Raman Spectroscopic and ab Initio Investigation of Aqueous Boric Acid, Borate, and Polyborate Speciation from 25 to 80 °C

Temperature-dependent Raman studies of the aqueous speciation of boric acid and sodium borate have been carried out at 25 and 80 °C. Normalized solvent-corrected reduced isotropic Raman spectra were obtained from perpendicular and parallel polarization measurements using perchlorate anion, [ClO4]−, as an internal standard. The intensity variations of these bands with concentration and temperature provided strong evidence that these arise from boric acid B(OH)3, borate [B(OH)4]−, and the polyborate species [B3O3(OH)4]−, [B4O5(OH)4]2–, and [B5O6(OH)4]−. A very weak high frequency shoulder on the borate band may indicate the presence of the diborate species [B2O(OH)5]−. Temperature- and concentration-independent quantitative Raman molar scattering coefficients (S) for the symmetric vibrational bands of boron-containing species were calculated, consistent with the mixed solvent electrolyte model reported by Wang et al. (Pure Appl. Chem. 2013, 85, 2117–2144) up to approximately 100 °C. The band assignments and scattering parameters reported here provide a framework for using reduced isotropic Raman spectroscopy as a research tool for measuring quantitative speciation of boric acid/polyborate solutions under conditions relevant to nuclear reactor primary coolant circuits and spent fuel bays, among other applications.