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

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.