Raman Spectral Studies of Molten Selenious Acid

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Abstract
Photoelectric Raman spectra of molten selenious acid, obtained in the temperature range of about 70° to 175°C, provide evidence for the existence of a new species H2Se2O5. Comparisons of Raman bands of molten selenious acid with those of very concentrated aqueous solutions of sodium biselenite indicate common frequencies. In concentrated aqueous solutions, the equilibrium 2HSeO3— = Se2O52—+H2O provides Se2O52— ions in considerable concentrations. Similarly, in molten selenious acid the reaction 2H2SeO3 = H2Se2O5+H2O produces H2Se2O5 molecules in quantity. Raman lines common to the two spectra arise from vibrations which are characteristic of the Se2O5 grouping. Such vibrations are those produced by the bending and stretching motions of the Se′–O–Se′ linkage. When the stoichiometric composition of the melt is between 50 and 66.67 mole% SeO2 in H2O, (H2SeO3)2 and H2Se2O5 are important species. At the higher compositions, however, one weak band of the melt was observed which is close in frequency to an intense band of solid selenium dioxide. The complexity of the Raman spectrum of that solid suggests considerable polymerization. Accordingly, (SeO2)n may make an increasing contribution as the composition is increased above 66.67 mole% SeO2 in H2O, although the contribution of that species to the Raman spectra of the melt is considered to be small at the lower compositions.

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