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 SeO₄^2—. 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 C_H2O 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 SeO₄^2— indicates that [H₂SeO₄^—]≈[OH₃⁺]. Below CH₂SeO₄ = 11–12M, Raman lines of SeO₄^2— appear, as [HSeO₄^—] continues to decrease. When CH₂SeO₄ has decreased to approximately 4.2M, [SeO₄^2—] attains a maximum value of about 0.91 mole liter^—1, and the concentration at which [SeO₄^2—] 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 SeO₄^2— have been assigned according to structures of C_2v, C_s, and T_d symmetry, respectively, and those assignments are compared with vibrational assignments of the closely related species H₂SO₄, HSO₄^—, and SO₄^2—. 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.