Anions Anodes

From the retrosynthetic point of view, electrochemical redox reactions are an easy way to accomplish the principle of redox-umpolung (polarity reversal) [2]. As can be seen in Scheme 22.1, oxidation of an electron-rich neutral compound will lead to an electrophilic cation radical, or starting with a nucleophilic anion, anodic oxidation may lead to an electrophilic cation. In the mirror image reductions, an electron-poor neutral compound is transformed to a nucleophilic anion radical, or an electrophilic cation will end up as a nucleophilic anion. 

We connect the DC supply to the electrode metal wires and adjust the voltage so that a suitable DC current flows. An electric fleld, E, is accordingly set up in the solution between the electrodes. Positive ions (e.g., Na ) migrate in the same direction as the E-field all of the way up to the cathode—they are cations. Negative ions (e.g., CP) migrate in the opposite direction in the same directions as the electrons in the wires—they are anions. Anode and cathode are defined from current flow direction and not necessarily from the polarity of the external voltage source. In the bulk of the electrolyte, no change... 

Faraday, a student of Davy, introduced the vocabuiary of eiectrochemistry (eiectrode, anion-anode and cation-cathode) and observed the iink between the mass of compound produced or consumed and the amount of charge passed (iaws of eiectroiysis). 

The presence in the FTlR-speetrum of the line corresponding to HCOO, is essential, for it proves the eorreetness of the earned out estimation of the current efficiency of the processes of the anodic destruction of amino acid anions. In a broader sense, this result confirms, in our opinion, the validity of a four-electron scheme of a "gentle" electrooxidation of glycine anions and the possibility of its appUcation to oxidation of a-alanine anion suggested in [19]. The appearance in the FTIR-spectra obtained at the potentials of aminoacid anions anodic oxidation, of the bands corresponding to the formation of adsorbed species (-CH2-NH2)ads [33], as well as the presence of HCOO and CN proposed in the [19] scheme electrooxidation of Gly and a-Ala, presenting the process (9) as follows ... 

Note that chemists tend to refer to positive ions as cations (attracted to the cathode m electrolysis) and negative ions as anions (attracted to an anode). In this section of the encyclopedia, the temis positive ion and negative ion will be used for the sake of clarity. 

As a furtlier example for tire meaning of ex situ investigations of emersed electrodes witli surface analytical teclmiques, results obtained for tire double layer on poly crystalline silver in alkaline solutions are presented in figure C2.10.3. This system is of scientific interest, since tliin silver oxide overlayers (tliickness up to about 5 nm) are fonned for sufficiently anodic potentials, which implies tliat tire adsorjDtion of anions, cations and water can be studied on tire clean metal as well as on an oxide covered surface [55, 56]. For tire latter situation, a changed... 

Capillary zone electrophoresis also can be accomplished without an electroosmotic flow by coating the capillary s walls with a nonionic reagent. In the absence of electroosmotic flow only cations migrate from the anode to the cathode. Anions elute into the source reservoir while neutral species remain stationary. 

The best known oxoanion of iron is the ferrate(VI) prepared by oxidizing a suspension of hydrous iron(III) oxide in concentrated alkah with potassium hypochlorite or by anodic oxidation of iron in concentrated alkah. Crystals of potassium ferrate [13718-66-6], K FeO, are deep purple, orthorhombic, and contain discrete tetrahedral [FeOJ anions. Barium ferrate [13773-23A] can be precipitated from solutions of soluble ferrate salts. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Cd(OH)2 is much more basic than Zn(OH)2 and is soluble ia 5 NaOH at 1.3 g/L as the anionic complex tetrahydroxocadmate [26214-93-7] Cd(OH) 4. Technical-grade Cd(OH)2 sold for 74/kg ia 1991 and its most important utihty is as the active anode ia rechargeable Ni—Cd and Ag—Cd storage batteries. The chemical reaction responsible for the charge—discharge of the batteries is (35) ... 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

The ferrous ions that dissolve from the anode combine with the hydroxide ions produced at the cathode to give an iron hydroxide precipitate. The active surface of ferrous hydroxide can absorb a number of organic compounds as well as heavy metals from the wastewater passing through the cell. The iron hydroxide and adsorbed substances are then removed by flocculation and filtration. The separation process was enhanced by the addition of a small quantity of an anionic polymer. 

The feed solution containing ions enters a compartment whose walls are a cation-exchange and an anion-exchange membrane. If the anion-exchange membrane is in the direction of the anode, as shown for the middle feed compartment, anions may pass through that membrane in response to an electrical potential. Tne cations can likewise... 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 22-61. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional elec trodialysis, many cehs may be stacked between the anode and the cathode. 

In this paper the electtode anodic reactions of a number of dihydropyridine (DHP) derivatives, quantum-chemical calculations of reactions between DHP cation-radicals and electrochemiluminescers anion-radicals (aromatic compounds) and DHP indirect ECL assay were investigated. The actuality of this work and its analytical value follow from the fact that objects of investigation - DHP derivatives - have pronounced importance due to its phaiTnacology properties as high effective hypertensive medical product. 

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

ANION A negatively charged atom or group of atoms, or a radical which moves to the positive pole (anode) during electrolysis. 

Anion A negatively charged ion in an electrolyte solution, attracted to the anode under the influence of a difference in electrical potential. Chloride is an anion. Anion exchange capacity Sum total of exchangeable anions that a soil can adsorb. Expressed as centimoles of negative charge per kilogram of soil. 

Anion-negatively charged ions that migrate toward the anode of a galvanic cell. 

---