Hydrogen overpotential on mercury

High-pressure cells mainly are used to contain low-boiling solvents such as liquid NH3 or liquid S02 at ambient temperature or to study solvents with moderate boiling points at elevated temperature.53 The dropping-mercury electrode (DME) can be readily operated at several thousand atmospheres, and one interesting result is that the hydrogen overpotential on mercury declines about 1 V over a range of 200°C. 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

The potential of this electrode is less sensitive to mercury purity than that of SCE. When using the electrode, the disadvantageous properties of mercury(l) sulfate (hydrolysis and relatively high solubility) should be taken into account. It is also known that mercury can dissolve in aerated dilute sulfuric acid however, this process is hindered by high overpotential of hydrogen evolution 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... 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

The variation of the overpotential with the current density for the reaction of hydrogen evolution on a mercury cathode in diluted sulfuric add at 25 °C is ... 

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... 

The hydrogen-tritium separation factor on platinum in acid solutions is higher than on mercury, reaching values of = 10 at low overpotentials and Vt = 20 at high values of Tj. 

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