Pressure-balanced Reference Electrode Research Articles

Redox potential measurements in the H2SO4-FeSO4-Fe2(SO4)3-H2O system at high temperature using an Ir electrode

An Ir electrode was evaluated as a suitable alternative to Pt for redox potential measurements at high temperature and high pressure. Measurements were performed using a flow-through electrochemical cell (FTEC) with a Ag/AgCl in 0.1m LiCl flow-through external pressure-balanced reference electrode (FTEPBRE), at a flow rate of 0.5mL/min, 35atm (500psia) and up to 200°C. The potential was measured for the H2SO4-FeSO4-Fe2(SO4)3-H2O system for concentrations of 1M H2SO4, 10−2M Fe(III), and 10−4–10−2M Fe(II) (ratios of [Fe(II)]/[Fe(III)] equal to 0.01, 0.1, and 1 respectively). The potentials measured with the Ir electrode were similar to those obtained using a Pt electrode, with a maximum difference of 8mV over the range of studied solutions and temperatures. The change in potential measured with the Ir electrode behaved according to the Nernst equation upon changes in the test solution, and were consistent with the corresponding estimated potential variation predicted by the Aqueous (OLI-AQ) model from the OLI® aqueous chemistry simulation software (www.olisystems.com).

Electrochemical oxidation of carbon monoxide, methanol, formic acid, ethanol, and acetic acid on a platinum electrode under hot aqueous conditions

Using a pressure cell equipped with an Ag ∣ AgCl ∣ 0.1 M KCl external pressure-balanced reference electrode (EPBRE), hydrogen, methanol, formic acid, carbon monoxide, ethanol, acetic acid, and glucose were electrochemically oxidized on a Pt electrode under hot aqueous conditions (365−525 K), and the polarization curves were obtained at a sweep rate of 1 or 10 mV s −1. The potential measured versus EPBRE was corrected to the RHE scale based on the experimentally or theoretically calculated pH of the solution at high temperature. During methanol and carbon monoxide oxidation, a strongly adsorbed intermediate presumably CO, was formed but it was oxidized at a lower potential than under ambient temperature. Formic acid was rapidly oxidized around 0 V versus RHE without formation of this adsorbed intermediate. Using a gas mixture of hydrogen and carbon monoxide, it was confirmed that the surface coverage by CO was decreased dramatically with a temperature increase from 425 to 475 K under hot aqueous conditions. Ethanol and acetic acid were also satisfactorily oxidized, but the trial to measure the electrochemical oxidation behavior of glucose was not successful due to the adhesion of char-like compounds to the electrode.

Flow-through electrochemical cell for accurate pH measurements at temperatures up to 400°C

Abstract We have developed a flow-through electrochemical cell (FTEC) and demonstrated that it operates successfully in both subcritical and supercritical aqueous environments at temperatures up to 400°C. The FTEC consists of a previously developed flow-through external (Ag ∣ AgCl) pressure-balanced reference electrode (FTEPBRE) (Lvov et al., J. Electroanal. Chem. 444 (1998) 186) and a modified flow-through platinum hydrogen electrode (FTPHE). The experimental measurements have been carried out for two solutions: (1) 10−3 mol kg−1 HCl+10−1 mol kg−1 NaCl and (2) 10−2 mol kg−1 HCl+10−1 mol kg−1 NaCl at flow rates between 0.1 and 1.4 cm3 min−1 and at temperatures of 25°C (18.7 MPa), 200°C (23.8 MPa), 350°C (24.2MPa), and 400°C (25.3 MPa). We have also carried out a detailed irreversible thermodynamic analysis of the FTEC and estimated different significant contributions to the measured potentials. Comparison of experimentally observed and theoretically calculated pH values shows an ability to use the FTEC for high precision potentiometric and pH measurements. We conclude that high temperature pH measurements with accuracy better than ±0.03 logarithmic units can now be accomplished. Also, we assume that valuable streaming potential data can be obtained using the FTEC for estimating the surface charge (zeta potential) on the oxide material capillary tube in hydrothermal solutions at temperatures up to 400°C.

Potentiometric pH measurements in high subcritical and supercritical aqueous solutions

An operational electrochemical thermocell is proposed for potentiometric pH measurements in high subcritical and supercritical aqueous solutions. The thermocell consists of the silver–silver chloride flow-through external pressure-balanced reference electrode (FTEPBRE) and the flow-through platinum–hydrogen pH sensor. Using this thermocell, we have measured the potentials of the thermocell for a few HCl(aq) and NaOH(aq) solutions of different concentrations and derived the corresponding pH differences (ΔpH) over a wide range of temperatures from 25 to 400°C at pressures between 27.5 and 33.8 MPa. Comparison of the experimentally derived and theoretically calculated ΔpH values clearly demonstrates an ability of the thermocell to measure pH with accuracy better than ±0.1 units, and this result in turn demonstrates the viability of the FTEPBRE as a versatile reference electrode for use at high subcritical and supercritical temperatures.

Advanced flow-through external pressure-balanced reference electrode for potentiometric and pH studies in high temperature aqueous solutions

A new flow-through external pressure-balanced reference electrode (FTEPBRE) has been developed for potentiometric and pH measurements in high temperature aqueous solutions. The unique feature of this advanced FTEPBRE is that the reference solution flows through the electrode so that the concentration of solution across the thermal liquid junction is well defined. Because the electrolyte concentration profile in the reference electrode is maintained constant, uncertainty in the thermal liquid junction potential (TLJP) can be eliminated at a given temperature and pressure. We have employed the silver⊢silver chloride |platinum|hydrogen non-isothermal electrochemical system for potentiometric measurements of potentials between HCI (aq) solutions of 0.01 and 0.001 mol kg −1 at temperatures up to 633 K and at pressures of 275 and 338 bar. The results of the measurements have been compared with calculations made using the available thermodynamic data, and good agreement, within ±3 to 10 mV, between the calculated and the measured potentials is obtained. We concluded that the advanced reference electrode can be used to make accurate potentiometric measurements (within a few mV) in aqueous systems over wide ranges of temperatures and pressures. Also we demonstrated an ability of the thermocell to measure pH with high accuracy of ± 0.04 to 0.09 over wide ranges of temperature and pressure.

流通条件下における岩石／熱水反応系のｐＨ測定

A pH-measuring system was daveloped for the study of rock/hot water reaction. This system consists of a main reaction autoclave and a pH-measuring cell, and the main autoclave is connected with the pH measuring cell by three sampling pipes. The pH of reaction solution can be measured directly at the same temperature and pressure as the main autoclave. A platinum-hydrogen test electrode and a silver-silver chloride external pressure-balanced reference electrode were used in the pH-measuring cell. The pHmeasuring cell showed a Nernstian pH response in the all temperature and pressure ranges (100-250°C and 50-200kgf/cm2) studied here. The temperature dependence of pH for calcite-water system was determined under the saturated solubility condition in the temperature range of 100-250°C using the pH measuring system.