Mercury cathodic limits

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

It is necessary to consider the factors which affect the limiting current with a dropping mercury cathode. 

Organic electroreductions at mercury cathodes in tetraalkylammonium (TAA+) electrolyte solutions at the limit of the cathodic potential window are described. Aromatic hydrocarbons, fluorides, ethers and heterocycles, as well as aliphatic ketones, alkenes and alkynes have been studied, using both aqueous and non-aqueous solvents. At these very negative potentials neither the TAA+ cation nor the mercury cathode are inert, instead they combine to form TAA-mercury. It is hypothesized, and supporting evidence is presented, that TAA-mercury serve as mediators in the organic electroreductions. The mediated reactions show remarkable selectivity in certain cases and it is shown that this selectivity can be improved by the choice of the TAA +. 

Electrochemical studies are usually performed with compounds which are reactive at potentials within the potential window of the chosen medium i.e. a system is selected so that the compound can be reduced at potentials where the electrolyte, solvent and electrode are inert. The reactions described here are distinctive in that they occur at very negative potentials at the limit of the cathodic potential window . We have focused here on preparative reductions at mercury cathodes in media containing tetraalkylammonium (TAA+) electrolytes. Using these conditions the cathodic reduction of functional groups which are electroinactive within the accessible potential window has been achieved and several simple, but selective organic syntheses were performed. Quite a number of functional groups are reduced at this limit of the cathodic potential window . They include a variety of benzenoid aromatic compounds, heteroaromatics, alkynes, 1,3-dienes, certain alkyl halides, and aliphatic ketones. It seems likely that the list will be increased to include examples of other aliphatic functional groups. 

The stoichiometric composition of Pyr-mercury was determined 8,9) by using thin mercury films, plated onto a platinum disk, as the cathodes. Under conditions where the amount of mercury constituting the cathode limited the amount of Pyr-mercury that could be formed, it was found that the composition is 1 TAA+/5 Hg. Similarly, it was found that the product of (CH3)4N+ consists of (CH3)4N+/5 Hg. In a recent work Bard and co-workers 10) used exhaustive electrolysis of (C4H9)4N+ at mercury in acetonitrile and suggested the stoichiometry (C4H9 )4N+/4 Hg. 

From the limited data available, it seems that terminal alkynes can be efficiently reduced to the corresponding alkenes at mercury cathodes in (C4H9)4N+ electrolyte solutions. The cathodic reduction can be carried out in an organic-aqueous medium in which base related complications, associated with other electron-transfer reductions, can be avoided. Efficient reduction of alkenes has not proven possible. In competition, both benzenoid and alkyne functionalities are reduced. Selectivity can be improved by controlling the water content of the medium so that a terminal alkyne can be converted to an alkene in the presence of a benzenoid aromatic functionality. 

The data in Tables 1 and 2 are given for platinum as both anode and cathode material. For mercury, which is the most commonly used cathode metal, the cathodic limits are normally displaced to somewhat more cathodic potentials than on platinum. Mercury is seldom useful as anode material, since it is oxidized at potentials above +0.4 V (SCE) and goes into solution. 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

In both polarographic and preparative electrochemistry in aptotic solvents the custom is to use tetraalkylammonium salts as supporting electrolytes. In such solvent-supporting electrolyte systems electrochemical reductions at a mercury cathode can be performed at —2.5 to —2.9 V versus SCE. The reduction potential ultimately is limited by the reduction of the quaternary ammonium cation to form an amalgam, (/ 4N )Hg , n = 12-13. The tetra-n-butyl salts are more difficult to reduce than are the tetraethylammonium salts and are preferred when the maximum cathodic range is needed. On the anodic side the oxidation of mercury occurs at about +0.4 V versus SCE in a supporting electrolyte that does not complex or form a precipitate with the Hg(I) or Hg(II) ions that are formed. 

With regard to electrode material, it can be seen (Tables 4 and 5) that cathodic limits on mercury are displaced by a few tenths of a volt to more negative potentials than on platinum. On the anodic side, the number of practically useful electrode materials is limited to noble metals and different types of carbon one case (anodic limit of pyridine nos. 35 and 36) shows that the anodic limit is lower on graphite than on platinum, and this seems to be a general trend for the comparison of carbon based anode materials, except possibly for vitreous carbon (Table 5) and bright (smooth, polished or shiny) platinum. 

We have just mentioned that one reason for a limited range of potentials in a particular SSE is the reactivity of the components of the SSE toward oxidation and reduction. It is also obvious that the limiting cathodic process in protic solvents, nos 1-9 in Table 4, must be reduction of protons or the equivalent, the proton donor. The unfavourable cathodic limit for reduction of protons can, however, be vastly improved by the use of mercury as the cathode material and a tetraalkylammonium salt as SSE (nos. 1 and 3). The reason for mercury being such a favourable material is its large overpotential (see Section 10) for the reduction of protons (hydrogen evolution reaction). We have already commented (p. 24) on the fact that the reduction of protons occurs many orders of magnitude faster on certain metals than on others, and this manifests itself by the overpotential, i.e., in order to make the reaction go at a measurable rate one has to increase the electrode potential from the equilibrium potential. Table 6 shows overpotentials for hydrogen evolution and... 

Table 3 Cathodic Limits of Ethylene Diamine (EDA) and Pyridine Using Dropping Mercury Electrode... 

V versus SCE [64], The cathodic limiting reaction is hydrogen evolution, thus forming the acid anion as the coproduct. The apparent electrochemical window of acetic acid is about 4 V [63], whereas that of formic acid is around 1 V [49], For methanol and ethanol, there are reports on limiting cathodic potentials around -2 V versus mercury pool electrode [65], and their accessible electrochemical window is around 2 V. The cathodic limiting reactions are probably hydrogen evolution and an alkoxide formation. 

Castner, Hamilton Young — (Sep. 11, 1858, Brooklyn, New York, USA - Oct. 11,1899, Saranac Lake, New York, USA) Castner studied at the Brooklyn Polytechnic Institute and at the School of Mines of Columbia University. He started as an analytical chemist, however, later he devoted himself to the design and the improvement of industrial chemical processes. He worked on the production of charcoal, and it led him to investigate the Devilles aluminum process. He discovered an efficient way to produce sodium in 1886 which made also the production of aluminum much cheaper. He could make aluminum on a substantial industrial scale at the Oldbury plant of The Aluminium Company Limited founded in England. However, - Hall and - Heroult invented their electrochemical process which could manufacture aluminum at an even lower price, and the chemical process became obsolete. Castner also started to use electricity, which became available and cheap after the invention of the dynamo by - Siemens in 1866, and elaborated the - chlor-alkali electrolysis process by using a mercury cathode. Since Karl Kellner (1851-1905) also patented an almost identical procedure, the process became known as Castner-Kellner process. Cast-... 

Polarography (discovered by Jaroslav Heyrovsky in 1922) is a technique in which the potential between a dropping mercury electrode and a reference electrode is slowly increased at a rate of about 50 200 mV min while the resultant current (carried through an auxihary electrode) is monitored the reduction of metal ions at the mercury cathode gives a diffusion current proportional to the concentration of the metal ions. The method is especially valuable for the determination of transition metals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, W, V, and Pt, and less than 1 cm of analyte solution may be used. The detection hmit is usually about 5 X 10 M, but with certain modifications in the basic technique, such as pulse polarography, differential pulse polarography, and square-wave voltammetry, lower limits down to 10 M can be achieved. 

The conditions at a dropping mercury cathode are clearly different from those at a stationary electrode, and the limiting current, or diffusion... 

The electroreduction of the complexes [Fe Cp( / -arene)][PF6] was first studied in basic aqueous medium and ethanol. It was found that the complexes [Fe ( 7 -C5H4R)( 7 -C6Me6)] (R = H or C02 ) are both initiators for ETC catalyzed selfdecomposition and redox catalysts for the reduction of water to dihydrogen on mercury cathode [352]. The reduction of water is a side reaction of the ETC catalytic process and limits its coulombic efficiency (Scheme 41). On such a cathode, the surtension of the reduction of water is very high, which makes this electrode especially suitable for the study of metal ions in such a medium. During this study, it was found that the most stable Fe complex [Fe ( / -C5H5)( -C6Me6)], also catalyzes the cathodic reduction of nitrates to ammonia in the same basic aqueous medium (pH 13) [353, 354]. 

Cold vapor mercury detection limits were determined with a FIAS(ji )-100 or FIAS-400 flow-injection system with amalgamation accessory. The detection limit without an amalgamation accessory is 0.2/ig/L with a hollow cathode lamp, 0.05 /ig/L with a System 2 electrodeless discharge lamp. (The Fig detection limit with the dedicated FIMS(ji )-100 or FIMS-400 mercury analyzers is <0.010/ig/L without an amalgamation accessory and <0.001 /ig/L with an amalgamation accessory.) Flydride detection limits shown were determined using an MFlS-10 Mercury/Flydride system. 

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. 

We have so far dealt with experimental data obtained on a mercury cathode. In the case of other electrodes poorly adsorbing hydrogen, where the discharge step is undoubtedly the rate-limiting stage, data are less comprehensive and, as a rule, less accurate. By far the most comprehensive and reliable data have been obtained for liquid gallium, some liquid alloys, and... 

The direct metallothermic reduction of pollucite ore with sodium metal is the primary commercial source of cesium metal. In the process, raw pollucite ore is reduced with sodium molten metal at ca. 650"C to form a sodium-cesium alloy containing some rubidium as impurity. Fractional distillation of this alloy in a distillation column at ca. 700"C produces 99.9 wt.% pure cesium metal. Cesium can also be obtained pyrometallurgiccdly reducing the chloride CsCl with calcium metal or the hydroxide CsOH with magnesium metal. Nevertheless, the electrolytic recovery of a cesium amalgam from an aqueous solution of cesium chloride can be achieved in a process similar to the chlor-alkali production with a mercury cathode. Afterwards, the cesium is removed from the amalgam by vacuum distillation. However, cesium metal is produced in rather limited amounts because of its relatively high cost (US 40,800 /kg)... 

Heyrovsky was aware that it is of importance not only to make a discovery, but that it is necessary to make ones colleagues aware of the results. He described the results of his first experiments on electrolysis with a dropping mercury cathode in Czech in the October issue of the journal Chemicke listy ( 7). He, nevertheless, realized that publication in the Czech language limits the news to a relatively small circle and so he prepared an English version dealing... 

---