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

Abstract

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.

Mechanism for hydrolysis of double six‐membered ring tetraborate anion

“…The Mulliken charges of B (1) and B (3) are nearly equal as shown in Table 1. The Mulliken charges of O (9) and O (12) are equal, and these values are more negative than those of O (5) and O (8) . Thus, it can be inferred that the OH of water molecules tends to bond to B (1) and B (3) and that H preferentially bonds to O (9) and O (12) during hydrolysis reaction.…”

“…In pathway A, a water molecule attacks to B (3) 4 2− ] to form dihydrogen bond, and this makes the structure more stable. The more negative ESP near BO 4 and the more negative Mulliken charges of O (9) and O (12) may be the reasons for forming dihydrogen bonds. Cheng et al studied the hydrolysis mechanisms of aluminum alkoxide Al(OR) 3 and found that two hydrogen atoms of water first bond to the oxygen atoms of OR group in the form of hydrogen bond to form intermediate product.…”

“…It is difficult to identify the hydrolysis position when the second water molecule reacts with A-IM2. Therefore, two hydrolysis pathways were designed to study the difference between these two hydrolysis options as shown in Figures 2 and 3 (23) of water bonds to the corner oxygen atom O (9) . The intermediate A-IM3 absorbs 122 kJ/mol of energy to form the transition state A-TS2.…”

“…The intermediate A-IM3 absorbs 122 kJ/mol of energy to form the transition state A-TS2. The H (23) …O (21) distance in A-TS2 increases to 1.237 Å, and both of the H (23) …O (9) and O (21) H (22) Figure S2) shows that there is a weaker covalent interaction between B (4) …O (9) and B (2) …O (8) , and this indicates that the double ring characteristic still exists in A-IM4 and that it is difficult to completely break the ring. The average B-O distances in each BO unit of A-IM4 are 1.481 Å, 1.486 Å, 1.435 Å, and 1.480 Å.…”

“…Water molecules easily attacks to B (2) and B (4) in A-IM4 because the B (2) -O (8) and B (4) -O (9) bonds are broken to form BO 3 unit as show in Figure 2. As seen in Table 1, B (4) has the highest positive Mulliken charge and O (9) has the lowest negative charge in A-IM4.…”