Aqueous Geochemistry at Gigapascal Pressures: NMR Spectroscopy of Fluoroborate Solutions

UC Davis UC Davis Previously Published Works Title Aqueous geochemistry at gigapascal pressures: NMR spectroscopy of fluoroborate solutions Permalink https://escholarship.org/uc/item/0283b3h3 Authors Ochoa, G Pilgrim, CD Kerr, J et al. Publication Date 2019 DOI 10.1016/j.gca.2018.09.033 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 244 (2019) 173–181 www.elsevier.com/locate/gca Aqueous geochemistry at gigapascal pressures: NMR spectroscopy of fluoroborate solutions Gerardo Ochoa a, Corey D. Pilgrim a, Julia Kerr a, Matthew P. Augustine a William H. Casey a,b,⇑ a Department of Chemistry, University of California, Davis, 1 Shields Ave, Davis, CA 95616, USA b Department of Earth and Planetary Sciences, University of California, Davis, 1 Shields Ave, Davis, CA 95616, USA Received 27 April 2018; accepted in revised form 29 September 2018; Available online 10 October 2018 Abstract Aqueous geochemistry could be extended considerably if nuclear-magnetic resonance (NMR) methods could be adapted to study solutions at elevated temperatures and pressures. We therefore designed an NMR probe that can be used to study aqueous solutions at gigapascal pressures. Fluoride solutions were chosen for study because 19F couples to other nuclei in the solutions (31P and 11B) in ways that make peak assignments unequivocal. Correspondingly, NMR spectra of 19F- and 11B were 19 collected on aqueous HBF4-NH4PF6 solutions to pressures up to 2.0 GPa. At pressure, peaks in the F spectra were clear and À À À assignable to the BF4 (aq), F (aq) and BF3OH (aq) ions, and these aqueous complexes varied in signal intensity with pres- 11 À À sure and time, for each solution. Peaks in the B spectra at pressure could be assigned to the BF4 (aq) and BF3OH (aq) spe- o À cies. Additionally, there is a single peak that is assignable to H3BO3(aq) and B(OH)4 (aq) in rapid-exchange equilibria. These peaks broaden and move with pressure in ways that suggest reversible interconversion of borate and fluoroborate species. The À 19 PF6 ion was found to provide a suitable F shift and intensity standard for high-pressure spectra because it was chemically inert. The positions and intensities of the doublet peak also remains constant as a function of pressure and pH. Addition of electrolytes considerably distorts the phase diagram of water such that the stability region of the aqueous solution expands to well beyond the 0.8 GPa freezing pressure of pure water; some fluoroborate solutions remain liquid until almost 2.0 GPa. Ó 2018 Elsevier Ltd. All rights reserved. Keywords: Spectroscopy; NMR; Aqueous solutions; High-pressure; Kinetics 1. INTRODUCTION This gap in technology inspired us to design a high-pressure NMR probe that can resolve signals from solute species in Geochemists developed models to predict solute specia- aqueous solutions at gigapascal pressures (Pautler et al., tion and mineral equilibria in aqueous solutions to 1200 °C 2014; Ochoa et al., 2015, 2016). and 6.0 GPa. These models are a primary geochemical tool The challenges to geochemists of high-pressure NMR on for uncovering reaction pathways in the Earth (see aqueous solutions are considerable. For example, the sam- Sverjensky et al., 2014; Sverjensky and Huang, 2015). The ple volume must be larger than a few microliters if induc- need for these models is compelling but the conditions are tion methods of detecting NMR signals are employed; well beyond the limits of conventional NMR spectroscopy. one needs 1020 spins in the RF coil. The need for such volumes precludes the facile use of diamond-anvil cells for detecting aqueous solutes as the volumes of the diamond ⇑ Corresponding author at: Department of Chemistry, University anvils are nanoliters or less (see Meier et al., 2017; Meier, of California, Davis, 1 Shields Ave, Davis, CA 95616, USA. 2018). Another challenge for aqueous solution geochemists E-mail address: [email protected] (W.H. Casey). https://doi.org/10.1016/j.gca.2018.09.033 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved. 174 G. Ochoa et al. / Geochimica et Cosmochimica Acta 244 (2019) 173–181 is the need to identify a new chemical-shift standard for the follows: water = 1.000 ± 0.004; (1) 1.090 ± 0.005, (2) chemical system that is studied. As pressures increase to a 1.030 ± 0.005, (3) 0.999 ± 0.004, and (4) 1.006 ± 0.005, few gigapascals at near-ambient temperatures, the relative with the uncertainties corresponding to one estimated stan- permittivity of water, and thus the Brønsted acid-base dard error of the mean. properties of solutes, changes dramatically. The signals from shift standards commonly change accordingly and 2.2. 19F and 11B NMR Spectroscopy thus become useless, or the compound simply decomposes. In this paper we collect information on aqueous solutes The design of the high-pressure NMR probe is described via 19F- and 11B NMR spectra at gigapascal pressures. Flu- in earlier papers (Pautler et al., 2014; Augustine et al., 2017) oroborate solutions were chosen not because the ions are and will not be discussed at length here. Internal pressure À common in geochemical fluids, but because the BF4 (aq) was estimated from gauge pressure on a laboratory hydrau- 19 11 ion has coupled F- and B NMR spectra (JFB = 15 Hz) lic press using an external calibration curve based upon flu- and because we could identify a chemical-shift standard, the orescence of a ruby coupled to a fiberoptic cable (Fig. S-1). À 19 PF6 (aq) ion, which has useful coupling between F and Uncertainties in pressure are estimated to be ±0.1 GPa (one 31P and is unreactive. The constant couplings allow the spe- estimated standard deviation). Metal parts of the microcoil ciation to be interpreted unequivocally because the peak geometry were protected from corrosion by the sample positions do not move with pressure or pH. The probe solution by coating them with a thin layer of epoxy and latex. design differs significantly from that described by Pautler The 19F spectra at pressure were collected at 42.23 MHz et al (2014) and Ochoa et al. (2016) in that the aqueous on an Aspect Imaging M2TM compact MRI instrument built solution is also used as the pressure-transmission fluid around a permanent 1.05 T magnet that was interfaced to a (see Augustine et al., 2017); thus the sample volume is TecmagTM Redstone spectrometer. Shimming was per- increased from 15 lLto0.4 mL. The pressures reached formed on the single 1H peak in the solvent water by (2.0 GPa) corresponds to pressures of the upper mantle, increasing the radio frequency (RF) to 44.89 MHz. Here albeit at near-ambient temperatures. an automated MatlabTM code controlled the Redstone pulse programmer (1H pulse parameters: p/2 pulse = 18 ls, relax- 2. EXPERIMENTAL METHODS ation delay = 3 s). An RF-pulse tip angle of 31.2° (4.1 ls) with a relaxation delay of 500 ms was used for all 19F spec- 2.1. Solution chemistry tra (see Ernst and Anderson, 1966). Data analysis of 19F spectra collected at pressure was accomplished by signal- All solutions were prepared from 18 MX water. For averaging 200 two-dimensional (2D) scans over a 15-h per- determining properties of the shift standard, two 1.0 M iod; each 2D scan contained 500 individual scans. This 2-D NH4PF6 solutions were created by weighing the appropriate method was necessary to compensate for drift of the mag- amount of reagent-grade NH4PF6 into a volume of H2O netic field. Compensation was accomplished using a Mat- TM À and then titrating to pH = 3.5 and pH = 10.0 with NaOH. lab code that adjusted the spectra using the PF6 (aq) Fluoroboric acid/ammonium hexafluorophosphate solutions signal as a standard. Additional high-resolution, ambient were created by weighing the appropriate amount of reagent- 19F spectra for all solutions were collected at 470.54 MHz grade NH4PF6 into a prepared solution of HBF4 in H2O. using an 11.7 T superconducting magnet interfaced to a The pH of the fluoroborate solutions varies both with Bruker DRX spectrometer. À 11 time and pressure, due to the slow hydrolysis of BF4 (aq) The B NMR spectra of solutions 1–3 were acquired at to fluoride and boric acid and because of the changing 74.41 MHz using at 6.95 T Oxford Instruments 78-mm-bore Brønsted acid-base chemistry with pressure. Therefore we superconducting magnet that was interfaced to a Tecmag denote as pH° the pH values initially measured at ambient OrionTM spectrometer. A single-pulse acquisition was used, pressure. The pH was adjusted with the addition of solid with p/2 pulse of 29 ls and a relaxation delay of 200 ms. 11 NaOH or aqueous NH4OH. Four separate solutions were Additional B NMR spectra on solution 2 were collected used in this study: (1) 2.28 M HBF4, 0.83 M NH4PF6, from ambient pressure to 1.3 GPa at 42.99 MHz using a 2.68 M NaOH, pH° = 13.0; (2) 0.57 M HBF4, 0.22 M Bruker spectrometer that was built around a 9.398 T super- NH4PF6, 0.83 M NH4OH, pH° = 8.1; (3) 0.59 M HBF4, conducting magnet. Shimming for these experiments were 0.22 M NH4PF6, 0.69 M NH4OH, pH° = 5.1; (4) 0.60 M accomplished by changing the RF to 61.42 MHz in order 2 HBF4, 0.22 M NH4PF6, 0.60 M NH4OH, pH° = 3.3. Solu- to detect the H signal of D2O in the solvent of solution 2 tion 1 was intentionally saturated with the mineral Villiau- at each pressure and shimming until the peak width was mite [NaF(s)] before running the pressure experiments and approximately 70 Hz at full-width-at-half-maximum (Sup- the composition is therefore as analyzed, not as prepared.

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