Reductions in aqueous solution

Selective Reduction. In aqueous solution, europium(III) [22541 -18-0] reduction to europium(II) [16910-54-6] is carried out by treatment with amalgams or zinc, or by continuous electrolytic reduction. Photochemical reduction has also been proposed. When reduced to the divalent state, europium exhibits chemical properties similar to the alkaline-earth elements and can be selectively precipitated as a sulfate, for example. This process is highly selective and allows production of high purity europium fromlow europium content solutions (see Calcium compounds Strontiumand strontium compounds). 

Figure 14.12 Variation of the reduction potentials of the couples O2/H2O and H /Hi (or O2/OH" and H2/H2O) as a function of pH (full lines). The broken lines lie 0.5 V above and below these full lines and give the approximate practical limits of oxidants and reductants in aqueous solution beyond which the solvent itself is oxidized to 02(g) or reduced to Hi(g).

In this study, the chemical reduction in aqueous solution using conventional and ultTMonic hydrothermal reduction method were conducted for the preparation of fine nickel powders from the aqueous solution of nickel salt by reducing with hydrazine. The differences in the reaction parametera and final product properties resulting from two methods were identified to find the effects of ultrasound. 

Barbier MJ, Beedelievre AM, Beedelievre J (1978) Electrochemical study of tellurium oxido-reduction in aqueous solutions. J Electroanal Chem 94 47-57... 

As described above, the mechanism for C02 reduction in aqueous solutions proposed by Eyring and co-workers45 has been widely accepted. However, there remains a rather large difference between theoretical and observed values for the Tafel slope,45 and... 

Rhodium and ruthenium complexes have also been studied as effective catalysts. Rh(diphos)2Cl [diphos = l,2-bis(diphenyl-phosphino)ethane] catalyzed the electroreduction of C02 in acetonitrile solution.146 Formate was produced at current efficiencies of ca. 20-40% in dry acetonitrile at ca. -1.5 V (versus Ag wire). It was suggested that acetonitrile itself was the source of the hydrogen atom and that formation of the hydride HRh(diphos)2 as an active intermediate was involved. Rh(bpy)3Cl3, which had been used as a catalyst for the two-electron reduction of NAD+ (nicotinamide adenine dinucleotide) to NADH by Wienkamp and Steckhan,147 has also acted as a catalyst for C02 reduction in aqueous solutions (0.1 M TEAP) at -1.1 V versus SCE using Hg, Pb, In, graphite, and n-Ti02 electrodes.148 Formate was the main... 

The 3A2u(da pa) state of Pt2 (P2 5)i+H8 is an extremely powerful one-electron reductant in aqueous solution. Preliminary experiments have shown that species such as Os(NH3)5C12 (ei/2 ... 

Nitrobenzene radical-anion is more stable in aprotic solvents than its aliphatic counterparts. Nitrobenzene shows two one-electron polarographic waves in acetonitrile with By, -1,15 and -1.93 V vj. see, Tire first wave is due to tlie formation of the radical-anion and this species has been characterised by esr spectroscopy [6]. Nitrobenzene radical-anion can also be generated in steady-state concentration by electrochemical reduction in aqueous solutions at pH 13 [7] and in dimethyl-formamide [8]. It is yellow-brown in. solution with A., ax 435 nm. Protonation initiates a series of reactions in which niti osobenzene is formed as an intermediate and... 

The demonstration that PMNs formed O2- in the respiratory burst necessitated the consideration of all the species which result when dioxygen is reduced one electron at a time (Fig. 1). Superoxide, the result of the reduction of dioxygen by one electron, appears to act mainly as a mild reductant in aqueous solutions. But when it coexists with H2O2, its spontaneous dismutation product, O can initiate a number of potentially injurious events [reviewed by Fridovich The primary means by which cells deal with superoxide anions appears to be through the catalysis of their dismutation by a family of metalloenzymes collectively designated superoxide dismutases. 

Ar3PCH2PhBr, the observed order in the ease of reduction, in aqueous solution at pH 7.00, correlates with the electron-withdrawing ability of the aryl groups (Ar = 2-furyl > 2-thienyl > phenyl > 1-methylpyrrolyl) the first reduction wave would correspond... 

About a third of the hydrogen manufactured is used in mining for the hydrometallurgical extraction of copper and other metals, the extraction of the metals from their ores by reduction in aqueous solution ... 

A study of the initially formed 94 used acetonitrile solvent.145 Rapid chemical follow-up steps were proposed. Reduction in aqueous solution was shown to proceed along similar lines.146 The energies of Hiickel molecular orbitals for alkylpyridinium ions indicated that no correlation exists between reduction potential and the quaternary nitrogen electron density.147 EPR spectra of the radicals (94) were analyzed.148... 

Silver perchlorate is reduced by hydrogen in a reaction which is second order with respect to silver ion concentration (Webster and Halpern, 51). The activation energy is 15.2 kcal./mole. This is the first demonstrated example of a reduction in aqueous solution which is not first order in metal salt concentration. 

Electrocatalytic possibilities with C02 reduction in aqueous solution are surprising. On copper at 0 °C, CH4 is the main product from electrolysis, and on a molybdenum cathode at room temperature, it is methanol (Hori, 1980). Using lead in a porous gas diffusive electrode, it has been possible to obtain HCOOH at 100 mA cm-2 (Hallmann, 1991). Macrocyclic compounds catalyze the reduction of C02 to CO to... 

The classic work on the mechanism of electrolytic C02 reduction in aqueous solution was carried out by Park, Anderson, and Eyring but was not published until 1969. They determined the Tafel slopes and reaction orders with respect to C02 and found for the latter, zero and for the former two regions, 0.09 in a lower current density and 0.20 in a higher one. They found these results (on Hg) to be consistent with the pathway ... 

The study of nanosized particles has its origin in colloid chemistry, which dates back to 1857 when Michael Faraday (1791-1867) set out to systematically investigate the optical properties of thin hhns of gold. Faraday prepared a suspension of ultra-small metaUic gold particles in water by chemically reducing an aqueous solution of gold chloride with phosphorus (Faraday, 1857). To this day, nanoscale metal particles are stiU produced by chemical reduction in aqueous solutions. 

Ferric chloride is readily reduced by suitable reagents to the ferrous salt. Metallic zinc or iron, or even nascent hydrogen, effects the reduction in aqueous solution. Alkali sulphides reduce it with deposition of sulphur, and alkali iodides 4 with liberation of iodine, thus —... 

Question 7.9 Explain why it is unlikely that Pm + compounds could be prepared by reduction in aqueous solution. (Hint consult the reduction potentials in Table 2.7). Assuming that this were possible, why would PmS04 be a good choice of compound to isolate Suggest a solvent that might be a good choice. 

This historical distinction is paralleled by the widely different chemistries exhibited by the two types of complex. The classical complexes are invariably octahedral and react so slowly that studies of their reactions are of kinetic rather than synthetic interest. They are impervious to both oxidation and reduction in aqueous solution. 

Independently whether partial oxidation of Met residues occurs during the synthesis or whether Met(O) derivatives are purposely used, the sulfoxide has to be quantitatively reduced in the final steps to regenerate the methionine peptides. For this purpose, various reagents were proposed for the reduction in aqueous solution upon final deprotection and in organic solvents generally prior to the deprotection step in the presence or absence of cystine disulfides. 

The easy hydrolysis of the imine interferes with its polarographic or preparative reduction in aqueous solution. Polarographically inactive unconjugated ketones may,... 

Arylaziridinium salts show a reverse structural effect on the polarographic halfwave potential, as electron-donating substituents make the reduction in aqueous solution easier and electron-attracting groups have the opposite effect [201]. A tetramerization of 1-benzylaziridine to a tetra-azacyclododecane may be induced anodically [202]. 

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