Metal-insoluble salt electrode

Metal-insoluble salt electrode Metal-insoluble salt electrodes are convenient for batteries. For example, the AgCl/Ag electrode consists of metallic silver, coated with the insoluble salt AgCl, and suspended in a solution or paste saturated with the salt. The electrode reaction is... 

From a thermodynamic viewpoint, oxide electrodes are frequently regarded as metal/insoluble salt electrodes " in which the activity of the metal ion is modified by interaction with the ligands, which in this case are the OH ions. Thus, reaction (6) may be regarded as a combination of the following ... 

Metal insoluble-salt These consist of a metal in contact with one of its slightly soluble salts this salt in turn is in contact with a solution containing the anion of the salt. An example is represented as Ag AgCl Or (c). The electrode process at such an electrode as AgCl (s) Ag + Cl" Ag + e- —> Ag (s) or overall, AgCl (s) + e- Ag (s) + Cl". The electrode reaction involves only the concentration of Cl" as a variable, in contrast with the Ag Ag electrode, which has the Ag concentration as a variable. The most frequently electrode of this type is the calomel electrode (see text for description). 

Metal/Insoluble Salt/Ion Electrodes, Electrode potentials are usually reported relative to the normal hydrogen electrode [NHE a(H ) = 1, 1] but they are... 

An insoluble salt electrode (also called a second-order electrode) consists of a metal covered by a porous layer of its insoluble salt. The whole assembly is immersed in asolution containing a corresponding anion. For example, a silver-silver chloride electrode is denoted Ag(s) AgCl(s) CH the electrode potential is a combination of the equation analogous to Eq. (7), and the solubility product of a sparingly soluble salt, Vs(AgCl) = fl(Ag+). fl(CH), is shown in Eq. (14) ... 

The metallic electrode described above is the simplest of the electrode types. Another type of electrode is the insoluble-salt electrode, in which a metal is covered with one of its insoluble salts silver chloride deposited on silver is one such example. The oxidation reaction in this case is... 

Potentiometric sensors can be classified based on whether the electrode is inert or active. An inert electrode does not participate in the half-cell reaction and merely provides the surface for the electron transfer or provides a catalytic surface for the reaction. However, an active electrode is either an ion donor or acceptor in the reaction. In general, there are three types of active electrodes the metal/metal ion, the metal/insoluble salt or oxide, and metal/metal chelate electrodes. 

At another type of active electrode, found in many batteries, the reaction is the conversion between a metal and an Insoluble salt. At the surface of this type of electrode, metal cations combine with anions from the solution to form the salt. One example is the cadmium anode of a rechargeable nickel-cadmium battery, at whose surface cadmium metal loses electrons and forms cations. These cations combine immediately with hydroxide ions in... 

In addition to metals, other substances that are solids and have at least some electronic conductivity can be used as reacting electrodes. During reaction, such a solid is converted to the solid phase of another substance (this is called a solid-state reaction), or soluble reaction products are formed. Reactions involving nomnetaUic solids occur primarily in batteries, where various oxides (MnOj, PbOj, NiOOH, Ag20, and others) and insoluble salts (PbS04, AgCl, and others) are widely used as electrode materials. These compounds are converted in an electrochemical reaction to the metal or to compounds of the metal in a different oxidation state. 

The same expression is obtained if electrodes of the second kind are considered as electrodes of the first kind, where the activity of the metal cations depends on the solubility product of the given insoluble salt (cf. Eq. 3.1.26) ... 

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. 

A special case is when the electrochem-ically active components are attached to the metal or carbon (electrode) surface in the form of mono- or multilayers, for example, oxides, hydroxides, insoluble salts, metalloorganic compounds, transition-metal hexacyanides, clays, zeolites containing polyoxianions or cations, intercalative systems. The submonolayers of adatoms formed by underpotential deposition are neglected, since in this case, the peak potentials are determined by the substrate-adatom interactions (compound formation). From the ideal surface cyclic voltammetric responses, E° can also be calculated as... 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

Using supporting electrolytes such as tetraalkylammonium salts, one may apply potentials as negative as -2.6 V vs. SCE in aqueous solutions, while in some nonaqueous systems even -3.0 V vs. SCE (aqueous) is accessible. Unfortunately, mercury electrodes have serious limitations in applications at positive potentials (with the exception of passivated mercury electrodes, which are described in Section VI), and this has led to extensive research in the development of solid metal and carbon electrodes. Oxidation of mercury occurs at approximately +0.4 V vs. SCE in solutions of perchlorates or nitrates, since these anions do not form insoluble salts or stable complexes with mercury cations. In all solutions containing anions that form such compounds, oxidation of the mercury proceeds at potentials less than +0.4 V vs. SCE. For example, in 0.1 M KC1 this occurs at +0.1 V, in 1.0 M KI at -0.3 V, and so on. 

For those redox couples that involve a metal ion plus the metal, the logical electrode system is the metal itself. In other words, if the measured quantity is to be cupric ion [copper(II)], a practical indicator electrode is a piece of copper metal. All second-class electrodes involve an active metal in combination with an insoluble compound or salt. Thus, the silver/silver chloride electrode actually is a silver/silver ion electrode system that incorporates the means to control the silver ion concentration through the chloride ion concentration [Eq. (2.14)]. A related form of this is the antimony electrode, which involves antimony and its oxide (an adherent film on the surface of the antimony-metal electrode) such that the activity of antimony ion is controlled by... 

Electrodes of the type metal in contact with an insoluble salt or oxide. 

Electrodes of this type are sometimes called electrodes of the second class and are applied in electrochemistry as the reference electrodes for measuring unknown potentials. They are formed by a metal in contact with its insoluble salt, which is immersed into a solution of a soluble electrolyte with the... 

For this reason the concentration of the anions has to be kept at a low value by the gradual addition of the respective salt to the solution, i. e. at a rate corresponding to its consumption for the precipitation of the insoluble salt. In addition, another anion is added to the electrolyte in a higher concentration which forms with the metallic cation a soluble compound the function of which is the conduction 9f current. The precipitating anions in the course of electrolysis are quickly consumed at the anode so that the actual conducting of current to the electrode is then carried out almost exclusively by the other anions. The oations not precipitated by the latter anions can then migrate into the bulk of the solution, where they meet the precipitating anions and form insoluble compounds. 

Basically, the final choice of the cation has to relate strictly to the application. The presence of cations such as Li+ or Na+ in solutions may lead to precipitation of insoluble surface films or noble metal electrodes and thus interfere with the basic electrochemical behavior of many redox couples on nonactive metal electrodes in polar aprotic solvents [9], The use of tetraalkyl ammonium salts eliminates this problem because the thermodynamics of insoluble salt precipitation on electrodes differs in the presence of these bulky cations from that developed in the presence of cations of alkaline or alkaline earth metals [6-9],... 

Similar to the behavior of nonactive metal electrodes described above, when carbon electrodes are polarized to low potentials in nonaqueous systems, all solution components may be reduced (including solvent, cation, anion, and atmospheric contaminants). When the cations are tetraalkyl ammonium ions, these reduction processes may form products of considerable stability that dissolve in the solution. In the case of alkali cations, solution reduction processes may produce insoluble salts that precipitate on the carbon and form surface films. Surface film formation on both carbons and nonactive metal electrodes in nonaqueous solutions containing metal salts other than lithium has not been investigated yet. However, for the case of lithium salts in nonaqueous solvents, the surface chemistry developed on carbonaceous electrodes was rigorously investigated because of the implications for their use as anodes in lithium ion batteries. We speculate that similar surface chemistry may be developed on carbons (as well as on nonactive metals) in nonaqueous systems at low potentials in the presence of Na+, K+, or Mg2+, as in the case of Li salt solutions. The surface chemistry developed on graphite electrodes was extensively studied in the following systems ... 

These electrodes behave as if they were reversible with respect to the common anion, e.g., the chloride ion in the above electrode. The electrode reaction involves the passage of the electrode metal into solution as ions and their combination with the anions of the electrolyte to form the insoluble salt, or the reverse of these stages thus, for the silver-silver chloride electrode,... 

Occasionally electrodes of the third kind are encountered these consist of a metal, one of its insoluble salts, another insoluble salt of the same anion, and a solution of a soluble salt having the same cation as the latter salt, e.g.,... 

The reverse process takes place at the anode in a cell of this type. Thus 1 — v equivalents of the anion migrate to the anode, where they react with the metal of the electrode to form the insoluble salt, leaving the concentration of the electrolyte unaltered. 

The reference electrodes have been divided according to the electrode reaction responsible for maintaining the constant potential In electrodes of the first kind, the potential of the metal electrode is determined by the concentration of the metal ions in electrodes of the second kind, the potential is determined by the concentration of an anion that forms an insoluble salt with the metal cation and in redox electrodes, the potential of an indifferent electrode is determined by the relative concentrations of the two components of a redox system. 

An electrode of the second kind may be used if the metal cation forms an insoluble salt with an anion, but from the point of establishing a satisfactory reference electrode of this kind, the most interesting constant is not the solubility constant, but the equilibrium constant for the reaction... 

---