Amalgams oxidation potentials

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

The theoretical decomposition voltage of sodium chloride may be calculated as the sum of the reversible oxidation potential of the chlorine electrode and the reversible reduction potential of the amalgam electrode. 

Low-valent rhenium complexes with Tp and bpy (bpy = bipyridine) ligands that do not include CO in the coordination sphere have been synthesized. Complexes of the type TpRe(bpy)Y (Y = Cl or cyclopentene bpy = 2,2 -bipyridyl) are derived from [TpRe bpy)Cl][OTf with the accompanying one or two electron reduction by Zn/Hg or Na/Hg amalgam, respectively (Scheme 60). " TpRe(bpy)(f/ -cyclopentene) is a potent 7i-base as evidenced by the II/I oxidation potential of —0.66 V versus NHE, 0.89 V more cathodic than TpRe(CO)(PMe3)( -cyclohexene). 

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

The electrode potential of aluminium would lead us to expect attack by water. The inertness to water is due to the formation of an unreactive layer of oxide on the metal surface. In the presence of mercury, aluminium readily forms an amalgam (destroying the original surface) which is. therefore, rapidly attacked by water. Since mercury can be readily displaced from its soluble salts by aluminium, contact with such salts must be avoided if rapid corrosion and weakening of aluminium structures is to be prevented. 

Thiols are easily oxidized to disulfides in solution, but this reaction occurs only very slowly at most electrode surfaces. However, use can be made of the unique reaction between thiols and mercury to detect these compounds at very favorable potentials. The thiol and mercury form a stable complex which is easily oxidized, in a formal sense it is mercury and not the thiol which is actually oxidized in these reactions. For the LCEC determination of thiols a Au/Hg amalgam electrode is used Using a series dual-electrode both thiols and disulfides can be determined in a single chromatographic experiment... 

After discontinuing the stirring, the potential is scanned positively, causing the amalgamated lead to be oxidized back into the solution (i.e., stripped out of the electrode). This oxidation of lead gives a current peak, ip, the magnitude of which is determined by the concentration of Pb in the mercury electrode which is in turn proportional to the amoimt of Pb " in the sample. 

The way that the stripping analysis is performed in practice is much more sophisticated. After the solution has been exhausted, all of the copper (as Cu (Hg) amalgam) resides on the surface of the mercury drop. The potential of the drop is changed from cathodic to anodic (we say that we have stepped the potential), and the copper is all oxidized back to Cu and the charge determined - as oxidation-The potential chosen should be more positive than E for the analyte couple by at least 0.2 V. 

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

In aqueous solution nobelium ions are most stable in the 2 oxidation state. In this oxidation state nobelium has a filled f-electron shell, 5f ", which is likely a major factor for its stability. The potential for the No(III)/No(II) couple has been calculated by Nugent et al. as 1.45 0.05 V [177]. A value of —1.4 to —1.5 V was determined by Silva and coauthors from experimental measurements [180]. David et al. have performed electrochemical amalgamation experiments for the reduction of No(II) to No(0) in aqueous acetate and citrate solutions [181]. They determined half-wave potentials of—1.709 0.006 V versus SCE in acetate and —1.780 0.004 V versus SCE in citrate. Their data was consistent with a reversible two-electron reduction process for which the data in acetate solution was taken as representative of a noncomplexing medium. The 1/2 value in acetate was converted to a value of —1.47 0.01 V versus SHE and subsequently used to derive a standard potential value of —2.49 0.06 V for the No(II)/No(0) couple. 

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