Aqueous solutions metal reduction from

Cementation is the process of recovery of metals from dilute aqueous solution by reductive precipitation using another metal with a more negative electrode potential, e.g., Cu + Fe° Cu° + Fe. The product, in this case cement copper, is relatively impure because of iron contamination. However, cementation can be used in conjunction with a solvent extraction flow sheet to remove small amounts of a metallic impurity, for example, removal of copper from a nickel solution by cementation with nickel powder. Here the dissolved nickel conveniently augments the nickel already in solution. 

Electroplating from non-aqueous solutions is useful when the surface of a substrate, which easily reacts with water, should be protected with a thin metal film. For instance, a superconductor, Ba2YCu307, has a strong oxidizing ability and its surface easily reacts with air. In order to protect the surface of the Ba2YCu307 with a metal film, a method to electroplate such metals as Ag, Cu, Pb and Sn from acetonitrile has been studied [20]. In aqueous solutions, the reduction of Cu3+ in Ba2YCu307 narrows the applicable potential range and only Ag can be deposited. 

The effect of neutral salts (e.g., NaCl) on the composition of borates precipitated from, or in equilibrium with, aqueous solutions doubtless arises from a reduction in water activity, metal borate complexation, and a shift in polyborate equilibria (Sections IV,A, B). The "indifferent or inert component method has frequently been used for the synthesis of borates. Potassium and sodium chlorides can be used to enhance the precipitation of specific nickel (48), aluminum (51), iron (49), and magnesium (151) borates. In the K20-B203-H20 system at 25°C (248), the presence of potassium chloride results in a reduced boric acid crystallization curve, lower borate solubilities, lower pH, and an extended B203 K20 range over which the pentaborate crystallizes. 

The second step in the procedure requires the working electrode to be anodicaUy polarized, yielding one of the dashed Hnes shown in Fig. 3.6. The electrode is then cathodicaUy polarized, and the other dashed Hne from Fig. 3.6 is obtained. The anodic polarization usuaUy results in the oxidation of the metal species, whUe the reaction resulting from cathodic polarization depends on the medium. In an aerated solution, the oxygen reduction reaction may be the prime cathodic reaction, while, in the case of deaerated aqueous solutions, hydrogen reduction could be the dominant reaction. In Fig. 3.6, the redox reaction is represented by a general reaction... 

The cyclic voltammograms (CV) of the bipyridine nitrosyl ruthenium complexes may reveal multiple couples resulting from redox processes centered at the metal, the nitrosyl ligand, and the bpy ligands. Two couples are evident in the CV electrochemical potential range of +1.0 to —1.0 V vs. Ag/AgCl of a 5/ira s-[Ru(bpy)2L(NO)] complexes in organic solvent or aqueous solution. The reduction peak at the more positive value is chemically reversible, whereas the second reduction peak is practically irreversible in the CV time scale such peaks correspond to the NO "° and pro-... 

Typical coprecipitation synthetic methods involve the following stages (i) nanomaterials formation takes place from aqueous solutions, or by reduction from non-aqueous solutions, electrochemical reduction and decomposition of metal-organic precursors with templates (ii) metal chalconides are formed by the reactions of molecular precursors (iii) microwave/sonication assists the coprecipitation to take place at the microscale with the following advantages ... 

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

The deposition of RE metals from aqueous solutions does not work because of the highly electropositive nature of the REE. Therefore, industrial production of RE metals is carried out by fused salt electrolysis or metaHothermic reduction. 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

Chromium metal can be electroplated from an aqueous solution of potassium dichromate. The reduction half-reaction is... 

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