A Raman spectroscopic and ab initio investigation of aqueous boron speciation under alkaline hydrothermal conditions: evidence for the structure and thermodynamic stability of the diborate ion.

Raman spectra of aqueous sodium borate solutions, with and without excess NaOH, NaCl, and LiCl, have been obtained from perpendicular and parallel polarization measurements acquired using a custom-built sapphire flow cell over the temperature range 25 to 300 °C at 20 MPa. The solvent-corrected reduced isotropic spectra include a large well-defined band at 865 cm-1 which overlaps with the boric acid B(OH)3 band at 879 cm-1, and becomes increasingly intense at elevated temperatures. This band does not correspond to the spectrum of any other previously reported aqueous polyborate ions, all of which have symmetric stretching bands at frequencies below that of borate, [B(OH)4]-, at 745 cm-1. Based on the classic high-temperature potentiometric titration study by R. E. Mesmer, C. F. Baes and F. H. Sweeton, Acidity measurements at elevated temperatures. VI. Boric acid equilibriums, Inorg. Chem., 1972, 11, 537-543, the new band was postulated to arise from a diborate ion, [B2(OH)7]- or [B2O(OH)5]-. Ab initio density functional theory (DFT), together with chemical modelling studies, suggest that it is most likely [B2(OH)7]-. Thermodynamic formation quotients derived from the peak areas showed variations with ionic strength as well as charge-balance discrepancies, which suggest one or more unidentified minor equilibrium species may also be present. The most likely candidate is the divalent diborate species [B2O2(OH)4]2- which is also predicted to have a band near 865 cm-1 and is postulated to be present as a sodium ion pair. These are the first quantitative Raman spectra ever reported for borate-rich solutions under such conditions and provide the first spectroscopic evidence of a diborate species at PWR reactor coolant temperatures.

Second ionization constant of sulfuric acid in H2O and D2O from 150 to 300 °C at p = 11.5 MPa using flow AC conductivity.

A custom-built flow-through AC conductivity instrument was used to measure the deuterium isotope effect on the ionization quotient of bisulfate from 150 to 300 °C, at p = 11.5 MPa. Standardized solutions of KCl, HCl, KOH, KHSO4, K2SO4, and H2SO4 were prepared in light and heavy waters and their conductivities were measured and fitted with the Quint-Viallard conductivity model to obtain single ion conductivities at infinite dilution for K+, Cl-, H+, OH-, HSO4-, and SO42-. These are the first conductivities of DSO4- and SO42- measured in heavy water at any temperature, and the first ionization constants for bisulfate reported in heavy water above 225 °C. The deuterium isotope effect on the chemical equilibrium constant, ΔpK2a = pK2a,D - pK2a,H, was found to increase with temperature, in contrast to the behaviour seen for other simple oxyacids.

Limiting Conductivities of Strong Acids and Bases in D2O and H2O: Deuterium Isotope Effects on Proton Hopping over a Wide Temperature Range.

The molar conductivity (Λ°) of hydrochloric acid, potassium hydroxide, and sodium hydroxide has been measured in both light and heavy waters from 298 to 598 K at p = 20 MPa using a high-precision flow-through alternating current (AC) conductance instrument. The results were used to explore the deuterium isotope effect on ionic transport by proton hopping mechanisms under hydrothermal conditions. Extrapolations of published transport number data to elevated temperature were used to calculate the individual ionic contributions (λ°) for H3O+, D3O+, OH-, and OD-, from which the excess molar conductivities due to proton hopping were calculated. These are the first reported values for the excess conductivities for D3O+ and OD- at temperatures above 318 K. The excess conductivities indicate a strong deuterium isotope effect whereby the transport of D3O+ by proton hopping is reduced by ∼33% relative to H3O+, and OD- is reduced by over 60% relative to OH-, over the entire temperature range. A well-defined maximum in the excess conductivities of D3O+ and H3O+ at ∼420 K suggests that the Eigen cation (H2O)4H+ and the Zundel transition-state cation (H2O)2H+ are destabilized at elevated temperatures as the three-dimensional, tetrahedrally hydrogen-bonded networks in water break down. The less pronounced maximum for OD- and OH- suggested that their Eigen and Zundel anions, (H2O)3OH- and (H2O)OH-, are less destabilized in the two-dimensional networks and chains that dominate the "structure" of liquid water under these conditions.

Third dissociation constant of phosphoric acid in H2O and D2O from 75 to 300 °C at p = 20.4 MPa using Raman spectroscopy and a titanium-sapphire flow cell.

A custom-built titanium-sapphire flow cell has been used with a confocal Raman microscope to collect solvent-corrected reduced isotropic spectra of sodium and potassium phosphate solutions in light and heavy water from 75 to 300 °C at 20.4 ± 0.4 MPa over a wide range of concentrations. The symmetric vibrational modes of PO43- and HPO42-/DPO42- in both solvents broadened and moved to lower wavenumbers with increasing temperature, suggesting that oxyanion-water hydrogen bond strengths increase at elevated temperatures. Raman scattering coefficients, measured relative to the trifluoromethanesulfonate ion, were used to determine thermodynamic equilibrium quotients for the reaction PO43- + H2O ⇌ HPO42- + OH- and its deuterium counterpart. Standard-state acid ionization constants were calculated using a modified Pitzer model and fitted as a function of temperature and solvent molar volume over the range of 25 to 300 °C from psat to 20 MPa. The deuterium isotope effect on the chemical equilibrium constant, ΔpK3a,m = pK3a,D,m - pK3a,H,m, was found to decrease from 1.045 ± 0.046 at 25 °C to 0.898 ± 0.073 at 250 °C. This behaviour is consistent with a model in which zero-point energy effects dominate at low temperatures and long-range solvent polarization becomes increasingly important as the temperature increases towards the critical point of D2O. These are the first experimental ionization constants to be reported in the literature for this reaction in light water above 50 °C and in heavy water at any temperature.

Second Dissociation Constant of Carbonic Acid in H2O and D2O from 150 to 325 °C at p = 21 MPa Using Raman Spectroscopy and a Sapphire-Windowed Flow Cell.

Solvent-corrected reduced isotropic spectra of carbonate and bicarbonate in light and heavy water have been measured from 150 to 325 °C at 21 MPa using a confocal Raman microscope and a custom-built titanium flow cell with sapphire windows. The positions of the symmetric vibrational modes of CO32- and HCO3-/DCO3- were compared to density functional theory (DFT) calculations with a polarizable continuum model in light and heavy water. The experimental Raman peak positions shifted linearly toward lower wavenumbers with increasing temperatures. Raman scattering coefficients, measured relative to a perchlorate internal standard, were used to determine equilibrium molalities of the carbonate and bicarbonate species. These yielded quantitative thermodynamic equilibrium quotients for the reaction CO32- + H2O ⇌ HCO3- + OH- and its deuterium counterpart. Ionization constants for HCO3- and DCO3-, K2a,H,m and K2a,D,m, calculated in their standard states using the Meissner-Tester activity coefficient model, were combined with critically evaluated literature data to derive expressions for their dependence on temperature and pressure, expressed as solvent molar volume, over the range 25 to 325 °C from psat to 21 MPa. These are the first experimental values to be reported for this reaction in light water above 250 °C and in heavy water above 25 °C. The value of the deuterium isotope effect on the chemical equilibrium constant, ΔpK2a,m = pK2a,D,m - pK2a,H,m, decreased from ΔpK2a,m = 0.67 ± 0.07 at 25 °C to ΔpK2a,m = 0.17 ± 0.13 at 325 °C and psat.

Investigation of Uranyl Sulfate Complexation under Hydrothermal Conditions by Quantitative Raman Spectroscopy and Density Functional Theory.

Quantitative first and second formation constants of aqueous uranyl sulfate complexes were obtained from Raman spectra of solutions in fused silica capillary cells at 25 MPa, at temperatures ranging from 25 to 375 °C. Temperature-dependent values of the symmetric O-U-O vibrational frequencies of UO22+(aq), UO2SO40(aq), and UO2(SO4)22-(aq) were determined from the high-temperature spectra. Temperature-independent Raman scattering coefficients of UO22+(aq) were calculated directly from uranyl triflate spectra from 25 to 300 °C, while those of UO2SO40(aq) and UO2(SO4)22-(aq) were derived from spectroscopic data at 25 °C and concentrations calculated using the formation constants of Tian and Rao ( J. Chem. Thermodyn. 2009 , 41 , 569 - 574 ), together with the Specific Ion Interaction Theory (SIT) activity coefficient model. Chemical structures and vibrational frequencies predicted from Density Functional Theory (Gaussian 09) were employed to interpret the Raman spectra. Values of the cumulative formation constants ranged from log ß1 = 3.23 ± 0.08 and log ß2 = 4.22 ± 0.15 at 25 °C, to log ß1 = 12.35 ± 0.22 and log ß2 = 14.97 ± 0.02 at 350 °C. This is the first reported use of high-pressure fused silica capillary cells to determine formation constants of metal ligand complexes from their reduced isotropic Raman spectra under hydrothermal conditions.

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, ClO4-, 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 < mNaST/ mBST < 0.254, displayed well-defined bands at 880, 747, 615, and 532 cm-1, corresponding to the species B(OH)3 > [B(OH)4]- > [B3O3(OH)4]- ≫ [B5O6(OH)4]-, respectively. The triborate ion, [B3O3(OH)4]-, 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, [B3O3(OH)4]-, 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 K31, m b= 2.259 ± 0.060, is larger than the value extrapolated from the results of Mesmer et al. by a factor of ∼3.

Non-Complexing Anions for Quantitative Speciation Studies Using Raman Spectroscopy in Fused Silica High-Pressure Optical Cells Under Hydrothermal Conditions.

This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO4(-)), perchlorate (CIO4(-)), perrhenate (ReO4(-)), and trifluoromethanesulfonate, or "triflate" (CF3SO3(-)) in acidic and neutral solutions at temperatures up to 400°C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO4(-) (stable) > ReO4(-) > CIO4(-) > CF3SO3(-), with half-lives (t1/2) > 7 h at 300°C. In neutral solutions, the order was HSO4(-) (stable) > CF3SO3(-) > ReO4(-) > CIO4(-), with t1/2 > 8 h at 350°C. CF3SO3(-) was extremely stable in neutral solutions, with t1/2 > 11 h at 400°C.

Raman and ab initio investigation of aqueous Cu(I) chloride complexes from 25 to 80 °C.

Temperature-dependent Raman studies of aqueous copper(I) chloride complexes have been carried out up to 80 °C, along with supporting ab initio calculations for the species [CuCl(n)(H2O)m](1-n), n = 0-4 and hydration numbers m = 0-6. Normalized reduced isotropic Raman spectra were obtained from perpendicular and parallel polarization measurements, with perchlorate anion, ClO4(-), as an internal standard. Although the Raman spectra were not intense, spectra could be corrected by solvent baseline subtraction, to yield quantitative reduced molar scattering coefficients for the symmetric vibrational bands at 297 ± 3 and 247 ± 3 cm(-1). The intensity variations of these bands with concentration and temperature provided strong evidence that these arise from the species [CuCl2](-) and [CuCl3](2-), respectively. The results from ab initio calculations using density functional theory predict similar relative peak positions and intensities for the totally symmetric Cu-Cl stretching bands of the species [CuCl2(H2O)6](-) and [CuCl3(H2O)6](2-), in which the water is coordinated to the chloride ions. A less intense Raman band at 350 ± 10 cm(-1) is attributed to the symmetric Cu-Cl stretching mode of hydrated species [CuCl(H2O)](0) with six waters of hydration. Temperature- and concentration-independent quantitative Raman molar scattering coefficients (S) are reported for the [CuCl2](-) and [CuCl3](2-)species.

Standard partial molar volumes of some aqueous alkanolamines and alkoxyamines at temperatures up to 325 degrees C: functional group additivity in polar organic solutes under hydrothermal conditions.

Apparent molar volumes of dilute aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N,N-dimethylethanolamine (DMEA), ethylethanolamine (EAE), 2-diethylethanolamine (2-DEEA), and 3-methoxypropylamine (3-MPA) and their salts were measured at temperatures from 150 to 325 degrees C and pressures as high as 15 MPa. The results were corrected for the ionization and used to obtain the standard partial molar volumes, Vo2. A three-parameter equation of state was used to describe the temperature and pressure dependence of the standard partial molar volumes. The fitting parameters were successfully divided into functional group contributions at all temperatures to obtain the standard partial molar volume contributions. Including literature results for alcohols, carboxylic acids, and hydroxycarboxylic acids yielded the standard partial molar volume contributions of the functional groups >CH-, >CH2, -CH3, -OH, -COOH, -O-, -->N, >NH, -NH2, -COO-Na+, -NH3+Cl-, >NH2+Cl-, and -->NH+Cl- over the range (150 degrees C

Apparent and standard partial molar volumes of NaCl, NaOH, and HCl in water and heavy water at T = 523 K and 573 K at p = 14 MPa.

Apparent molar volumes, Vphi,2, of aqueous NaCl, NaOH, NaOD, HCl, and DCl in water and heavy water were determined at T = 523 and 573 K and p = 14 MPa with a high-temperature platinum vibrating-tube densimeter in the aquamolality range 0.25

Standard partial molar volumes of aqueous glycolic acid and tartaric acid from 25 to 350 degrees C: evidence of a negative Krichevskii parameter for a neutral organic solute.

Apparent molar volumes have been determined using a high-pressure vibrating-tube densimeter for aqueous solutions of glycolic acid (HGly = HOCH(2)COOH) and tartaric acid (H(2)Tar = HOOCCH(OH)CH(OH)COOH) at temperatures from 25 degrees C to 350 degrees C and pressures as high as 20 MPa. The resulting standard partial molar volumes (HGly,aq) are relatively independent of temperature until 315 degrees C, at which point (HGly,aq) deviates sharply toward negative values. This suggests that the Krichevskii parameter, A(Kr) = lim(x(2) --> 0) , which describes the discontinuities in standard partial molar properties at the critical point of water, is negative. Almost all aqueous nonelectrolytes are characterized by positive Krichevskii parameters. This is the first negative value reported for any organic molecule that is not an ion or zwitterion and only the third ever observed directly for a neutral species (the others are B(OH)(3) and H(3)PO(4)). The standard partial molar volumes for H(2)Tar(aq) are also relatively independent of temperature until 275 degrees C, suggesting a similar behavior. However, the onset of thermal decomposition prevented measurements at temperatures above 300 degrees C.