Triborate Formation Constants and Polyborate Speciation under Hydrothermal Conditions by Raman Spectroscopy using a Titanium/Sapphire Flow Cell.

Solvent-corrected reduced isotropic Raman spectra of aqueous boric acid + sodium borate solutions have been obtained from perpendicular and parallel polarization measurements in a novel custom-made titanium flow cell with sapphire windows over the temperature range 25 to 300 °C at 20 MPa using the perchlorate anion, ClO, as an internal standard. The reduced isotropic spectra of solutions yielded the first reported quantitative speciation results for polyborate ions in equilibrium with boric acid and borate in high-temperature aqueous solutions above 200 °C. The spectra obtained from solutions at low sodium/boron ratios, 0 < m/ m < 0.254, displayed well-defined bands at 880, 747, 615, and 532 cm, corresponding to the species B(OH) > [B(OH)] > [BO(OH)] ≫ [BO(OH)], respectively. The triborate ion, [BO(OH)], was found to be the major polyborate species in these boric acid-rich solutions in the range 25 to 300 °C. Thermodynamic formation constants for the triborate species, [BO(OH)], calculated from the peak areas, are in agreement with the literature values reported by Mesmer et al at 50, 100, and 200 °C to within the combined experimental uncertainties. At 300 °C, the value for the formation constant, log K = 2.259 ± 0.060, is larger than the value extrapolated from the results of Mesmer et al. by a factor of ∼3.