Electrochemistry reference electrodes, types

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Electrodes of this type are sometimes called electrodes of the second class and are applied in electrochemistry as the reference electrodes for measuring unknown potentials. They are formed by a metal in contact with its insoluble salt, which is immersed into a solution of a soluble electrolyte with the... 

As it is not possible to determine the absolute potential of an electrode, the electrode potential must always be referred to an arbitrary zero point, defined by the potential of a chosen reference electrode. Thus, it is very important always to quote the type of reference electrode used in electrochemistry. Differences in operating potentials reported in the literature are often attributable to the use of different reference electrodes. 

Reference electrodes of the second kind are based on three phases in mutual contact where the electrode potential is a function of the common anion (An ) activity in solution. One such example is the AglAgClICP (silver-silver chloride) reference electrode and as stated in previous chapters this type of electrode is well known in aqueous electrochemistry. The electrode potential of the half-cell is given by ... 

Use of this type of reference electrode requires good solubility and high concentrations of the anion, An, in the IL. This is not always straightforward for ILs, where limited or low solubility of anions such as halides (e.g. CU) may be encountered. As a result this type of reference electrode is not commonly used within IL electrochemistry. Another requirement is that the metal halide solid, typically on the surface of the metal wire, should be stable to the IL and should have very low solubility within the IL, otherwise unstable reference electrode potentials will arise. This can be mitigated by having a suitably high concentration of the An in the IL phase. 

In IL electrochemistry use of the two commonly reported types of reference electrodes shows significant difference in electrochemical responses. Using the methodology developed by Oldham [19], the level of can be calculated as a function of distance of the reference electrode tip from a disk working electrode by ... 

Although strictly speaking this type of reference electrode has not been used in IL electrochemistry, sufficient data does exist for this type of reference electrode to be constructed. Compton et al. have shown that H2 can be oxidised in a number of ILs at a platinum electrode (Table 7.5) [33-35]. Additionally for selected ILs the reduction of can also occur [35]. The formal potential determined for the H" H2 redox couple in selected ILs is presented in Table 7.5. 

Quasi-reference electrodes (Chap. 14) are by and large the most common type of reference electrodes reported in ionic Uquid electrochemistry. However, since the potential of this reference electrode is unknown, and the stability of this reference electrode can vary from electrode to electrode and solution to solution, accurate potentials cannot be measured with this type of reference electrode. Quasireference electrodes can also show considerable drift, especially when using silver wire directly immersed into the IL solutirm [15]. If using a quasi-reference electrode in IL electrochemical measurements for accurate determination of potentials, the electrode should be calibrated against an lUPAC recommended redox couple [15]. However, in many IL electrochemical reports, this is not performed and as such any potential data reported cannot be cOTifirmed by other laboratories. 

The calomel reference electrode is a metal/salt-type electrode, so electrochemistry of the Hg/Hg2Cl2 and Ag/AgCl electrodes are similar. The electrochemical halfreaction of the Hg/Hg2Cl2 reference electrode is... 

EPR spectrometers use radiation in the giga-hertz range (GHz is 109 Hz), and the most common type of spectrometer operates with radiation in the X-band of micro-waves (i.e., a frequency of circa 9-10 GHz). For a resonance frequency of 9.500 GHz (9500 MHz), and a g-value of 2.00232, the resonance field is 0.338987 tesla. The value ge = 2.00232 is a theoretical one calculated for a free unpaired electron in vacuo. Although this esoteric entity may perhaps not strike us as being of high (bio) chemical relevance, it is in fact the reference system of EPR spectroscopy, and thus of comparable importance as the chemical-shift position of the II line of tetra-methylsilane in NMR spectroscopy, or the reduction potential of the normal hydrogen electrode in electrochemistry. 

The final method of coupling enzyme reactions to electrochemistry is to immobilize a biocatalytic material directly at the electrode surface. This biocatalytic material can be an immobilized enzyme, bacterial particles, or a tissue slice, as shown in Fig. 8. The biocatalyst converts substrate (analyte) into product, which is measured by the electrode. Electrodes of this type can be potentiometric or Faradaic, and are often referred to as biosensors. ... 

PC-doubling Reactions PC doubling refers to a type of charge-transfer reaction in which both bands of the semiconductor are involved, thus emphasizing the distinctive features of semiconductor electrochemistry. The first examples of such reactions relate to the photoan-odic oxidation of species such as formate and tartrate at wide band gap -type electrodes [74, 75]. A photon generates an electron-hole pair in the semiconductor. The electron and hole are separated by the electric field of the depletion layer. The electron is detected as photocurrent in the external circuit. The hole oxidizes a species from solution producing an intermediate... 

When a metal is placed in contact with an electrolyte, a potential difference is observed at the liquid-metal interface, as noted in Chapter 2. This is similar to the work-function potential difference which occurs when two dissimilar metals are brought into contact, or the potential difference associated with a semiconductor p n junction. The value of potential difference associated with a metal electrode-electrolyte interface is a function of the metal and contacting electrolyte. Theoretical treatment of this situation is complex and one should refer to a text on electrochemistry such as those by Macinnes (1961) or Newman (1973). Certain types of electrodes are extremely sensitive to various trace impurities in the contacting electrolyte and may react quite differently in seemingly similar circumstances. 

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