The redox electrode

This electrode is made up of a platinum wire dipping into an aqueous solution of ions of a metal in two oxidation states. Alternatively the solution could be that of a complex of a metal in two oxidation states. The electrode is reversible to the oxidised and reduced forms of the species in solution which appear in the electrode reactions. 

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

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).

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. 

Strategy. Ecu +.cu is to be measured with a copper wire as the redox electrode, thereby allowing the activity of the copper ion to be calculated with the Nemst equation (equation (3.9)) as follows ... 

A rapid potentiometric flow inject technique for the simultaneous determination of oxychlorine species (e g., CIO2 ) was developed by Ohura et al. (1999). The analytical method is based on the detection of a large transient potential change of the redox electrode due to chlorine generated via the reaction of the oxychlorine species (e.g., CIO2 ). The detection limit for C102 is 3.4 ppb. 

Electrodes of the first kind differ distinctly from the redox electrodes in that in the latter case, both oxidation states can be present in variable concentrations, while in electrodes of the first kind, one of the oxidation states is the electrode material. 

Figure 4.22 Simultaneous idealized representation of ETSM processes. Curve a (solid line) represents the current flow in the cell for a redox cycle, while curve b (dotted line) represents the concurrent frequency shift associated with the adsorption (fwward arrow) and desorption (reverse arrow) of mass at the electrode surface during the redox electrode reaction. The reference electrode is a saturated calomel electrode (SCE).

Eh is often confused with the closely related redox potential or oxidation-reduction potential (ORP) measurement. ORP is measured by placing a redox electrode into water or sediment the redox electrode is a piece of metallic platinum, which acquires a more negative potential with respect to its reference electrode under reducing conditions where electron activities are higher. ORP is the voltage measured between this redox electrode and a reference electrode placed in the same environment. It provides a useful, approximate characterization of redox conditions in the aquatic environment, although it... 

Third, the Fe(III) and Fe(II) activities in each system at each time are calculated by the procedure outlined below, and an ORP calculated using the Nernst equation (Eh ). Eh is then compared to Eh, the Eh recorded by the redox electrodes (corrected to SHE). The solubility product for the precipitated solid ferric hydroxide phase is also calculated for each data point using the pH and computed values of Fe " or one of its hydrolysis species. 

In the redox electrode, an inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction. This type of electrode was mentioned in Chapter 12. 

Figure 12.20 A typical redox titration curve for hydrogen peroxide measurement. The left ordinate is the millivolt reading from the redox electrode the right ordinate is the first derivative of the millivolt curve. Volume (mL) of titrant (permanganate) is shown on the abscissa. The endpoint shown is calculated with the first derivative method—the maximum of the pink curve. Used with permission from the author.

The quantities redox potential and standard redox potential have been introduced so far in an abstract fashion. Now, we can combine electrodes to establish an electrochemical cell, and this can be used to measure directly the potentials mentioned. For every redox couple, we must find a way to represent this couple by an electrode. The Zn/Zn couple already is a special sort of electrode, namely a metal/metal ion electrode, also called an electrode of the first kind. Other types are gas electrodes, which can be designed with couples like nos. 4, 8 and 14 in Table 2.5 by combining the redox couples with a body made of an inert metal. A similar combination is useful for couples where both partners are dissolved species. The resulting electrode type is the redox electrode. A symbolic notation for electrodes is given for the examples listed in Table 2.6. A phase boundary is symbohzed by a vertical dash or just by a slash. 

The electrode potential depends on E (0, R) and the ratio of the activities of the oxidised and reduced forms in equilibrium. E (0, R) is the potential of the redox electrode when a (ox) = a (red) it is not necessary that the activities should be unity as for other types of electrode. From a knowledge of E (0, R) the potential of any mixture of oxidised and reduced forms can be calculated (figure R.l). 

The redox electrode model of photosynthesis illustrating the mechanism of photon-induced charge separation and electron transfer processes in a pigmented BLH separating two aqueous solutions. P and P denote the pigment in the ground and excited state, respectively. A = electron acceptor, D = electron donor, A and are reduced and oxidized forms. Subscripts o and i indicate the outer and inner side of the membrane, and P is oxidized P. (Adapted from Nature, 219, 272, 1968 227, 1232, 1970). 

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