Redox Electrodes—Inert Metals

In the redox electrode, an inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction. This type of electrode was mentioned in Chapter 12. 

The inert metal used is usually platinum. The potential of such an inert electrode is determined by the ratio at the electrode surface of the reduced and oxidized species in the half-reaction  

A very important example of this type of electrode is the hydrogen electrode, Pt H2, H+  

The pressure of gases, in atmospheres, is used in place of activities. If the hydrogen pressure is held at 1 atm, then, since for Equation 13.19 is de- 

The vapor pressure of water above Calculate the pH of a solution whose potential at 25°C measured with a hydrogen 

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The classical generation of an activity-sensitive voltage is spontaneous in a solution containing both non redox ions and redox ions. Classical electrodes of types 1,2, and 3 respond by ion exchange directly or indirectly to ions of the same material as the electrode. Inert metal electrodes (sometimes called type 0)—Pt, Ir, Rh, and occasionally carbon C—respond by electrons exchange from redox pairs in solution. Potential differences are interfacial and reflect ratios of activities of oxidized to reduced forms. 

Historically, indicator electrodes have been metals which form a redox couple with the analyte, such as a Ag electrode for the determination of Ag", or a chemically inert metal which responds to the activity ratio of a soluble redox couple, such as a Pt electrode for Fe /Fe. Whereas simple indicator electrodes of this type perform well for the analysis of relatively pure samples, they are often subjwt to interferen< when apphed to complex samples such as those of biological origin. 

Oxidation-reduction electrodes, abbreviated to redox electrodes, consist of an inert metal (Pt, Au, Hg) immersed in a solution containing two forms... 

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... 

Consider an inert metal with a fractal electrode immersed into an electrolyte containing a redox couple. We assume semi-infinite diffusion of the oxidized species Ox and the reduced species Red in the electrolyte. For the sake of simplicity we assume that the solution contains initially (at the time, t = 0) only the oxidized form and the bulk and surface concentrations are identical, i.e., cox=cL- The electrode is initially subjected to an electrode potential where no reaction takes place. The only reaction occurring when the potential is lowered, is the reversible reduction of Ox to Red, i.e., Ox + ze = Red. In addition, it is assumed that the overall reaction is limited by diffusion of Ox in electrolyte. 

Using a different convention, a simple metal in contact with its cations is also commonly termed an electrode of the first kind, or a class I or first-order electrode, while an electrode covered with an insoluble salt, e.g. AgCI I Ag for determining u(Cr), is termed an electrode of the second kind, or a class II or second-order electrtxle. In this latter convention, inert electrodes fur following redox reactions (cf. Chapter. 4) are somewhat confusingly termed redox electrodes. 

Analyses using inert electrodes are experimentally identical to those using redox electrodes but are less useful in practice since we usually want to know how much metal is in solution, and use of the Nernst equation (equation (3.8)), e.g. for the case of Fe and Fe " ", will merely tell us the ratio of the respective redox states in solution. 

Figure 9.1 Cross-sections of a typical inert or redox electrode, embedded (a) within glass, and (b) within epoxy resin. Type (b) is preferred if the metal is liable to melt or react when very hot.

In most circumstances, the emf or current from a redox or inert electrode will be the same whether or not the metal is encased within a protective... 

It was shown that both reference electrodes as well as metallic electrodes can be prepared in house by the analyst. For such systems, the best results are obtained if the metal of the inert or redox electrode is first bonded within a... 

RedOx electrode potentials are the result of an exchange of electrons between metal and electrolyte. In Section 5.4 we have shown that the metal/metal-ion electrode potentials are the result of an exchange of metal ions between metal and electrolyte. In the RedOx system the electrode must be made of an inert metal, usually platinum, for which there is no exchange of metal ions between metal and electrolyte. The electrode acts as a source or sink for electrons. The electrolyte in the RedOx system contains two substances electron donors (electron-donating species) and electron acceptors (electron-accepting species). One example of a RedOx system is shown in Figure 5.4. In this case the electron donor is Fe ", the electron acceptor is Fe , the electrode is Pt, and the electrode process is... 

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. 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

In redox electrodes an inert metal conductor acts as a source or sink for electrons. The components of the half-reaction are the two oxidation states of a constituent of the electrolytic phase. Examples of this type of system include the ferric/ferrous electrode where the active components are cations, the ferricyanide/ferrocyanide electrode where they are anionic complexes, the hydrogen electrode, the chlorine electrode, etc. In the gaseous electrodes equilibrium exists between electrons in the metal, ions in solution and dissolved gas molecules. For the half-reaction... 

Another form of redox reference electrode is similar to the electrode of the first kind. In this case the inert metal (e.g., Pt, Au, or C) is used as the inner electrode and a stable and soluble redox couple is placed inside the inner reference electrode compartment. A normal liquid junction is used in this type of reference electrode. Unlike the electrode of the first kind, the redox reference electrode is relatively immune to changes in concentration inside the reference electrode compartment because it is the ratio of the reduced/oxidized form of redox couple that determines the potential and not the absolute concentrations. However, redox reference electrodes are sensitive to changes of concentration of oxygen and other redox species. 

Moderately doped diamond demonstrates almost ideal semiconductor behavior in inert background electrolytes (linear Mott -Schottky plots, photoelectrochemical properties (see below), etc.), which provides evidence for band edge pinning at the semiconductor surface. By comparison in redox electrolytes, a metal-like behavior is observed with the band edges unpinned at the surface. This phenomenon, although not yet fully understood, has been observed with numerous semiconductor electrodes (e.g. silicon, gallium arsenide, and others) [113], It must be associated with chemical interaction between semiconductor material and redox system, which results in a large and variable Helmholtz potential drop. 

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]. 

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. 

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 electrochemistry of metalloproteins has developed markedly (4-6) over the past 15 years. It has mainly been concerned with the electrochemistry at solid electrodes gold, upon which are adsorbed redox-inactive promoters, i.e., molecules that bind both to the electrode and the protein and edge-plane graphite, with or without redox-inert metal ions in solution. 

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. 

Platinum and gold are examples of inert metals used to record the redox potential of a redox couple dissolved in an electrolyte solution. The hydrogen electrode is a special redox electrode for pH measurement. It consists of a platinum or gold electrode that is electrolyticaUy coated (platinized) with... 

It is convenient to classify metallic indicator electrodes as electrodes of the first kind, electrodes of the second kind, or inert redox electrodes. 

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... 

Because there is generally no difficulty in finding a.suitable indicator electrode, redox titrations are widely used an inert metal such as platinum is usually satisfactory for the electrode. Both the oxidized and reduced forms are usually soluble and their ratio varies throughout the titration. The potential of the indicating electrode will vary in direct proportion to log as in the calculated potential for the... 

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. 

The last type of metallic electrode is the redox indicator electrode. This electrode is made of Pt, Pd, Au, or other inert metals, and serves to measure redox reactions for species in solution (e.g., Fe /Fe, Ce /Ce" ). These electrodes are often used to detect the endpoint in potentiometric titrations. Electron transfer at inert electrodes is often not reversible, leading to nonreproducible potentials. Although not a metal electrode, it should be remembered that carbon electrodes are also used as redox indicator electrodes, because carbon is also not electroactive at low applied potentials. 

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