6 research outputs found
Reference systems for assessing viability and accuracy of pH sensors in high temperature subcritical and supercritical aqueous solutions
This paper describes an effort in developing reference systems to be used in high temperature subcritical and supercritical aqueous solutions for assessing the viability and accuracy of high-temperature pH sensors. Each of these reference systems consists of a couple of three-component aqueous solutions in which the concentration of NaCl is greater than the concentration of another solute, such as HCl and NaOH. The viability of the reference systems was tested using the recently published experimental data employing a flow-through electrochemical cell at temperatures from 25°C to 400°C and densities from 0.17 to 1 g cm
−3 [S.N. Lvov, X.Y. Zhou, and A.V. Bandura, 1999a, Estimation of Isothermal Potential for HCl/NaCl and NaOH/NaCl Systems in High Temperatures Supercritical Aqueous Solutions, J. Supercrit. Fluids, in preparation; S.N. Lvov, X.Y. Zhou, and D.D. Macdonald, 1999, Flow-Through Electrochemical Cell for Accurate pH Measurements at Temperature up to 400°C, J. Electroanal. Chem., 463, 146–156]. The applicability of the method was tested for dilute (0.01 and 0.001 mol kg
−1) HCl aqueous solutions having background NaCl electrolyte in the amount of 0.1 mol kg
−1. The most significant property of the reference systems is that the pH difference, and hence, the cell potential, can easily be estimated with sufficient accuracy using only the analytical concentrations of the electrolytes without considering the speciation calculations, given the fact that the association and dissociation constants at supercritical temperatures have not been accurately determined yet
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Progress on yttria-stabilized zirconia sensors for hydrothermal pH measurements
Electrochemical cells are reviewed and a new design is evaluated for potentiometric pH measurements to above 300 °C. The new design system minimizes the effects of metal corrosion on measured pH. In addition, a recently developed [Zhou, X.Y., Lvov, S.N., Ulyanov, S.M., 2003. Yttria-Stabilized Zirconia Membrane Electrode, US Patent #6, S17, 694] flow-through, yttria-stabilized zirconia (YSZ) pH sensor has been further tested. The Nernstian behavior and precision of the YSZ electrode were evaluated by measuring the potentials vs. H
2–Pt electrode at 320 and 350 °C. Also, using the YSZ electrode, the association constants of HCl(aq) at 320 and 350 °C have been determined from the potentials of a HCl(aq) solutions at 0.01 to 0.001 mol kg
−1. The results, p
K
320=−1.46±0.46 and p
K
350=−2.35±0.25, in good agreement with literature data, both demonstrate the effective use of the cell and YSZ electrode for pH measurements to about ±0.05 pH units, and confirm the Nernstian behavior of the YSZ electrode in acidic HCl solutions up to 350 °C. Commercial YSZ tubes available for high-temperature pH sensing are, however, far from ideal because of irregular compositions, phase structures, and interstitial materials. A consequence is the premature structural decay of YSZ tubes in acidic solutions at elevated temperatures. In spite of the long-term decay, YSZ sensors respond rapidly to changes in pH, apparently limited only by the rate of mixing of solutions within the cell. This system makes the measurement feasible above 300 °C of mineral hydrolysis equilibrium constants and their free energy changes within uncertainties of about ±1.0 kJ