Anodic oxides impurities

Siher(Il) oxide, AgO, is a black solid, Ag Ag 02, obtained by anodic or persulphate oxidation of an AgNOs solution. Continued anodic oxidation gives impure Ag203. Argentates, e.g. K.AgO, containing silver(I) are known. 

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

Less important oxides are Ag203, obtained impure by extended anodic oxidation of silver, and Ag30, obtained hydrothermally from Ag/AgO at 80°C, 4000 bar, which is a metallic conductor with the anti-BiI3 structure containing an hep array of silvers with oxide ions occupying 2/3 of the octahedral holes [32]. 

N 24.12% brick red solid mp, decomps when heated over 300°. Insol in w and the usual organic solvents as well as weak acids and alkalies. Comm prepn (Ref 3) is from thiocyanic acid and/or thiocyanates either by anodic oxidation or by interaction with hydrogen peroxide or halogens. The yield is impure because it contains both H and O. The S content varies between 45 and 55%. Lab prepn of the pure polymer is by reacting the Na salt of 5-chlor-3-mercapto 1,2,4-thiodiazols with either acet, ethanol or w (Refs 1 2)... 

The principle of the electrorefining process is basically simple plutonium is oxidized at a liquid metal anode containing impure metal feed and the resulting Pu+3 ions are transported through molten salt to a cathode where pure metal is produced. 

An electrochemical cell in which electrolysis takes place is called an electrolytic cell. The arrangement of components in electrolytic cells is different from that in galvanic cells. Typically, the two electrodes share the same compartment, there is only one electrolyte, and concentrations and pressures are far front standard. As in all electrochemical cells, the current is carried through the electrolyte by the ions present. For example, when copper metal is refined electrolytically, the anode is impure copper, the cathode is pure copper, and the electrolyte is an aqueous solution of CuS04. As the Cu2f ions in solution are reduced and deposited as Cu atoms at the cathode, more Cu2+ ions migrate toward the cathode to take their place, and in turn their concentration is restored by Cu2+ produced by oxidation of copper metal at the anode. 

FIGURE 12.14 A schematic representation showing the electrolytic process for refining copper. The anode is impure copper. The Cu2 ions produced by oxidation of the anode migrate to the cathode, where they are reduced to pure copper metal. A similar arrangement is used for electroplating objects. 

The anodic oxidation of solution components (e.g., organic impurities)... 

A group of scientists have studied current transients in biased M-O-M structures.271,300 The general behavior of such a system may be described by classic theoretical work.268,302 However, the specific behavior of current transients in anodic oxides made it necessary to develop a special model for nonsteady current flow applicable to this case. Aris and Lewis have put forward an assumption that current transients in anodic oxides are due to carrier trapping and release in the two systems of localized states (shallow and deep traps) associated with oxygen vacancies and/or incorporated impurities.301 This approach was further supported by others,271,279 and it generally resembles the oxide band structure theoretically modeled by Parkhutik and Shershulskii62 (see. Fig. 37). 

Barrier anodic oxides covering the surface of aluminum etched foil are usually formed in borate or phosphate solutions. To improve capacitor characteristics, high-purity aluminum is desirable with as low a concentration of impurities as is acceptable in terms of cost. 

The equilibrium potential observed is depicted in the Figure 1 where we illustrate the potential-current behavior for the cathodic reaction (the oxygen reduction) coupled with a possible unspecified anodic oxidation of organic impurities. At electrochemical equilibrium i. = i and no net current flows in the circuit,... 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

The impurity gives a signal that disturbs the measuring system. An example is shown in Fig. 10.1 [10], The residual current-potential curves were obtained with a platinum electrode in propylene carbonate (PC) containing various concentrations of water. Because water is amphiprotic, its cathodic reduction and anodic oxidation are easier than those of PC, which is aprotic and protophobic. Thus, the potential window is much narrower in the presence of water than in its absence. Complete removal of water is essential for measuring electrode reactions at very negative or positive potentials. 

Silver(I) oxide, [CAS 20667-12-3]. AgjO. is made by action of oxygen under pressure on silver at 300°C, or by precipitation of a silver salt with carbonate-free alkali metal hydroxide it is covalent, each silver atom (in solid AgjO) having two collinear bonds and each oxygen atom four tetrahedral ones two such interpenetrating lattices constitute the structure. Silver(I) oxide is die normal oxide of silver. Silver(II) oxide, AgO, is formed when ozone reacts with silver, and thus was once considered to be a peroxide, Silvcr(III) oxide, Ag203, has been obtained in impure state by anodic oxidation of silver. 

FLINAK is purified by treatment with the HF released by ammonium bifluoride (NH4HF2) the HF converts oxide impurities in the melt to H20 [7]. In this purification procedure, the fluoride salt mixture is combined with 15 wt% NH4HF2 and heated to about 500°C in a graphite crucible. The molten mixture is poured into a platinum container and heated to 750°C. Hydrogen is passed through the molten mixture for approximately 2 days. Further purification can be achieved by con-trolled-potential electrolysis at an applied potential of about 3 V between a tungsten cathode and glassy carbon anode. 

FIGURE 18.19 Electrorefining of copper metal, (a) Alternating slabs of impure copper and pure copper serve as the electrodes in electrolytic cells for the refining of copper, (b) Copper is transferred through the CuS04 solution from the impure Cu anode to the pure Cu cathode. More easily oxidized impurities (Zn, Fe) remain in solution as cations, but noble metal impurities (Ag, Au, Pt) are not oxidized and collect as anode mud. 

At the impure Cu anode, copper is oxidized along with more easily oxidized metallic impurities such as zinc and iron. Less easily oxidized impurities such as silver, gold, and platinum fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions are reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution. 

Chen K.Y., Shen P.K., Tseung A.C. Anodic oxidation of impure H2 teflon-bonded Pt-Ru/W03/C electrodes. J. Electrochem.Soc. (1995) 142(10) L185-L187. 

The principle of the electrorefining process is basically simple plutonium is oxidized at a liquid metal anode containing impure metal feed and the resulting Pu ions are transported through molten salt to a cathode where pure metal is produced. The transport salt is usually eutectic NaCl-KCl but NaCl-CaCl2 can also be used. As liquid plutonium metal builds up on the cathode it drips off into an annular channel surrounding the anode cup where it coalesces into a pool of metal and is recovered after the cell is cooled. The entire chemical process is performed in a molten salt bath. 

Usually only one electrode product is recovered as a product of commercial value the product of the counterelectrode must be removed as waste or as a low-value byproduct. During anodic oxidations, hydrogen gas is evolved and has to be freed of solvent and low-boiling by-products. In the case of cathodic reductions, the gas given off is oxygen, sometimes containing halogens and acids as impurities. 

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