Raman Spectral Studies of Aqueous Solutions of Selenic Acid

Abstract

Relative integrated Raman intensities of aqueous solutions of selenic acid have provided concentrations of the species H2SeO4, HSeO4—, and SeO42—. When water is added to supercooled selenic acid at 25°C, the molecular H2SeO4 concentration, [H2SeO4], which is approximately 18 mole liter—1 in the pure acid, decreases rapidly and approaches zero, as the stoichiometric concentration of selenic acid, CH2SeO4, approaches 11–12M. Simultaneously, the HSeO4— concentration, [HSeO4—], increases, but it falls below the stoichiometric water molarity CH2O as dilution progresses. When CH2SeO4 = CH2O = 14.5M, [HSeO4—]<CH2O, indicating that the reaction H2SeO4+H2O = HSeO4—+OH3+ is incomplete. With further dilution, [HSeO4—] attains a maximum value of about 12.9 mole liter—1, at CH2SeO4 = 13.5M, and near CH2SeO4 = 11–12M, where [H2SeO4]≈0, the absence of intense characteristic Raman lines of SeO42— indicates that [H2SeO4—]≈[OH3+]. Below CH2SeO4 = 11–12M, Raman lines of SeO42— appear, as [HSeO4—] continues to decrease. When CH2SeO4 has decreased to approximately 4.2M, [SeO42—] attains a maximum value of about 0.91 mole liter—1, and the concentration at which [SeO42—] is maximal, compares favorably with the concentration corresponding to a maximum in the electrical conductivity, CH2SeO4 = 3.7M. Fundamental vibrational frequencies of H2SeO4, HSeO4—, and SeO42— have been assigned according to structures of C2v, Cs, and Td symmetry, respectively, and those assignments are compared with vibrational assignments of the closely related species H2SO4, HSO4—, and SO42—. Other comparisons between Raman spectra of nearly pure selenic and sulfuric acids, however, reveal additional frequencies at 932 and ∼1200 cm—1, respectively, produced by intermolecular vibrations. Such additional vibrations suggest the existence of extended, strongly hydrogen-bonded networks, in which symmetric vibrations of the SeO2 or SO2 groups of individual H2SeO4 or H2SO4 molecules are out-of-phase in neighboring molecules. In addition, another frequency of aqueous solutions of selenic acid at ∼800 cm—1, obtained from careful analysis of band contours, presumably is also produced by intermolecular vibrations between OH3+ and HSeO4—, in accordance with the low electrical conductivity and the high viscosity of solutions in which those ions are predominant.