Redox Reference Electrode

Since the monitoring circuit draws negligible current, the reference electrode redox couple need not be perfectly reversible (i.e., need not have a large exchange current density) and need not have a very large surface area. 

However, there are reports in the literature, briefly summarized [146] on the formation of complex species HgS , Hg(HS)2, and HgS(HS)2, which may play some role in the formation of potential. Though in principle the HglHgSIS system could be used as a reference electrode redox couple, its applicability is in practice very limited. This system may be considered as a quasi-reference electrode one, in view of the extremely small solubility product of HgS. Therefore, the potential of such electrode may be not well reproducible and not sufficiently stable in time. The wider use of such electrode would need more profound and extensive studies on its nature and performance. 

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

Calomel Electrodes Calomel reference electrodes are based on the redox couple between Hg2Cl2 and Hg (calomel is a common name for Hg2Cl2). 

Silvcr/Silvcr Chloride Electrodes Another common reference electrode is the silver/silver chloride electrode, which is based on the redox couple between AgCl and Ag. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

In general, low level detection is masked by the noise level inherent in any measuring device. Electrochemical methods are susceptible to electrical interference from external sources, variations in reference electrode parameters resulting from aging or contamination, and interference from redox... 

Fig. 21-8 Reference electrodes (Pt to redox system) for high pressures and temperatures (max. 250 C and 100 bar).

The redox potential is determined with a probe consisting of a platinum electrode and a Hg/Hg2Cl2 Cl or Ag/Ag Cl—Cl reference electrode. If Ep is the potential of the platinum probe, Ej the potential of the reference electrode and tne redox potential of the soil (in mV on the hydrogen scale) then... 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

Redox switching, 126 Reference electrodes, 100, 105, 142 Reflectance spectroscopy, 44 Resistance, 22, 105 Resolution 50, 71 Reverse pulse polarography, 68 Reversible systems, 4, 31 Reticulated vitreous carbon, 114, 115 Riboflavin, 37... 

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.)... 

Reference electrodes for non-aqueous solvents are always troublesome because the necessary salt bridge may add considerable errors by undefined junction potentials. Leakage of components of the reference compartment, water in particular, into the working electrode compartment is a further problem. Whenever electrochemical cells of very small dimensions have to be designed, the construction of a suitable reference electrode system may be very difficult. Thus, an ideal reference electrode would be a simple wire introduced into the test cell. The usefulness of redox modified electrodes as reference electrodes in this respect has been studied in some detail... 

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

A classification of electrodes has already been given in Section 1.3.1. The function of the indicator electrode is to indicate by means of its potential the concentration of an ion or the ratio of the concentrations of two ions belonging to the same redox system. Under non-faradaic conditions, the relationship between the potential and these concentrations is given by the Nemst or the more extended Nernst-Van t Hoff equation, as explained below. As a single potential between an electrode and a solution cannot be measured in the absolute sense but only in a relative manner, a reference electrode is needed its function is merely to possess preferably a constant potential or at any rate a known potential under the prevailing experimental conditions. Often both electrodes cannot be placed in the same solution, so that a second solution... 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

Most manufacturers not only have taken the above requirements and possibilities into account, but also deliver for convenience combined electrodes that contain both the indicator and reference electrodes this is the case for glass electrodes (see Fig. 2.13) and other ISEs as well as for redox electrodes. 

Cyclic voltammetry was performed on precursor polymer thin films cast on platinum electrodes in order to assess the possibility of electrochemical redox elimination and consequently as an alternative means of monitoring the process. All electrochemical experiments were performed in a three-electrode, single-compartment cell using a double junction Ag/Ag+(AgN03) reference electrode in 0.1M... 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

In principle the ISO-NOP sensor works as follows. The sensor is immersed in a solution containing NO and a positive potential of —860 mV (vs Ag/AgCl reference electrode) is applied. NO diffuses across the gas permeable/NO-selective membrane and is oxidized at the working electrode surface producing a redox current. This oxidation proceeds via an electrochemical reaction followed by a chemical reaction. The electrochemical reaction is a one-electron transfer from the NO molecule to the electrode, resulting in the formation of the nitrosonium cation ... 

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