Indifferent electrode

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

In potentiometric measurements the simplest approach to the liquid-junction problem is to use a reference electrode containing a saturated solution of potassium chloride, for example the saturated calomel electrode (p. 177). The effect of the diffusion potential is completely suppressed if the solutions in contact contain the same indifferent electrolyte in a sufficient... 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

Assume that both the initial substances and the products of the electrode reaction are soluble either in the solution or in the electrode. The system will be restricted to two substances whose electrode reaction is described by Eq. (5.2.1). The solution will contain a sufficient concentration of indifferent electrolyte so that migration can be neglected. The surface of the electrode is identified with the reference plane, defined in Section 2.5.1. In this plane a definite amount of the oxidized component, corresponding to the material flux J0x and equivalent to the current density j, is formed or... 

The first two terms on the right-hand side of this equation express the proper overpotential of the electrode reaction rjr (also called the activation overpotential) while the last term, r)c, is the EMF of the concentration cell without transport, if the components of the redox system in one cell compartment have concentrations (cOx)x=0 and (cRed)x=0 and, in the other compartment, Cqx and cRcd. The overpotential given by this expression includes the excess work carried out as a result of concentration changes at the electrode. This type of overpotential was called the concentration overpotential by Nernst. The expression for a concentration cell without transport can be used here under the assumption that a sufficiently high concentration of the indifferent electrolyte suppresses migration. 

The work with both DME and RDE requires the use of a base (supporting or indifferent) electrolytey the concentration of which is at least twenty times higher than that of the electroactive species. With UME it is possible to work even in the absence of a base electrolyte. The ohmic potential difference represents no problem with UME while in the case of both other electrodes it must be accounted for in not sufficiently conductive media. The situation is particularly difficult with DME. Usually no potentiostat is needed for the work with UME. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

An awkward situation arises when dealing with a dilute solution where it has been observed that the depletion of the electrode layer ultimately leads to an enhancement of the resistance of the solution and thereby affecting subsequently an alteration in the Ohm s Law potential drop (I x R) in the cell. This ultimately gives rise to a doubtful observed potential operative at the electrode. In order to overcome this serious anomaly, it is a normal practice to add an excess of an indifferent electrolyte to the system, such as 0.1 M KC1, which renders the solution to remain stable at a low and constant resistance, whereas the migration current (Im) of the species under examination almost vanishes i.e., I = Id. 

Ideal potentiometric measurements, especially in analytical chemistry, would require that the potential of the reference electrode be fixed and known, and that the composition of the studied solution affect only the potential of the indicator electrode. This would occur only if the liquid-junction potential could be completely neglected. In practice this situation can be attained only if the whole system contains an indifferent electrolyte in a much larger concentration than that of the other electrolytes, so that the concentration of a particular component in the analysed solution, which is not present in the reference electrode solution, has only a negligible effect on the liquid-junction potential Such a situation rarely occurs, so that it is necessary to know or at least fix the liquid junction potential... 

Electrochemically generated nickei(lll) oxide, deposited onto a nickel plate, is generally useful for the oxidation of alcohols in aqueous alkali [49]. The immersion of nickel in aqueous alkali results in the formation of a surface layer of nickel(ll) oxide which undergoes reversible electrochemical oxidation to form nickel(lll) oxide with a current maximum in cyclic voltammetry at 1.13 V vj. see, observed before the evolution of oxygen occurs [50]. This electrochemical step is fast and oxidation at a prepared oxide film, of an alcohol in solution, is governed by the rate of the chemical reaction between nickel oxide and the substrate [51]. When the film thickness is increased to about 0.1 pm, the oxidation rate of organic species increases to a rate that is fairly indifferent to further increases in the film thickness. This is probably due to an initial increase in the surface area of the electrode [52], In laboratory scale experiments, the nickel oxide electrode layer is prepared by prior electrolysis of nickel sulphate at a nickel anode [53]. It is used in an undivided cell with a stainless steel cathode and an alkaline electrolyte. 

Impurities, effect on electrode kinetics, 1091, 1120 Indicator electrodes, 1111 Indifferent ions, see supporting electrolyte Infrared... 

Unless there is a large excess of indifferent ions that assume the burden of carrying the current (as indeed was assumed above), the electron acceptors and donors do not move only by diffusion or convection they also move under the influence of the electric field. In fact, this is generally the case unless one has diminished the fraction of the current in the solution which reactants need for carrying, by adding an excess of ions of another kind that do not undergo electrodic reaction, e.g., the indifferent electrolyte. How must the current-potential equations be modified ... 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

Example. The surfaces of dispersed Agl particles can be considered similarly to an Ag-Agl-aqueous solution reversible electrode (i.e., each phase contains a common ion that can cross the interface). Here both Ag+ and I- will be potential determining ions because either may adsorb at the interface and change the surface potential. In this case, NaN03 is an example of an indifferent electrolyte as far as the electrode potential goes. 

Overpotential Departure from equilibrium (reversible) potential due to passage of a net current. Concentration overpotential results from concentration gradients adjacent to an electrode surface. Surface overpotential results from irreversibilities of electrode kinetics. Supporting (inert or indifferent) electrolyte Compounds that increase the ionic conductivity of the electrolyte but do not participate in the electrode reaction. 

For measurement of redox couples, a frequently overlooked but convenient reference electrode is a conventional glass pH electrode (assuming that the sample solution system contains a constant level of acidity). Such an electrode provides an extremely inert and stable reference potential that is completely indifferent to most redox species. However, the glass electrode requires the use of an electrometer amplifier such as that contained in pH meters. 

Under the open circuit conditions, the illumination of BiOHal electrodes with a full spectrum of mercury quartz lamp causes the shift of the electrode potential to the anodic direction (typical for p-type semiconductors). In this case, in the indifferent electrolytes (neutral or acidified KHal solutions) saturated with oxygen, the high positive potential values are attained for BiOCl and BiOBr electrodes (Table 6.2). 

The mechanism of the passage of direct current tlirough a conductor of the second class which is connected with a migration of ions and chemical reactions on electrodes can be most easily explained by an example. Let us imagine a vessel with two indifferent platinum electrodes A and C electrically connected to a source of direct current B and immersed into an aqueous solution of common salt in water (Fig. 1.). 

To yireparc electrodes both metals and nonmctals can be applied. Iodine in a solid state in contact with iodide solution can, for example, become an electrode reversible with respect to the iodide ions. When making such an electrode the iodine is placed into a glass vessel and the iodide solution poured over it current connection is made by a platinum wire, which is in contact with the iodine and acts as an indifferent electrode. Equilibrium at the electrode is attained according to the formula ... 

The phenomenon of limiting current can appear at anodes as well as at cathodes. When an indifferent electrode is immersed into a solution of ferrous salt or any other anodically oxidizable substance the limiting current density will be attained at a certain terminal voltage. The limiting ourrent density will depend not only on the maximum number of ions (Fe++) transferred by diffusion or even migration in a unit of time from the bulk of the solution to the electrode but also on the maximum transport-rate of the products (Fe+++) in the opposite direction. 

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