Reference electrode openings

Give some ways in which reference-electrode openings are constructed in order to minimize cross contamination between the reference-electrode electrolyte and the electrochemical-cell electrolyte. Can the openings be made too small Explain. 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

The simplest procedure in studying galvanic corrosion is a measurement of the open-circuit potential difference between the metals in a couple in the environment under consideration. This will at least indicate the probable direction of any galvanic effect although no information is provided on the rate. A better procedure is to make similar open-circuit potential measurements between the individual metals and some appropriate reference electrode, which will yield the same information and will also permit obser-... 

Measurements of open-circuit potentials relative to some reference electrode have been assumed on occasion to provide a means of rating metals as to their relative resistance to corrosion on the basis that the more negative the measured potential, the higher will be the rate of corrosion, but this assumption is obviously invalid, since it disregards polarisation of the anodic and cathodic areas. 

Open-circuit Potential the potential of an electrode (relative to a reference electrode) from which no net current flows, so that the anodic and cathodic reactions occur at an equal rate. 

Figure 9 shows the first and second cycle of a cyclic voltammogram of a 0.2 molal (mol kg"1) solution of lithium bis[2,2 biphenyldiolato(2-)-0,0 ]borate in PC at a stainless steel electrode. The sweep covers the potential range from open circuit potential ER versus a lithium reference electrode up to 4500 mV versus Li and back to ER. The first cycle shows... 

It is worth emphasizing that Eq. (5.22) is valid under both open-circuit and closed-circuit conditions and that it holds for any part of the surfaces of the catalyst and the reference electrodes. Thus, referring to the metal electrode surfaces in contact with the electrolyte (region E) it is ... 

Figure 5.36. Effect of electrochemical O2 pumping on the Zr 3dj XPS spectra of Pt/YSZ at 400°C (a) Zr 3d5/2 spectrum shift from AUWr=0 (solid curve) to AUwr=1. 2 V (dashed curve) (b) effect of overpotential AUv/r on the binding energy, Eb) and kinetic energy, (AEk--AEb) shifts of Zr 3dS/2 (filled circles, working electrode grounded) and Pt 4f7/2 (open circle, reference electrode grounded).6 Reprinted with permission from the American Chemical Society.

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

When the area A of the eleetrode/solution interface with a redox system in the solution varies (e.g. when using a streaming mercury electrode), the double layer capacity which is proportional to A, varies too. The corresponding double layer eharging current has to be supplied at open eireuit eonditions by the Faradaic current of the redox reaction. The associated overpotential can be measured with respect to a reference electrode. By measuring the overpotential at different capaeitive eurrent densities (i.e. Faradaic current densities) the current density vs. eleetrode potential relationship can be determined, subsequently kinetic data can be obtained [65Del3]. (Data obtained with this method are labelled OC.)... 

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

Figure16.6 (a) Schematicdrawingofexperimental set-upforthe evaluation of the interfacial tension under potential control, (b) Relative change in contact angle as a function ofthe potential after the substrate was inserted into (open circles) and pulled from the nitrobenzene phase. Insets are schematic drawings of the side views of the contact lines. The potential was described with respect to the Au/AuO f reference electrode.

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

The dropping mercury electrode had a drop time of 3-4s under an open head of 50 cm Hg. A saturated calomel electrode was the reference electrode. Before recording, the solutions were shaken well. After recording, the electrodes were well rinsed with distilled water and wiped dry. A starting potential of 0.2 V was used and the solutions were degassed with dry nitrogen for 2 min prior to recording. A full-scale sensitivity of 10 pA was used. 

The saturated calomel reference electrode is an example of a constant-potential electrode. A drawing and a photograph of a typical SCE available commercially are shown in Figure 14.4. It consists of two concentric glasses or tubes, each isolated from the other except for a small opening for electrical contact. 

Figure 8.12. Open-circuit potential for the solution 1 g/L paraformaldehyde, pH 12.50, 25°C, 1M KCl, Cu electrode, SCE reference electrode, EDTA variable. (From Ref. 31, with permission from the Electrochemical Society.)...

Another form of this definition [equation (3.6.15)] has sparked much debate in the scientific community [121-124]. In this approach Vapp (or Vbias) is taken as the absolute value of the difference between the potential at the working electrode measured with respect to a reference electrode (Vmeas) and the open circuit potential (Voc) measured with respect to the same reference electrode under identical conditions (in the same electrolyte solution and under the same illumination). In the case of a semiconductor photoanode where oxygen evolution takes place the efficiency is calculated as ... 

V°rev = 1.229V is the standard state reversible potential for the water splitting reaction and Vaoc is the anode potential at open circuit conditions. Term Vmeas-Vaoc arises from the fact that Voc represents the contribution of light towards the minimum voltage needed for water splitting potential (1.229V) and that the potential of the anode measured with respect to the reference electrode Vmeas has contributions from the open circuit potential and the bias potential applied by the potentiostat (i.e. Vmeas= Vapp+Vaoc). The term Vmeas-Vaoc makes relation (3.6.16) independent of the electrolyte pH and the type of reference electrode used. Thus the use of V°rev in relation (3.6.16) instead of VV or V°hz as in the case of relation (3.6.15) is justified. 

For the measurement of the open circuit potential of the catalyst during the oxidation reaction a Pt rod measuring electrode and a Ag/AgCl/KCljat reference electrode were applied (13,14). 

Figure 6.2-13 UPD phenomena of Ge on Au(l 11) in dry [BMIM] PFe" two-dimensional islands with an average height of 250 + 20 pm start growing upon a potential step from the open circuit potential to -500 mV vs. the Pt quasi reference electrode (picture from [59] - with permission of the Peep owner soeietes).

Fig. 3.5,19 Open-circuit voltage (K. ) for CdS formed in 9-layer CdAr films as a function of particle size (estimated from UV/visible absorption spectra). The electrolyte was 1.0 M Na2S03 at pH 7.25, and platinum and standard calomel electrodes were used as the counter and reference electrodes, respectively. (From Ref. 5.)...

Figure 15-14 Solid colored circles show the drift in apparent pH of a low-conductivity industrial water supply measured continuously by a single electrode. Individual measurements with a freshly calibrated electrode (black circles) demonstrate that the pH is not drifting. Drift is attributed to slow clogging of the electrode s porous plug with AgCI(s). When a cation-exchange resin was placed inside the reference electrode near the porous plug, Ag(l) was bound by the resin and did not precipitate. This electrode gave the drift-free, continuous reading shown by open diamonds. [From S. Ho, H. Hachlya. K. Baba. Y. Asano. and H. Wada, Improvement of the Ag I AgCt Reference Electrode and Its Application to pH Measurement," talonta 1995,42.1685.]...

The chromatographic detector at the opening of this chapter has a Cu working electrode, a stainless steel auxiliary electrode, and a Ag AgCl reference electrode. 

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