Half-redox equilibrium

Therefore, the potential of one electrode depends systematically on the activities of all the species that participate in the half-redox equilibrium. Furthermore, these species are not necessarily those that exchange electrons. 

It is easy to write the expression of a half-redox equilibrium in basic medium starting from its expression in acidic medium. It is sufficient to write the latter in the direction of a reduction and to add to both members the same number of hydroxide ions as the number of protons that exist on the left-hand side. With this thought experiment, the protons are transformed into water. Thus, in order to find... 

Obviously, the difference ii°(AuCl4 /Au(s)) - (0.06 4/3)log (Cl ) plays the part of a standard potential symbolized by (Au Cl4 /Au(s)). Another example of analytical interest is provided by the couple iodate lOs /iodine dichloride ICl2, whose half-redox equilibrium is... 

The internal redox indicators are dissolved in the titrand solution. They are organic dyes whose oxidized and reduced forms are different colors. They are derivatives for which their oxidation-reduction is achieved rapidly. Their half-redox equilibrium may be symbolized by... 

In a very oxidizing medium, the orthophenanthroline/Fe + complex of a pale blue color is formed from ferroin. The corresponding half-redox equilibrium is... 

Antimony +III in the antimonyl cation SbO+ belongs to the redox couple Sb20s/Sb0+ and obeys the half-redox equilibrium... 

From a mechanistic standpoint, it is interesting to notice that in hydrochloric acid, the oxidization with four exchanged electrons by IO3 first occurs through an iodatometry with six exchanged electrons. This means that the first half-redox equilibrium to be operative is... 

Bromate ions are strong oxidizing agents. During the course of the reaction, they are reduced into bromide ions according to the half-redox equilibrium... 

Let s recall that the half-redox equilibrium corresponding to the couple Sb+ /Sb+ is... 

Chromous ions Cr enter into the half-redox equilibrium... 

Remark Bi, as bismuthic acid HBiOs, is a very strong oxidant in acidic medium. It enters into the half-redox equilibrium through which the cation bismuthyle BiO+ is formed ... 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

As a consequence when the difference between equilibrium potentials of the two half redox reactions is low, the modifying metal, during the preparation of a bimetallic catalyst by direct redox reaction, will be deposited selectively on specific sites of the parent metal (i.e. sites that are highly oxidizable such as comers, edges, etc.). However, the equilibrium potentials are defined by Nemst s law which provides facilities to fit the potential values by changing the concentrations of the oxidized and reduced forms (eqs 2 and 2 ) and so induces selective deposition of the modifier on the parent catalyst. 

Redox equilibrium — An equilibrium system may involve one or more - redox reactions taking place in solution. Such reactions may be written as a combination of individual half reactions (- half-cell reactions), for example... 

All electrodes depend on oxidation and reduction, but the term oxidation-reduction electrode, or redox electrode, is usually reserved for the case in which a species exists in solution in two oxidation stages. This electrode is denoted M(s) Ox, Red, where M is an inert metal (usually platinum) serving as an electron carrier and making electrical contact with the solution. The half-cell equilibrium can either be simple (e.g., Fe + + e = Fe +) or be affected by other... 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

Using the generic half-cell equation Ox + ne Red as a model, redox equilibrium can be described by the equilibrium expression... 

The voltage of a single metal electrode in the half-cell of an electrochemical cell, such as the copper and zinc electrodes of the Daniell cell, cannot be measured. If the metal electrode is connected to a voltmeter using a wire and the wire placed in the solution to complete the circuit, another redox equilibrium and electrode potential will be generated. The voltage will be the difference of the two electrode potentials, not the voltage of the first metal electrode in equilibrium with its ions in aqueous solution. 

The first step consists of identifying the two half-reduction couples that are facing each other and affixing to them the judicious stoichiometric coefficients in such a way that the equilibrium in electrons should be satisfied. This step is called the redox balance. In the given example, the two half-redox reactions are... 

Other classes of electrodes do exist. Of fundamental importance is the hydrogen electrode. Within the electrode, gaseous dihydrogen is in equilibrium with the proton in solution in the presence of platinum. Platinum exchanges electrons with dihydrogen and with protons and takes the potential value of the solution at equilibrium. The redox couple is H+/H2(g) and the half-redox reaction is... 

The equilibrium constant of this reaction is about 10 °. This reaction results from the superimposition of the following half-redox equilibria ... 

The half-redox couple CH3COOH/CH3CH2OH obeys the equilibrium... 

At the same pH value, the half-redox Sn /Sn equilibrium that must be considered... 

In half-cells that do not contain a metal, electrical contact with the solution is made by using platinum wire or platinum foil as an electrode. The redox equilibrium is established at the surface of the platinum. The platinum electrode is inert so plays no part in the reaction. 

As an illustrative example for calculating the redox equilibrium between a NAC and a naturally occurring reductant, we consider the reduction of nitrobenzene at 25 °C in a 5 mM aqueous H2S solution buffered at pH 7.0. We assume that nitrobenzene (NB) is reduced to aniline (AN) and that hydrogen sulfide is oxidized to elemental sulfur (S(s)). From Figure 3 we get the EgCw) values for the two half-reactions ... 

Ladder diagrams can also be used to evaluate equilibrium reactions in redox systems. Figure 6.9 shows a typical ladder diagram for two half-reactions in which the scale is the electrochemical potential, E. Areas of predominance are defined by the Nernst equation. Using the Fe +/Fe + half-reaction as an example, we write... 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

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