Inert metal electrode

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The underlying problem in testing the validity of the additivity principle in corrosion, mineral extraction, and electroless plating is that the electrode metal itself forms part of one of the half-reactions involved, e.g., zinc in equation (5) and copper in equations (8) and (12). A much better test system is provided by the interaction of two couples at an inert metal electrode that does not form a chemical part of either couple. A good example is the heterogeneous catalysis by platinum or a similar inert metal of the reaction... 

A metallic electrode consisting of a pure metal in contact with an analyte solution develops an electric potential in response to a redox reaction occurring at its metal surface. Common metal electrodes such as platinum, gold, palladium or carbon are known as inert metal electrodes whose sole function is to transfer electrons to or from species in solution. Metal electrodes corresponding to the first kind are pure metal electrodes such as Ag, Hg and others that respond directly to a change in activity of the metal cation in the solution. For example, for the reaction... 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

Electrochemical measurements of potential with inert metallic electrodes are not reliable indicators of Eh levels in most natural water systems, and even the concept of an Eh level is meaningless unless restrictive conditions are satisfied. Analytical determinations can in proper circumstances give more reliable information about Eh levels than potential measurements can. 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Consider a half-reaction of first order occurring at an inert metallic electrode ... 

Fig. 4.2 Electron transfer at an inert metallic electrode. The potential applied to the electrode alters the highest occupied electronic energy level, EF, facilitating (a) reduction or (b) oxidation.

Peters equation — Obsolete term for the - Nernst equation in the special case that the oxidized and reduced forms of a redox pair are both dissolved in a solution and a reversible potential is established at an inert metal electrode. Initially Nernst derived his equation for the system metal/metal ions, and it was Peters in the laboratory of -> Ostwald, F. W. who published the equation for the above described case [i]. The equation is also sometimes referred to as Peters-Nernst equation [ii]. 

Because of the possible complexity of the electrode process no general treatment is possible, but some of the main concepts may be illustrated in terms of a simple model in which we shall assume a definite kinetic path. Let us consider an electrolysis system that consists of an inert metal electrode in contact with oxidized and reduced forms of a dissolved ionic species which can react at the electrode according to the stoichiometric equation... 

The Eh of natural waters has been calculated theoretically 2,3 ), measured with inert metal electrodes, calculated from analyses of individual redox species (3 7) and measured by equilibration with known redox couples, 3 ). Eh measurements have been used qualitatively as an operational parameter and quantitatively as an indication of a dominant redox couple. The qualitative use of Eh, advocated by ZoBell ( ), has resulted in a great many measurements ( ). As a quantitative tool, the use of Eh has not enjoyed widespread success. 

An electrolytic cell consists of a pair of inert metallic electrodes in a solution buffered to pH = 5.0 and containing nickel sulfate (NiS04) at a concentration of 1.00 M. A current of 2.00 A is passed through the cell for 10.0 hours. 

Redox electrodes currently in use are either (1) inert metal electrodes immersed in solutions containing redox couples or (2) metal electrodes whose metal functions as a member of the redox couple. 

Conductometry is an electrochemical technique used to determine the quantity of an analyte present in a mixture by measurement of its effect on the electrical conductivity of the mixture. It is the measure of the ability of ions in solution to carry current under the influence of a potential difference. In a conductometric cell, potential is applied between two inert metal electrodes. An alternating potential with a frequency between 100 and 3000 Hz is used to prevent polarization of the electrodes. A decrease in solution resistance results in an increase in conductance and more current is passed between the electrodes. The resulting current flow is also alternating. The current is directly proportional to solution conductance. Conductance is considered the inverse of resistance and may be expressed in units of ohm (siemens). In clinical analysis, conductometry is frequently used for the measurement of the volume fraction of erythrocytes in whole blood (hematocrit) and as the transduction mechanism for some biosensors. 

The potential for this couple can be measured with an inert metallic electrode immersed in a solution containing both iron species. The potential depends on the logarithm of the ratio between the molar concentrations of these ions. 

Platinum electrode Used extensively in electrochemical systems in which an inert metallic electrode is required. 

Fig. 5 Electron transfer at an inert metal electrode. The Fermi level of electrons in metal is altered by the applied potential so that the electron can move from the electrode to the molecule (A) or from the molecule to electrode (B).

In this section we only consider electron transfer processes between a redox couple dissolved in the electrolyte and an inert metal electrode such as platinum. Here an inert electrode means that we work in a potential range where essentially no other electrochemical reactions take place. Considering a single electron transfer step as given by... 

It follows from Section 4 that solvated electrons are generated during cathodic polarization of inert metal electrodes, e. g., in liquid ammonia and hexamethylphosphotriamide solutions of alkali metals salts. 

Under irradiation (curve 2), a photoanodic current, /ph, flows as long as the potential of the electrode is more positive than fb, so that electron/hole pair separation can occur. Thus the onset of the photocurrent is near Ef y (unless surface recombination processes move the onset potential toward more positive values). The photo-oxidation of R to O occurs at less positive applied potentials than those required to carry out this process at an inert metal electrode (curve 3). This is possible because the light energy helps to drive the oxidation process hence such processes are frequently called photoassisted electrode reactions. 

Photoeffects are generally not observed at n-type materials for redox couples located at potentials negative of In this case, the bands are bent downwards, the majority carrier tends to accumulate near the surface (i.e., an accumulation layer forms), and the semiconductor behavior approaches that of an inert metal electrode. Similarly, a (hole) accumulation layer forms in a p-type material for couples located positive of Ef y. 

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