Metal amalgam formation

An alternative which is attractive for large scale work is the electrochemical reduction of aldonolac-tones.42 43 Particular attention has been paid to the electroreductive synthesis of ribose from ribonolac-tone because of the importance of the former in the synthesis of riboflavin (vitamin 62). Processes generally involve a mercury cathode and maintenance of an acidic pH, often with the assistance of a phosphate or borate buffer. It has been reported that alkali metal ions are also necessary, suggesting that the reduction occurs via metal amalgam formation. However, other accounts make no mention of metal... 

If a mercury cathode is expected to be necessary, the aprotic solvent-alkali-metal salt system appears to be inconvenient since many compounds are cathodically cleaved, reduced, or/and deprotected at potentials beyond that of alkali-metal amalgam formation. nevertheless, in certain cases the use of lithium salts as an electrolyte possessing strong electrophilic properties appears necessary in order to avoid the possibility of a Hofmann degradation of the tetraalkylammonium ion by electrogenerated bases. Under such experimental conditions, the cathodic synthesis of some aza and aza-oxa ligands [31] has been successfully achieved from the corresponding and readily obtained poly-... 

Terbium metal is obtained from its anhydrous trifluoride, TbFs, or trichloride, TbCls, by thermal reduction with calcium, carried out in a tantalum crucible. Terbium produced by such methods may contain traces of calcium and tantalum. High purity metal can be prepared by various methods such as vacuum remelting, distillation, amalgam formation, floating zone melting, and various chemical processes. 

Amalgam formation shifts the equilibrium potential of a metal (polarographic... 

We can get the formal potential for the amalgam formation by Eq. (5.18). Data on the diffusion coefficients of metals in mercury (DM(Hg)) are available in the IUPAC report [Galus, Z. Pure Appl. Chem. 1984, 56, 636],... 

Many potentiometric studies have been carried out to obtain information about the compositions and formation constants of metal complexes in non-aqueous solutions. Ion-selective electrodes and metal (or metal amalgam) electrodes are used as indicator electrodes. 

Because alkali metals are stabilized by amalgam formation, the reductions of alkali metal ions occur at 0.5 to 1.0 V more positive potential than those of tLtbT ions. 

Anodic stripping voltammetry (ASV) is the most common version of stripping analysis. It involves the reduction of a metal ion to the metal (which usually dissolves in mercury, i.e., amalgam formation), as the preconcentration step ... 

Many lithium salts, such as lithium perchlorate and the halides, are soluble in nonaqueous solvents. The reduction potential of Li depends on the electrode and the solvent. At a mercury cathode amalgam formation takes place, whereas formation of lithium metal occurs at platinum in aprotic media. Lithium metal is less reactive than sodium, and in some solvents sodium attacks the solvent whereas lithium is unreactive. A small water content in an aprotic solvent may react with lithium (or Li may react with hydroxyl ions formed at the cathode) to form lithium hydroxide, which may cover the electrode with an insoluble, insulating layer. 

Carbon dioxide may be reduced by alkali metal amalgams in alcohol to alkali metal formate [115] in a yield of 85-90% the rate increases with the CO2 pressure and is dependent on the alkali metal, Na(Hg) < Li(Hg) < K(Hg). 

Binding of cations by cryptands and/or crowns hinders the cathodic reduction of cations. Thus, in propylene carbonate the presence of excess cryptand-222 markedly shifts the half-wave potential for the formation of alkali metal amalgams A maximum shift of about 1 V was observed for sodium ions. On adding 18-crown-6 to glyme and ethylenediamine, deposition of sodium is replaced by electrochemical generation of solvated electrons. 

I. Clean sodium metal (11.5 g.) is cut into 5-mm. cubes. The cubes are speared with a pointed glass rod and rapidly introduced below the surface of warm (30-40°C) pure mercuiy (1150 g. = 85 ml.) contained in a 500-ml. wide-neck Erlenmeyer flask. The flask is covered with cardboard to prevent spattering during the rather vigorous amalgam formation reaction. 

Note, however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential First, the product of reduction may be stabilized by amalgam formation in metal ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and hnally, it can be shown that the potential Ey is the sum of two terms ... 

In each case platinum electrodes are used mercury electrodes reduce the cathodic limit owing to amalgam formation and the anodic limit is greatly restricted by dissolution of the metal. 

Mercury (Hg) is a well-known poison for nanoclusters because of either amalgamation formation with the metal nanoparticles or physicoabsorbtion on the nanocluster surface. To test the poisoning with Hg during silaesterification reactions (Scheme 3.8), typically, in a Schlenk tube, Pd(OAc)2 (0.004 g, 0.02 mmol) and acetic acid (0.06 mL, 1.0 mmol) are dissolved in 4 mL of benzene and the solution is examined by UV-visible spectroscopy. A peak at 400 nm suggestive of Pd(OAc)2 is noted. PMHS (0.06 mL, 1.0 mmol, 33-35 Si-H units) was then added to the above solution. 

In cases in which the target cannot be usefully preconcentrated by electrolysis at electrode surfaces, such as transition metals with numerous high stable oxidation states or metals which show complicated amalgam formation, AdSV (Chapter 2.3) can be a valuable alternative. Exploiting the enhanced mass transport afforded by insonation in this way offers improved levels of sensitivity. 

Since the early 1980s, a number of methods have been developed to analyze various compoimds by stripping voltammetry following the adsorptive accumulation of the species on the electrode. The adsorptive accumulation procedure has enabled analysts to measure extremely low concentrations of species that cannot be accumulated by amalgam formation. Species measured following adsorptive accumulation include metal complexes, organic compounds, and many biological macromolecules ... 

In a mercury cell (Fig. 9.6), chlorine directly evolves at the anode (a DSA). But at the cathode, the formation of hydroxide ions is prevented by using liquid mercury. Thus, the hydrogen evolution reaction (HER) is replaced by the reduction of sodium ions and the formation of sodium-mercury metallic amalgam ... 

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