Electrode metal-insoluble salt-anion

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). 

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... 

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 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. 

Active metals such as lithium and sodium can be used as stable reference electrodes in nonaqueous solutions in which they are apparently stable. To a limited extent this may be true for the Mg/Mg2+, Ca/Ca2+ and A1/A13+ couples as well (though they must be checked separately for each specific solution). It is important to note that in most of the commonly used nonaqueous systems, the above active metals are thermodynamically unstable and react readily with the solvent, the salt anions and the unavoidably present atmospheric contaminants. However, the active metals are apparently stable in many systems because the above reaction products, which are usually insoluble (metal salts), precipitate as protective passivating surface films. These films prevent further corrosion of the active metals in solutions [21], Hence, the active metal covered by the surface films may... 

Most of the commonly used salts in nonaqueous systems comprise anions that are reactive and may be reduced at noble metal electrodes at low potentials. In the presence of cations such as Li+, salt anion reduction may precipitate insoluble surface species on the electrodes and thus become the dominant surface film forming process. The criteria chosen here for the reactivity of the various salt anions used are the onset potential of their reduction on noble metal electrodes and to what extent their reduction on the electrodes dominates the surface film chemistry. In this respect, the commonly used salt anions can be divided into three... 

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. 

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 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... 

During electrochemical deposition of metal oxides, metal hydroxides and metals, current are passed between an anode and cathode in a cell containing weakly alkaline electrolyte. The anion of the electrolyte is such that it does not form an insoluble salt with the metal anode. Metal ions issuing from the metal anode make contact with hydroxyl ions in solution and form finely divided oxides or hydroxides. The oxides or hydroxides are removed and chemically reduced to finely divided metal particles. The voltage necessary for carrying out the oxidation of the metal to metal ions is reduced through the use of an electrode as cathode, thereby reducing the cost of the process. 

This electrode is sometimes called an electrode of the second kind. It consists of a bar of metal immersed in a solution containing a solid insoluble salt of the metal and anions of the salt. There are a dozen common electrodes of this kind we cite only a few examples. 

There are follow-up reactions that may include nucleophilic attacks of the anions on the electrophilic solvait molecules, polymerization, and formation of insoluble salts by ionic reactions of the anions with the metal cations. Insoluble salts and other products (e.g., polymeric species) precipitate on the electrode and form an initial surface layer. Further reduction of solution species requires electron transport through the surface layer, and thereby, the follow-up reduction processes are much more selective than the initial ones. Therefore, the composition of the surface layer changes from the metal-film interface to the film-solution interface, i.e., it comprises multilayer surface films. Further... 

Apart from the role it plays in the redox phenomenon, the metal also plays that of an electrical contactor. The electrodes of the first kind are written conventionally as M M +. In this notation a vertical bar represents a phase boundary class II electrodes a metal in equilibrium with a saturated solution of a slightly soluble salt. They consist of a metal covered by a porous coat of one of its insoluble salts, the entire object dipping into a solution containing the anion giving the... 

To these sets of primary and secondary reactions related to solvents, one has to add the eontributions of salt anion reduction, which usually forms metal halides and M AXy species (A is the main high oxidation-state element in the salt anion and X is a halide, such as chloride or fluoride). Most of the produets of aetive metal surface reactions are ionic compounds that are insoluble in the mother solution, and therefore, precipitate as surface films. It should be added to this picture that possible polymeric species can be formed, espeeially in alkyl carbonate solvents, whose reduction forms polymerizable species sueh as ethylene or propylene. Hence, the surface films formed on active metal electrodes are very complicated. They have a multilayer structure perpendicular to the metal surface, and a lateral, mosaic-type composition and morphology (i.e. containing mixtures and islands of different compounds and grains). Such a structure may induce very non-uniform current distribution upon metal deposition or dissolution processes, which leads to dendrite formation, a breakdown of the surface films, etc. These situations are demonstrated in Fig. 13.6 active metal dissolution leads to the break-and-repair of the surface films, thus forming mosaic-type structures. 

A precipitation of insoluble salts on an electrode surface can be initiated and controlled by varying the concentration of metal ions. The most effective method is electrodeposition of mercuric and mercurous salts induced by an anodic polarization of a mercury drop electrode in a solution of anions [32-35] ... 

Second-class electrodes, that is those whose response is dependent on the change in concentration of an anion which gives an insoluble salt with the metal ion of the indicator electrode, provide a general means for monitoring the concentrations of anions. Some of those half reactions, which are well behaved electrochemically and provide means for the potentiometric monitoring of anion species, are summarized in Table I. This table also includes a tabulation of redox reactions that are useful to monitor the concentration of ligands that can complex metal ions. Consideration of these indicates one of the difficulties with absolute potentiometric measurements as a mea-... 

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