Standards overvoltages

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

The interfadal overvoltage referred to the standard redox potential is defined, generally, by Eqn. 10-41 ... 

TABLE 8. Decomposition potentials, d , anodic overvoltage for a current density 0.06 A cm , jo.o6 > standard oxidation potentials 0 for the oxidation of RMgBr in Et20 solutions and the Itond dissociation energy, D(R—MgBr), of the C—Mg bond in RMgBr... 

This description of the photoelectrochemical event was originally based on the use of an illuminated semiconductor electrode in a standard three electrode cell configuration. The theory can easily be extended, however, for practical applications to short-circuited cells prepared by deposition of inert metal with low overvoltage characteristics on a powdered semiconductor Such a metallized powder is shown in... 

The direct electrolysis of a number of organic substrates requires a considerable overvoltage in order to proceed at a reasonable rate. The rate of an electron transfer in solution is high when the standard potentials of the reacting systems have suitable values. 

Standard lead-acid batteries lose a small amount of water by evaporation, but the major mechanism for water loss is by electrolysis to form hydrogen and oxygen, as described by eqs (5.5) and (5.6). The presence of small quantities of foreign elements lowers the overvoltages for these processes and leads to an accelerated water loss it has been shown that the elements antimony, arsenic, cobalt, manganese, nickel, platinum and tellurium all have a deleterious effect, even at low levels. Two general approaches have been taken in the development of MF batteries ... 

Sodium ion acts as a spectator ion and is not involved in the electrode reactions. Thus, the sodium chloride solution is converted to a sodium hydroxide solution as the electrolysis proceeds. The minimum potential required to force this nonspontaneous reaction to occur under standard-state conditions is 2.19 V plus the overvoltage. 

Nernst equation 778 overvoltage 794 sacrificial anode 792 salt bridge 766 standard cell potential... 

Impulse — Traditionally used to indicate a short duration overvoltage event with certain rise and fall characteristics. Standards have moved toward including the term impulse in the category of transients. 

It is natural that the possibility of ionic reaction can be reliably predicted from the standard potential series only when the activities of all components taking part in the reaction equal unity. At other activities the mutual relations of substances in the potential series can be changed. There is a second limitation, namely, no retardation of the reactions by various foreign phenomena (e. g. by overvoltage or mechanical pasivity of the surface, due to the existence of oxide films). 

In cases where the difference in the standard potential is not so great, both curves can be made to run closer or farther apart by a suitable choice of reaction conditions lhus tho simultaneous deposition of both cations can be either facilitated or made more difficult. It is possible, for example, to carry out a simultaneous cathodic deposition of tin and lead from the solution of their chlorides it only requires a suitable adjustment of respective concentrations of their salts in the solution because tho standard potentials of both these metals are very near to each other (nSn — —0.14, tcpi, — —0.13). If an acid solution is used hydrogen ions should be discharged theoretically prior to both metals (tc h2 = 0), yet in fact, tho hydrogen overvoltage at both metals is so high that no hydrogen will be evolved at all. 

The standard equilibrium potential at the anode related to reaction (XXIV-7 is 7c° = 0.356 V. As oxygen is evolved owing to overvoltage from neutral solutions as late as the potential is about 1.2 V and from alkaline solution at about 0.8 V, the oxidation of ferrocyanide to ferricyanide can proceed with a 100 per... 

The first factor determines the tendency for dissolution to occur while the second and third, which are closely related, determine the rate of dissolution. The use of the standard electrode potentials as a measure of nobility is well known. The recognition that the exchange current density is a measure of the reversibility of a process and therefore a quantity characteristic of the reactivity of the system is more recent (13,32). As indicated by the Tafel relations, the exchange current density is a direct measure of the rate of the electrode reaction for any given value of the activation overvoltage (33). The values of iG may then be taken as a criterion for the electrochemical activity of a system. 

An examination of equation (1) shows that there are, in general, three ways in which the discharge potentials of two cations may be brought together (i) if the standard potentials are approximately equal and the overvoltages are small (ii) if the standard potentials are different, but the overvoltages vary suflSciently to compensate for this difference and (iii) if the differences in reversible potential and overvoltage are compensated for by differences in the activities of the ions. Examples of the three types of behavior are known. 

Separation of Metals by Electrolysis.—The complete separation of one metal from another is important in quantitative electro-analysis the circumstances in which such separation is possible can be readily understood from the preceding discussion of simultaneous deposition of two metals. The conditions must be adjusted so that the discharge potentials of the various cations in the solution are appreciably different. If the standard potentials differ sufficiently and there are no considerable deposition overvoltages, complete separation within the limits of analytical accuracy is possible this is, of course, contingent upon the metals not forming compounds or solid solutions under the conditions of deposition. Since the concentration of the ions of a deposited metal decreases during electrolysis, the deposition potential becomes steadily more cathodic, and may eventually approach that for the deposition of another metal. For example, if the ionic concentration is reduced to 0.1 per cent of its original value, the potential becomes 3 X 0.0295 volt more cathodic for a bivalent metal and 3 X 0.059 volt for a univalent metal, at ordinary... 

Figure 5.4 Pyramids of growth on an Ag(lOO) face obtained by applying a short overvoltage pulse on an initially flat crystal face in the standard system Ag(100)/AgNO3 [5.7]. The pyramids mark the emergence points of the screw dislocations. The quadratic symmetry of the pyramids corresponds to the (100) nature of the face. Face areay4(ioo) = 2 x 10" cm. ...

Figure 5.3 Oscillations of the overvoltage at constant current density t = - 65 pA cm on a quasiperfect cubic face of silver in the standard system Ag (100)/AgNO3 [5.6-5.9]. Tune scale 2 s div voltage scale 5 mV div. The product of the current density and the period of oscillations gives an amount of electricity equal to 9mon ...

Figure 5.7 Current-time record following a voltage pulse excitation on a quasi-perfect cubic face prepolarized at a subcritical overvoltage of //growth = - 4 mV in the standard system Ag (100)/AgNO3. Current scale 10 nA div time scale 0.5 s div" //nuc = - 10 mV, pulse duration fnuc = 80 ps. Electrode areaA = 2.2 x 10 cm. The current-time integral gives an electricity amount of one monolayer.

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