Reference electrode impedance

The impedance of an RE can be measured with any potentiostat with built-in electrochemical impedance spectroscopy (EIS) capabilities or with the addition of a separate frequency response analyzer (ERA). One method is described below. 

The RE and an inert CE of sufficient size are placed in a beaker containing electrolyte. The RE is connected to the WE terminal of the potentiostat, and the CE is coimected to both the RE and the CE terminals, An impedance scan is set up from at least 10 kHz down to 1 Hz, with no DC potential (V c) applied (Vdc = ocp where Vqcp is open circuit potential) and an AC waveform (Vj c) of 5-10 mV amplimde. 

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Figure 9.1 Equivalent circuit of an electrochemical cell. A, Auxiliary electrode R, reference electrode W, working electrode Rc, compensated resistance R , uncompensated resistance Rr, reference electrode impedance Zf, faradaic impedance Cdl, doublelayer capacitance.

Under the assumptions that the gain of the operational amplifier Av is large and that the reference electrode impedance Z f is small, the dominant stochastic error in the regulation signal consists of additive contributions from the reference electrode and the operational amplifier as... 

Figure 4.23 High-frequency reference electrode impedance problems in electrochemical impedance spectroscopy. EIS with (A) single reference electrode and (B) double reference electrode (high-frequency shunt).

Gamry Instruments. Reference Electrode Impedance Check. [Online]. Available at http // www.gamry.com/App Notes/ReferenceElectrodes/Reference Impedance Check.htm. Accessed 28 Oct. 2004. 

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Cells need not necessarily contain a reference electrode to obtain meaningfiil results as an example, if the two electrodes in figure A2.4.12 are made from the same metal, M, but these are now in contact with two solutions of the same metal ions, M but with differing ionic activities, which are separated from each other by a glass frit that pennits contact, but impedes diffusion, then the EMF of such a cell, temied a concentration cell, is given by... 

The potential of the working electrode, which changes as the composition of the electrochemical cell changes, is monitored by including a reference electrode and a high-impedance potentiometer. 

By means of a resistance in the circuit the spontaneous corrosion reaction can be made to proceed at a predetermined rate, and the rate can be measured by means of an ammeter A. At the same time the potentials of the individual electrodes can be measured by means of a suitable reference electrode, a Luggin capillary and high-impedance voltmeters and Kj. At equilibrium there is no net transfer of charge (/ = A = 0). the e.m.f. of the cell is a maximum and equals the difference between the reversible potentials of the two electrodes... 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Logarithmic scale for expressing acidity or alkalinity of water (7.0 to 0 indicates increasing acidity 7.0 to 14 indicates increasing alkalinity). Measured by means of a glass electrode/reference electrode pair immersed in the water sample under test. The potential difference depends upon the pH which is then displayed on a pH meter (high input impedance, millivoltmeter). 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

The technique of AC Impedance Spectroscopy is one of the most commonly used techniques in electrochemistry, both aqueous and solid.49 A small amplitude AC voltage of frequency f is applied between the working and reference electrode, superimposed to the catalyst potential Uwr, and both the real (ZRe) and imaginary (Zim) part of the impedance Z (=dUwR/dI=ZRc+iZim)9 10 are obtained as a function of f (Bode plot, Fig. 5.29a). Upon crossplotting Z m vs ZRe, a Nyquist plot is obtained (Fig. 5.29b). One can also obtain Nyquist plots for various imposed Uwr values as shown in subsequent figures. 

Nonfaradaic components associated with the uncompensated resistance between reference electrodes (7 ) and the double layer capacitance (Qi) can be accurately determined by AC impedance measurements. In this technique, a small AC potential perturbation is superimposed to the DC bias, and the resulting AC current is measured as a function of the frequency of modulation. The simplest circuit considered for a polarizable... 

FIG. 7 Simplified equivalent circuit for charge-transfer processes at externally biased ITIES. The parallel arrangement of double layer capacitance (Cdi), impedance of base electrolyte transfer (Zj,) and electron-transfer impedance (Zf) is coupled in series with the uncompensated resistance (R ) between the reference electrodes. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

FIGURE 10.1 A schematic diagram for a typical electrode system for potentiometric pH measurements. A potential is established on the pH sensitive membrane-solution interface of a pH electrode that responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The cell potential, which is proportional to the pH in the test solution, is measured using a high input impedance voltmeter between the pH electrode and the reference electrode. 

The measured impedance, Zr, generally includes the electrolyte resistance between the working electrode and the reference electrode, Rx, and hence... 

Problems with reference electrode/electrolyte. The sample solution can react with the electrolyte. The greatest source of error in biological solutions is through the formation of insoluble silver sulfide, often at the ceramic frit. A blackened spot is usually observed in a pH electrode that has been in service for a few weeks. This precipitate can impede the free flow of electrolyte and cause the probe response to become sluggish and cause large errors in the measured pH. 

Won t a reference electrode (RE) impede the diffusion pathways if placed directly in front of the working electrode ... 

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