In-situ measurement of dissolved H₂ in aqueous fluid at elevated temperatures and pressures

In-situ measurement of Hz in aqueous fluid under hydrothermal conditions has long been a major concern in experimental geochemistry owing to the important role of H₂ in a wide range of homogenous and heterogeneous equilibria. Although numerous efforts have been made, an effective and reliable H₂ sensor for geochemical applications is lacking, especially one suitable for measurement at supercritical conditions of water. In this paper, we discuss a sensor which was developed for in-situ measurement of dissolved H₂ in aqueous fluid at temperatures and pressures greater than previously possible. In general, the H₂ sensor consists of two electrodes and a Hg/HgO internal reference element: a platinum electrode and a yttrium-stabilized zirconia (YSZ) membrane electrode. The electrochemical cell can be depicted as follows:. Pt|H₂, H⁺, H₂O|ZrO₂(Y₂O³)| HgO|Hg. A titanium flow reactor was used to test and calibrate sensor response and sensitivity. Dissolved H₂ concentration and the rate of fluid flowing through the reactor were controlled by a modified HPLC pump and computer system, respectively. Dissolved H₂ concentrations of fluid leaving the reactor were determined by GC analysis. ΔE_YH of the fluid in the reactor was, at the same time, monitored and recorded by an electrometer with high input impedance. The experiments were performed at 400 bars, and temperatures up to 400°C. Results revealed good Nemstain response, even for unusually low dissolved H₂ concentrations. Thus, at 400°C and 400 bars, the following calibration line was obtained:. ΔE_YH (V) = 0.972 + 0.054 log mH₂ (M/kg) (r = 0.9974, N = 9), where mH₂ ranged from 0.096 to 5.75 mM/kg. The experiments also demonstrated excellent stability of the sensor during the month-long operation. The high temperature H₂ sensor may have important applications for field monitoring dissolved H₂ in mid-ocean ridge vent fluids. It is also ideally suited for direct determination of redox state in hydrothermal experiments at a wide range of conditions.