A, , standard redox potential

The thin-layer configuration and its associated diffusion problems means that it is possible to oxidise (or reduce) all of the electroactive species in the thin layer before they can be replenished to any marked degree. Consider, for example, the 0"+/0 couple, with a standard redox potential well within the "electrochemical window of the solvent, so that the current in the absence of the couple is small and can easily be accounted for. With the electrode pushed against the window the potential is stepped cathodic enough to ensure the rapid reduction of the 0" + and the current measured as a function of time, the concentration such that the time for the current to reach zero, or a steady residual value, is small. If the area under the I ft curve is A ampere seconds, then the charge passed Q = A coulombs. Thus, the number of moles of 0"+ reduced, N0, is given by ... 

To leam why the ideal choice of a redox indicator is one which has a standard redox potential as close as possible to the end point of the titration system. 

By assuming that n = 1, the limits for this particular indicator are E 0.059ln (100) V, i.e. a working range of c. 120 mV. The ideal choice of indicator is seen to be one which is characterized by a standard redox potential as close as possible to the end point of the titration system. 

All these results can be explained in terms of the model proposed above (cf. Fig. 11). Namely, with ferrous oxalate having a standard redox potential E° (Ox/R) of —0.2 V (SCE), which is a little more negative than the E of the surface trapped hole located ca. 0.5 V above E , the surface trapped hole is effectively quenched by the rapid reduction, and the photoanodic current flows without decomposition. With ferrocyanide, having an E(0x/R) of 0.2 V (SCE), which is more positive than the E of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface potential created will level it down to the E(0x/R) of the redox couple. At this point, the rates of nu-cleophillic attack of H2O and OH to the surface trapped holes are still low and the electrode decomposition is prevented. 

It has been pointed out by some authors CL, 2) that for a semiconductor having a thermodynamic decomposition potential, E in between Ec and E , a redox couple with a standard redox potential, E°, more negative than E is needed in order to operate the photoanode without decomposition. Then, the maximum photovoltage attainable is Us - Ej, which is often much lower than Eg -A-x. For GaP, this is only 0.8 V (4) (Fig. 11). S... 

How can a simple cofactor, such as heme, give rise to a wide spectrum of protein functionalities While the Fe(III)/Fe(II) couple has a standard redox potential of 0.77 V, when complexed with a protoporphyrin to form free heme, it may decrease to —0.115 V [3-5]. When heme is introduced into a protein matrix, redox potential shows an impressive variation of around 1 V. The electrochemical data for structurally characterized heme proteins involved in electron transfer and redox catalysis has been compiled at the Heme Protein Database (HPD, http //heme.chem. columbia.edu/heme) [6]. The database comprises not only peroxidases but also catalases, oxidases, monooxygenases, and cytochromes. From b-type heme with histidine-tyrosine ligation (E° = 0.55 V) to c-type heme with histidine-methionine... 

If all chemical species are in a standard state, the oxidation of H2O2 into 2 or 2 is thermodynamically allowed with any oxidizer having a standard redox potential higher than 0.69 or 1.18 V, respectively. 

Figure 11.17 (a) Standard redox potentials in absolute scale at... 

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