Hydrogen on mercury

The mercury cell operates efficiently because of the higher overpotential of hydrogen on mercury to achieve the preferential formation of sodium amalgam. Certain trace elements, such as vanadium, can lower the hydrogen overpotential, however, resulting in the release of hydrogen in potentially dangerous amounts. 

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... 

In addition to carbon and the noble metals, the principle material used for the working electrode is mercury, especially for capillary and film electrodes, Because of the high overvoltage of hydrogen on mercury, the latter electrodes ensure a relatively wide potential range for voltammetric/ polarographic determinations in various supporting electrolyte solutions. If capillary electrodes are used, the electrode surface can be renewed easily and reproducibly as with the DME, the SMDE and the HMDE. 

Fig. 39. Influence of the adsorption of diphenylamine on catalytic evolution of hydrogen on mercury.

The bicyclic amine 11-methyl-l l-azabicyclo[5.3.1]hendecanc (71) provided a model system in which the hydrogens on the equivalent a-tertiary-carbon atoms cannot be trans to the nitrogen-mercury bond in the mercur-ated complex and in which epimerization at these a carbons is impossible (77). This bicyclic system is large enough to accommodate a... 

For further example, the decomposition of hydrogen peroxide, 2H202, , - 02. j + 2H20, which hardly occurs in aqueous basic solutions, catalytically proceeds on mercury electrodes as a mixed electrode process of anodic and cathodic reactions shown in Eqn. 11-3 ... 

Water has no effect on mercury, nor does molecular hydrogen. However, atomic hydrogen readily combines with mercury vapors forming hydride, when exposed to radiation from a mercury arc. 

Fig. 7.73 The reaction order of the hydrogen-evolution reaction (on mercury) in respect to a hydrogen ion is equal to 1, as seen from the slope of the straight line.

Detection.—Sulphur in the free state is readily recognisable by its general appearance and characteristics, and especially by its combustion to sulphur dioxide. Both in mixtures and compounds the presence of the element can be demonstrated by heating with charcoal and an alkali carbonate,2 or even better, on a small scale, by heating with an equal bulk of sodium or potassium,3 or with powdered iron 4 in each case some of the sulphur is converted into sulphide, which may be detected by the action of an aqueous extract on mercury or silver, or on sodium nitroprusside the metals are blackened, whilst the nitro-prusside is very sensitive in giving a purple coloration (see p. 62). Alternatively, the solution of the alkali sulphide may be acidified and tests applied for hydrogen sulphide to the vapours evolved on warming. 

The importance of double layer structure on electrode kinetics was first shown by Frumkin for the hydrogen evolution reaction on mercury [44]. As a result of the structure of the electrochemical interface, the pre-electrode plane, i.e. the plane where the reactant undergoes electron transfer to become product, is such that the concentration of the reactant ion is different from that in the bulk solution and the corresponding potential difference with respect to the solution, (less than the applied electrode—solution potential difference ([Pg.34]

The first quantum mechanical treatment for the radiationless electron transfer was developed by Gurney [68] for hydrogen evolution on mercury. In his model, Gurney assumed that the intermediate H " is not... 

One of the most important reasons for the application of mercury to the construction of working electrodes is the very high overpotential for hydrogen evolution on such electrodes. Relative to a platinum electrode, the overpotential of hydrogen evolution under comparable conditions on mercury will be -0.8 to -1.0 V. It is therefore possible in neutral or (better) alkaline aqueous solutions... 

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