title: Determining pH at elevated pressure and temperature using in situ ¹³C NMR.
abstract: We have developed an approach for determining pH at elevated pressures and temperatures by using (13)C NMR measurements of inorganic carbon species together with a geochemical equilibrium model. The approach can determine in situ pH with precision better than 0.1 pH units at pressures, temperatures, and ionic strengths typical of geologic carbon sequestration systems. A custom-built high pressure NMR probe was used to collect (13)C NMR spectra of (13)C-labeled CO2 reactions with NaOH solutions and Mg(OH)2 suspensions at pressures up to 107 bar and temperatures of 80 °C. The quantitative nature of NMR spectroscopy allows the concentration ratio [CO2]/[HCO3(-)] to be experimentally determined. This ratio is then used with equilibrium constants calculated for the specific pressure and temperature conditions and appropriate activity coefficients for the solutes to calculate the in situ pH. The experimentally determined [CO2]/[HCO3(-)] ratios agree well with the predicted values for experiments performed with three different concentrations of NaOH and equilibration with multiple pressures of CO2. The approach was then applied to experiments with Mg(OH)2 slurries in which the change in pH could track the dissolution of CO2 into solution, rapid initial Mg(OH)2 dissolution, and onset of magnesium carbonate precipitation.
authors.affiliation: Department of Chemistry, ‡Department of Energy, Environmental, and Chemical Engineering, and §Department of Physics, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: J Andrew
authors.initials: JA
authors.lastname: Surface
authors.forename: Fei
authors.initials: F
authors.lastname: Wang
authors.forename: Yanzhe
authors.initials: Y
authors.lastname: Zhu
authors.forename: Sophia E
authors.initials: SE
authors.lastname: Hayes
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Giammar
authors.forename: Mark S
authors.initials: MS
authors.lastname: Conradi
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry; D057965:Carbon Sequestration; D066241:Carbon-13 Magnetic Resonance Spectroscopy / Q000379:methods*; D005844:Geology / Q000379:methods; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000737:chemistry; D011312:Pressure; D013696:Temperature
chemical_list: C005479:magnesium carbonate; D002245:Carbon Dioxide; D008274:Magnesium