Electrodes, redox

Such a redox couple is exemplified by an aqueous solution of Fe and Fe salts  

Each of the above combinations is referred to as a half reaction. 

In the simple system the electrons are passing via the metal electrode and the electrode could be carrying an excess or deficit of electrons. This is called 

The reaction quotient (0 is the mathematical product of the concentrations of the products of the reaction divided by the mathematical product of the concentrations of the reactants. 

By connecting two half cells and measuring the potential difference between them it should be possible to measure an unknown electrode potential, providing the electrode potential of the other (reference) electrode is known. This is the dilemma - how does one ascertain the electrode potential of the very first reference electrode As this is clearly impossible, the standard hydrogen electrode (Section 3) was arbitrarily chosen as the reference against which all other electrode potentials would be scaled. 

When both the oxidised and the reduced forms of a redox couple are dissolved in a solution, an inert metal can attain a potential, which only depends on the ratio of the activities of these two species. The inert electrode operates during the measurement as an electron source or drain (Fig. II.9.2a). As an example for a 

In the case of an electrochemical equilibrium, the Nernst equation follows  

2 for the meaning of the formal potential (Ce +,Ce +). Such inert indicator electrodes are very suitable for the indication of redox titrations. 

Another example of a redox electrode with a homogeneous redox equilibrium is the so-called quinhydrone electrode for pH measurements reported by Biilmann in 1921 [1]. Quinhydrone is a charge transfer complex consisting of quinone and hydroquinone in a 1 1 ratio. If this compound is placed into an aqueous solution, the chemical equilibrium appears as follows [2]  

At a platinum wire immersed into this aqueous solution, an electrochemical equilibriiun is established according to the electron transfer reaction  

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The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

The redox (electrode) potential for ion-ion redox systems at any concentration and temperature is given by the Nernst equation in the form... 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

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. 

Fig. 6. Band edge positions of several semiconductors ia contact with an aqueous electrolyte at pH 1 ia relation to the redox (electrode) potential regions (vs the standard hydrogen electrode) for the oxidation of organic functional groups (26,27).

Fig. 2.33 Potential difference Kbetween a redox electrode and a nickel electrode immersed in an alkali chloride melt 700°C, argon atmosphere ...

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

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. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

J Marlow, (a) Care and Maintenance of pH and redox electrodes (b) Care and Maintenance oflon-Selective Electrodes. In International Laboratory, Vol XII, Part 2, 1987... 

The abbreviations for the investigation methods are also taken from the nomenclature of Sillen and Martel 1 (76) aiex = anion exchange cal = calorimetry ciex = cation exchange dis = distribution between two phases est = estimate red = e.m.f. with redox electrode sp = spectrophotometry. Our selected data, rather limited in number, arise from the present status of the IAEA assessment of inorganic complexes of the actinides (12). 

In accordance with the classification of electrodes in Section 1.3.1 one can distinguish between redox electrodes (inert type) and electrodes of the 1st, 2nd and 3rd kinds (active type). 

Some special redox electrodes. Within the group of redox electrodes, attention should be paid to the hydrogen and oxygen electrodes, and also to the quinhydrone electrode and its tetrachloro version. 

Redox electrodes with more complicated reactions. In many redox systems hydrogen ions take part, which means that the pH also influences the redox... 

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. 

Oxidation-reduction electrodes, abbreviated to redox electrodes, consist of an inert metal (Pt, Au, Hg) immersed in a solution containing two forms... 

Quinhydrone, a solid-state associate of quinone and hydroquinone, decomposes in solution to its components. The quinhy drone electrode is an example of more complex organic redox electrodes whose potential is affected by the pH of the solution. If the quinone molecule is denoted as Ox and the hydroquinone molecule as H2Red, then the actual half-cell reaction... 

While the laws governing electrode potentials in non-aqueous media are basically the same as for potentials in aqueous solutions, the standardization in this case is not so simple. Two approaches can be adopted either a suitable standard electrode can be selected for each medium (e.g. the hydrogen electrode for the protic medium, the bis-diphenyl chromium(II)/ bis-diphenyl chromium(I) redox electrode for a wide range of organic... 

Fig. 3. Same as fig. 1 after removing all points below detection limits of Fe(lll) and of redox electrode measurements.

A redox electrode acts as a reagent as well as an electron conductor, as the metal of an electrode can also be one component part of a redox couple. 

Most electrodes are metallic. Sometimes the metal of an electrode can also be one component part of a redox couple. Good examples include metallic iron, copper, zinc, lead or tin. A tin electrode forms a couple when in contact with tin(IV) ions, etc. Such electrodes are called redox electrodes (or non-passive). In effect, a redox electrode has two roles first, it acts as a reagent and, secondly, it measures the energy of the redox couple of which it forms one part when connected to a voltmeter. 

Some metals, such as aluminium or magnesium, cannot function as redox electrodes because of a coating of passivating oxide. Others, such as calcium or lithium, are simply too reactive, and would dissolve if immersed in solution. 

Figure 2. End view of ecosystem chamber showing detail of daphnid chamber and positioning of redox electrodes...

Aeration was controlled at one of three rates by connecting either a dissolved oxygen (DO) or redox electrode, as appropriate (Section 2.4), to a meter containing a trip amplifier. The trip amplifier operated at two set points and switched two solenoid valves to allow the entry of air at a high or low flow rate through a pipe that emerged under the... 

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