In Situ Measurement of pH Under High Pressure

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Abstract
In situ measurement of pH under high pressure is complicated by the need to remain functional under extreme pressures, electrically noisy environments, and the difficulty in inferring chemical equilibrium conditions under pressure. We describe a pH sensor wherein the current flowing at a given input voltage is directly related to hydrogen ion concentration and can be operated within extreme environments. The sensor was tested at 25 °C for in situ pH measurement of several buffer solutions under high pressure up to 784.6 MPa. We found generally an increase in the acidity of the buffer solutions with increasing pressure. In all cases, pH changes were found to be less than predicted using inferred reaction volume data from the literature. Sensor response and pH changes were found to be completely reversible upon depressurization at the same temperature.