selenious acid as crystals and as aqueous and alcoholic solutions of varying concentrations as well as those sodium biselenite and sodium selenite are investigated. The solid has yielded a spectrum rich in sharp and intense lines, of the following frequencies :—124, 199, 254, 287, 299, 364, 524, 597, 706, 862, 889, 909 and 940 cm.^-1 The lines broaden out and undergo variations both in frequency shift and intensity.

The changes in relative intensities of the two lines, 706 and 597, are particularly remarkable and are explained by postulating two types of molecules namely the symmetrical form

and the unsymmetrical form

in dynamic equilibrium in the acid. In the solid and in the solution in methyl alcohol the latter is the more predominent type and in the aqueous solution almost the whole of the acid is converted to the former type. Thus the Raman effect data are made use of for the first time to explain the pseudo or the true nature of the acid.

The spectra of aqueous solutions of different concentrations indicate that the dissociation of the acid is weak and that, the proportion of molecules dissociated to HSeO₃′ ions remains practically constant throughout. The line 895 is assigned to the HSeO₃′ ions. The results are compared to those of its analogue, sulphurous acid. The other lines in the spectrum are attributed to vibrations of definite molecular species by comparing them to the lines in the sodium salts of the acid.

Three low frequencies of the solid at 124, 199 and 256 are discussed in the light of the existing theories and their probable assignment is indicated.

The influence of change of state of the Raman spectra of the acid is briefly discussed. The broadening of the lines in the liquid is explained as due partly to association in the molecules and partly to the composite nature of the bands.