Experimental and theoretical constraints on pH measurements with an iridium oxide electrode in aqueous fluids from 25 to 175°C and 25 MPa

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In-situ measurement of pH at elevated temperatures and pressures is of major importance for investigating chemical and biochemical systems in extreme environments. Based on the performance of the newly developed IrO x electrode at 25°C, we initiated a series of experiments to test the electrode at elevated temperatures (100 to 175°C) and high pressure (25 MPa). The experiment was carried out in a titanium flow-through reactor. Our results revealed good pH response at 100, 150 and 175°C, with good Nernstian slopes at 100 and 150°C. Although a greater-than-Nernstian response was observed at 175°C, the factors that cause this difference are attributed to the accuracy of calculations of the distribution of aqueous species rather than alteration of the IrO x surface. A key problem that may limit applications of the IrO x electrode at elevated temperatures and pressures is the noticeable shift in E° during the 175°C (25 MPa) experiments and between experiments with similar conditions at 150°C. The results of tests from 25°C to elevated temperatures provide highly useful information on the reversibility and functionality of the IrO x -pH sensor with implications for the suitability of its use under challenging chemical and physical conditions.

Original languageEnglish (US)
Pages (from-to)1051-1062
Number of pages12
JournalJournal of Solution Chemistry
Issue number8
StatePublished - Aug 2008

Bibliographical note

Funding Information:
Acknowledgements The authors thank colleagues at University of Minnesota: Dr. K. Ding, Dr. Q. Fu and Dr. D. Foustoukos for many helpful discussions, R. Knurr for his assistance in chemical analysis, E. Frahm for electron microprobe analysis, Z. Zhang for the contribution and efforts in making the Ir pH electrodes, and M. Griffith for technical support in the hydrothermal laboratory. We are also grateful to Dr. D.A. Palmer and two anonymous referees for helpful comments and suggestions that contributed substantially to improvement of this paper. U.S. National Science Foundation Grant OCE-0549457 supported our research on sensor development. Financial support for Y. Pan from the Murthy-Noruk Fellowship from the Department of Geology and Geophysics, University of Minnesota is also gratefully acknowledged.


  • Aqueous fluids
  • Elevated temperatures and pressures
  • Iridium oxide
  • Nernst constraints
  • pH measurements
  • pH sensor


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