Overpotential of hydrogen

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 high overpotential of hydrogen is an advantage of DME while this property can be exploited only with mercury-covered RDE and UME. On the other hand, the dissolution of mercury at rather low positive potentials is a disadvantage of DME which is not shared by RDE and UME made of nobler metals than mercury. 

Fig. 23. Dependence of the anodic overpotential of hydrogen-consuming Raney-nickel anodes of equal catalyst loading on the particle size of the catalyst.

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... 

It is known that a number of metals which do not amalgamate with mercury are capable of lowering the overpotential of hydrogen production at mercury electrodes and indeed catalytic hydrogen waves attributable to metal deposition can be observed in polarograms of simple salts of Rh, jj. 394,395 pj 394,395 Qy which amalgamate with... 

Kvokova and Lainer electrodeposited pure Re and Re-Cr alloy on Mo substrate. For the deposition of Re itself, two baths were used one containing perhhenic acid, and the other containing potassium perrhenate. In the first bath, the discharge of hydrogen ions was enhanced. The authors attributed the low overpotential of hydrogen on Re to its lattice parameter (a = 2.758 A). However, a justification to this theory has not been proposed. For both deposition of Re and Re-Cr alloy, the concentration polarization was foimd to be insignificant compared to the activation polarization. 

It is therefore essential that the lead—acid battery does not contain substances that lower the overpotentials of hydrogen and oxygen evolution, so as to preserve its capacity on open circuit stay and during storage. 

That is E depends on the sulfuric acid concentration. The rate of the cathodic reaction (13.12) is very slow, because of the very high overpotential of hydrogen evolution on lead electrode of high purity grade. And it is the reaction of hydrogen evolution that exerts the strongest influence on the rate of the self-discharge process, i.e. it practically determines this rate. 

The current density (/) of steady state permeation, therefore, is directly proportional to 0. Further, the coverage is strictly dependent on the overpotential and the mechanism of HER. The overpotential of hydrogen r is generally measured by the Tafel equation ... 

The examples illustrate the diversity as well as the common features of a paired electrosynthesis. One can start with one or two substrates to generate one or two products. Electrode processes can be mediated or direct. Undivided and divided cells are employed in paired electrosyntheses. But as in the BASF phthahde example, it is crucial for the synthesis of glyoxylic acid, sorbitol, and methyl ethyl ketone that the cathodic process is the reduction of the substrate and not the reduction of protons because in these cases protons are generated at the anode and the electrolysis takes place in aprotic solvent. Therefore effects that minimize the overpotential of hydrogen have to be omitted. Reaction control is important in all described examples, and consequently the cell and the setup have to fit for each case. Work-up and product isolation are significant for a successful synthesis and can be even more challenging in a paired synthesis. 

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. 

One of the most commonly interfering processes with electroplating is the evolution of hydrogen. The overpotential of hydrogen evolution varies enormously from one cathode metal to the other from zero on platinum under standard conditions to two volts on mercury it also varies with the pH of the solution, in alkaline solutions this potential is more negative than in acid. 

Let us consider the data on the effect of small additions of water on the overpotential of hydrogen evolution from acidic acetonitrile solution. Typical results are presented in Figure 4.13. 

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