Thermal Liquid Junction Research Articles

Construction and characterization of a solution-filled external pressure balanced reference electrode

This paper aims at the details of construction and characterization of a solution-filled reference electrode. Using heat transfer analysis, the temperature profiles in the heat exchanger region of the reference electrode were obtained. Using these profiles, and available data on Soret coefficients, the thermal liquid junction potential was calculated as a maximum of 4.74mV at the initial temperature of 473K. The calculations for liquid junction potential and IR drop were also performed. Theoretical calculations of electrode potential were used to validate the calculations.

Theoretical Analysis of the Electrode Potential of the Newly Designed KCl Buffered External Ag/AgCl Electrode

Pressure-balanced external Ag/AgCl electrodes have been extensively used for electrochemical measurements in high-temperature aqueous experiments. A simple design is suggested that can be available in temperatures where polymer-based materials cannot be applied to, as an electrode body. In this design, the poly(tetrafluoroethylene) tube-ceramic tube junction at the low-temperature region and ceramic tube-ceramic plug junction jointed by brazing in the hot-temperature region play crucial roles. The potential of the KCl-buffered external Ag/AgCl electrode was characterized by estimating the thermal liquid junction potential (TLJP) occurring from the thermal diffusion of buffered ionic species existing in the filling solution. The potential of the thermoelectrochemical cell, was divided into the sum of the isothermal potentials and TLJP. The TLJP for the Soret steady state was estimated by adopting Macdonald et al. ’s work [J. Electrochem. Soc., 126, 1618 (1979)] and by calculating the heat of transport after Agar et al. ’s theory [J. Phys. Chem., 93, 2079 (1989)], and Kirkwood et al. ’s linear response theory [J. Chem. Phys., 18, 857 (1960)]. The potential depression at the low-temperature region, which resulted from the thermal diffusion, was also calculated. The calculated potential of the KCl-buffered external Ag/AgCl electrode together with the experimental data was presented. © 2004 The Electrochemical Society. All rights reserved.

Evaluation of Thermal Liquid Junction Potential of Water-Filled External Ag/AgCl Reference Electrodes

Pressure-balanced external Ag/AgCl reference electrodes have been extensively used for corrosion monitoring in both pressurized water reactor and boiling water reactor environments. In order to prolong the electrode lifetime, pure water is often employed as the electrode filling solution. Characterization of the potential of the water-filled external Ag/AgCl reference electrode was performed by estimating a thermal liquid junction potential (TLJP) originating from the thermal diffusion of ionic species in the filling solution. The potential of the thermoelectrochemical cell, Ag/AgCl vs. that of the standard hydrogen electrode at temperature was expressed as the sum of the isothermal potentials and TLJP. The TLJP was analyzed for the Soret steady state based on irreversible thermodynamics by calculating the heat of transport after Agar et al’s theory and estimating the limiting ionic conductance from Quist et al’s work. Calculated potential of the water-filled external reference electrode was compared with experimental data, showing a qualitative agreement. © 2003 The Electrochemical Society. All rights reserved.

Potentiometric pH measurements in acidic sulfate solutions at 250 °C relevant to pressure leaching

A flow-through yttria-stabilized zirconia (FTYSZ) electrode has been employed for pH measurements at 250 °C in concentrated electrolyte systems relevant to hydrometallurgical processing of nickeliferous laterites. Experiments were conducted using a custom-made flow-through titanium electrochemical cell specially designed for high-temperature potentiometric pH measurements. In the present work, we assess the accuracy and validity of pH measurements with theory at 250 °C on concentrated sulfate process solutions. First, binary sulfuric acid–water solutions were measured at concentrations ranging from 0.05 to 0.5 mol kg −1 and showed 0.3 pH unit increase on the average between 25 and 250 °C. The measurements were then extended to ternary and quaternary systems containing 0.0076–0.038m Al 2(SO 4) 3 and 0.0276–0.276m MgSO 4, respectively. Autoclave passivation against corrosion was successfully achieved using sodium chromate or hydrogen peroxide. In this technique, the potentials are measured against a Ag/AgCl flow-through external reference electrode (FTEFE). A reference solution of 0.1 mol kg −1 NaCl(aq) flows permanently through the electrode at a constant flow rate, so that the thermal liquid junction and streaming potentials are kept constant. The experimental results were compared with theory using available thermodynamic data for estimating the quality of pH measurements. An average difference of 0.15 pH unit was observed between measured and theoretical values at temperature.

Characterization of Water-Filled Ag/AgCl Reference Electrode

Pressure-balanced external Ag/AgCl electrode has been extensively used for both Pressurized Water Reactor (PWR) and Boiling Water Reactor (PWR) environments. The use of KCI-based buffer solution often becomes the source of electrode potential drift due to slow leakage through its porous plug, typically made of zirconia. It is reported that results of our effort to improve the stability of electrode potential by using high purity water as the filling solution in which ion activity can be established and maintained at the solubility of AgCl even with the sustained leakage for a long period. Stability tests have been made in boron and lithium mixture solution at . The electrode potential remained stable within 10 mV over one week period. And after a thermal cycle between 288 to the potential shift of Ag/AgCl electrodes did not exceed 15 mV By using the limiting equivalent ionic conductances and Agar's hydrodynamic theory, the thermal liquid junction potential (TLJP) of the electrode has been predicted. The calculated values for the water-fiued Ag/AgCl electrode potential, in which the chlorine concentration in the filling solution was derived from the measured data at ambient temperature, had a good agreement with the experimental values.

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.

Estimation of the thermal liquid junction potential of an external pressure balanced reference electrode

The accurate measurement of pH and potential in high-temperature high-pressure aqueous solutions using an external pressure balanced reference electrode (EPBRE) requires the development of accurate methods for calculating the thermal liquid junction potential (TLJP). Using well-developed irreversible thermodynamic techniques, as well as Agar's hydrodynamic theory to calculate the ionic entropies of transport, we show that the TLJP makes a very significant contribution to the measured potential and can vary by more than ± 150 mV depending on the temperature and composition of the non-isothermal electrolyte bridge. Our calculations of the TLJP are compared with experimentally measured data and good agreement is obtained. The development of an advanced EPBRE, in which the composition of the non-isothermal liquid junction is maintained accurately at a well-defined concentration, is considered to be an appropriate way to solve the problem of performing accurate potentiometric measurements in high-temperature high-pressure aqueous systems.

Technical Note:A Long-Lived External Ag/AgCl Reference Electrode for Use in High Temperature/Pressure Environments

High temperature pressure Ag/AgCl electrodes resistant to the contaminating effects of H/sub 2/ and H/sub 2/S were described in 1979. But the external reference electrode suffers from a thermal liquid junction potential (TLJP) and the development of the Soret effect (thermo-diffusion of the electrolyte). This paper describes a design to improve electrode life. A silvered-tube replaces the silver wire electrode. A liquid chromatograph minipump is used to slowly inject KC1 solution into the electrode to maintain a ''front'' of fresh solution. This prevents ageing effects, and the Soret effect. Construction specifications are given for: corrosion resistant tubing, electrical gland end, zirconium yarn, titanium tubing, polyimide collar, PTFE tubing, minipumps for chromatograph, zirconia plug, and silver chloride.

ChemInform Abstract: SILVER‐SILVER CHLORIDE THERMOCELLS AND THERMAL LIQUID JUNCTION POTENTIALS FOR POTASSIUM CHLORIDE SOLUTIONS AT ELEVATED TEMPERATURES

Chemischer InformationsdienstVolume 11, Issue 7 Physical Inorganic Chemistry ChemInform Abstract: SILVER-SILVER CHLORIDE THERMOCELLS AND THERMAL LIQUID JUNCTION POTENTIALS FOR POTASSIUM CHLORIDE SOLUTIONS AT ELEVATED TEMPERATURES D. D. MACDONALD, D. D. MACDONALDSearch for more papers by this authorA. C. SCOTT, A. C. SCOTTSearch for more papers by this authorP. WENTRCEK, P. WENTRCEKSearch for more papers by this author D. D. MACDONALD, D. D. MACDONALDSearch for more papers by this authorA. C. SCOTT, A. C. SCOTTSearch for more papers by this authorP. WENTRCEK, P. WENTRCEKSearch for more papers by this author First published: February 19, 1980 https://doi.org/10.1002/chin.198007024Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume11, Issue7February 19, 1980 RelatedInformation

Solute-solvent interactions bioinorganic chemistry

The role of solvation in the electron transfer reactions of metalloproteins has been examined. Measurements of the rates of electro transfer reactions between metalloproteins and various inorganic complexes at different temperatures show that the mechanisms employed by hydrophobic and hydrophilic substrates vary significantly. Reactions of blue copper proteins and cytochromes with oxidants such as Co(dipic) − 2 (dipic = dipicolinate) and Co(phen) 3+ 3 (phen = 1,10-phenanthroline) exhibit large positive activation enthalpies and small (i some cases positive) activation entropies. One interpretation of these results is that substantial solvation changes accompany the interaction of the protein with the substrate in the activated complex. Specifically, penetration of hydrophobic substrates into hydrophobic protein interiors is thought to lead to displacement of ordered solvent molecules, thereby increasing entropy. This effect is apparently even larger in electron transfer reactions involving proteins that are believed to be physiological partners. Solvation of metalloproteins is believed to play a critical role in preventing close approach to reagents with hydrophilic electron transfer surfaces to the protein redox centers. In several cases electron tunneling between redox centers occurs over distances that are determined by the primary solvation shell. These distances have been calculated from electron transfer rate constants determined for several proteins. Measurements of the redox potentials and the standard enthalpies and entropies of several electron transfer reactions of cytochromes, iron-sulfur proteins, and blue proteins have been made. The determination of thermodynamic parameters for protein redox couples was accomplished by measuring the temperature dependence of the electrode potential of suitably designed electrochemical cells. If the electrochemical cell is in a nonisothermal configuration, the experimental temperature dependence, dE°/dt, of the electrode potential can be separated into the following components. dE°/dT = dφ tlj/dT + dφ tc/dT + dφ m f/dT φ tlj is the Galvani potential differences across the thermal liquid junction within the KCl salt bridge, φ tc is the ‘thermocouple’ potential difference between the hot and cold regions of the working electrode, and φ m f is the Galvani metal-solution potential difference at the working electrode. Since S° red - S° ox = F·(dφ m f/dT) where F si the Faraday. of dφ tc/dT and dφ tlj/dT are constant or can be neglected, partial molal entropy differences for the redox couples of interest can be obtained directly from measurements of dE°/dT. It has been proposed that in the nonisothermal electrochemical cell configurations under consideration dφ tc/dT < 14 μV° and dφ tlj/dT < 20μV°. Since these values are well below the experimental precision of the dE°/dT measurements, we neglect dφ tc/dT and dφ tlj/dT in nonisothemal experiments. Furthermore, since dφ tc/dT and dφ tlj/dT are constant for a given experimental arrangements, the relative values of dE°/dT for various systems will be unaffected even if these two sources of the observed temperature dependences are not negligible. We have determined the enthalpy and entropy differences for oxidation states changes in metalloproteins using an optically transparent thin-layer electrode (OTTLE) in a nonisothermal electrochemical cell configuration. The most striking result is that large negative values of Δ S° are obtained for proteins with buried copper or iron sites. Possible interpretations of these Δ S° values include conformational effects as well as solvation changes in the oxidized and reduced states.

Silver‐Silver Chloride Thermocells and Thermal Liquid Junction Potentials for Potassium Chloride Solutions at Elevated Temperatures

Potentials for the symmetrical thermocell (298.15°K) have been determined for concentrations ranging from 0.0050 to 0.505 mol/kg and for temperatures as high as 548.15°K. Thermodynamic analysis of the cell has permitted evaluation of the initial state thermal liquid junction potentials over the same range of conditions, and it was found that increases in an approximately parabolic manner with . Analysis of the entropy of transport function indicates that the variation of with can be accounted for by the change in the dielectric constant of the medium.

Irreversible Thermodynamics in Electrochemistry

The Onsager thermodynamics of irreversible processes provides a unified approach to the study of electrolytic systems out of equilibrium, such as concentration cells with transference, the initial and final emf's of thermal cells, and thermal diffusion (Soret effect) of ions. For concentration cells, the method justifies the classical results obtained by Nernst from equilibrium considerations. For nonisothermal systems, the product is the thermodynamic driving force of a process, where is the entropy transported reversibly from the “hot” to the “cold” heat reservoir (in the limiting case of equal temperatures) by the occurrence of a reversible process in the system. The emf of a concentration cell, the initial emf of a thermal cell, and the Soret coefficient, all properties of systems out of equilibrium, are determined, according to the Onsager relations, by the ratios of the reversible fluxes of matter to electricity, of entropy to electricity, and of entropy to matter, respectively, as they occur in systems completely at equilibrium.The transport entropy can become, according to circumstances, the entropy of electrical transport, or the entropy of diffusion transport. in turn involves the entropies of electrochemical transport (for the electrode reactions) and of migration transport (for the thermal liquid junction). The contribution of an ion (or electron) to and to gives rise to the ionic entropies of electrochemical transport and of migration transport , respectively. The latter contributes also to . The sum is known as the entropy of the moving ion and is measurable for a single ionic species. The initial thermal emf involves the transport entropy , the Soret coefficient involves , while the final thermal emf involves where is the transference number of the ion to which the electrodes are not reversible.For the hydrogen ion, , if is taken as zero in a saturated potassium chloride salt bridge. For other ions, , and the mass action dependence is normal. For most ions, appears to be small, i.e., about 0–3 cal/deg, and the mass action dependence is abnormal and low. In dilute solutions, shows a Debye dependence on concentration. These facts are consistent with the electrostatic interpretation of as the entropy of depolarization of the solvent dielectric when the ion moves away. For ions transferred by a chain mechanism (H+ and OH−), is significantly larger. The transport of ions across biological membranes plays an important role in life processes. Since living systems are not necessarily uniform in temperature, ionic transport entropies may have a role to play in the function of the biological cell.

The Temperature Coefficients of Electrode Potentials: The Isothermal and Thermal Coefficients—The Standard Ionic Entropy of Electrochemical Transport of the Hydrogen Ion

The temperature coefficient of the potential of an electrode can be defined experimentally by reference to (a) the potential of the SHE at the same temperature, (b) the potential of the same electrode at some fixed temperature. These two definitions give rise to the “isothermal” and “thermal” temperature coefficients of electrode potentials. The isothermal coefficient is where ΔS is the reaction entropy of the “SHE//Electrode” cell. The thermal coefficient is where is the entropy transported from the hot to the cold heat reservoir by the passage of n faradays of positive electricity through the cell from the cold to the hot electrode (before the onset of thermal diffusion). The entropy is divided into an entropy of electrochemical transport which determines the electrode temperature effect, and an entropy of migration transport which determines the thermal liquid junction potential.By assuming that is negligible in a saturated potassium chloride bridge, we have deduced that the thermal temperature coefficient of the SHE is +0.871 mv/°C [hot electrode (+) terminal] at 25°C. The standard ionic entropy of electrochemical transport of hydrogen ion is −4.48 cal/deg. Thermal temperature coefficients are computed for calomel, silver chloride, and copper sulfate electrodes and are compared with experiment. Thermal and isothermal temperature coefficients are computed and tabulated for nearly 300 standard electrode potentials.