Determining pH at Elevated Pressure and Temperature Using in Situ 13C NMR

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• 作者：J. Andrew Surface ; Fei Wang ; Yanzhe Zhu ; Sophia E. Hayes ; Daniel E. Giammar ; Mark S. Conradi
• 刊名：Environmental Science & Technology
• 出版年：2015
• 出版时间：February 3, 2015
• 年：2015
• 卷：49
• 期：3
• 页码：1631-1638
• 全文大小：306K
• ISSN：1520-5851

文摘

We have developed an approach for determining pH at elevated pressures and temperatures by using ¹³C 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 ¹³C NMR spectra of ¹³C-labeled CO₂ reactions with NaOH solutions and Mg(OH)₂ suspensions at pressures up to 107 bar and temperatures of 80 掳C. The quantitative nature of NMR spectroscopy allows the concentration ratio [CO₂]/[HCO₃^{鈥?/sup>] 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鈥?/sup>] 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.}