Hydrogen Evolution on Mercury

Mercury electrodes have been studied more than any other type of electrode, because of their ease of purification and the high degree of reproducibility attainable with them. All aspects of hydrogen evolution on mercury have probably been studied at one time or another. On the basis of all experimental evidence, it is commonly accepted that in this case, the first charge-transfer step is rate-determining, and is followed by fast ion-atom recombination 

The coverage by adsorbed hydrogen atoms must be very low, since none has ever been detected, even at the highest overpotentials measured. This also rules out atom-atom recombination as the fast second step, since the rate of the reaction 

The Tafel slope for this mechanism is 2.3RT/p F, and this is one of the few cases offering good evidence that Uc = P, namely, that the experimentally measured transfer coefficient is equal to the symmetry factor. A plot of log x versus F, where X is the dimensionless rate constant, given by 

The hydrogen-evolution reaction on the HDME is the best-known case in which p is experimentally shown to be independent of potential and to have a value close to 0.50. This is probably the best experimental evidence favoring the use of this value of Pc in the analysis of more complex electrode reactions. 

The exchange-current density for this system depends on the composition of the solution, but generally it is in the range 10 —10 ° Acm . Mercury is often 

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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... 

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... 

Another instance of a similar phenomenon is found in the work of Bockris and Parsons where the exchange current for hydrogen evolution on mercury was found to reach a minimum in approximately equimolar H2O + MeOH solutions, both pure solvents exhibiting values an order of magnitude higher. Simultaneously the transfer coefficient increased from 0.5 in water to 0.6 in MeOH. For this system equilibrium data are available to show that MeOH is concentrated at the interface whilst water is the predominant ion solvator. 

In order to experimentally detect barrierless discharge, the reaction of hydrogen evolution on mercury was chosen in the first place, since the hydrogen adsorption energy on this metal is low hence, one could expect a sufficient endothermicity of the discharge act. 

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. 

Let US evaluate the kinetic parameters corresponding to this reaction sequence. For the first charge transfer as the rate-determining step, we already know the result, since it is equivalent to step (7.1) for hydrogen evolution on mercury. If the second step is assumed to be rate-determining, we also know the result, (cf. Eqs. (6.18) and... 

The effect of the condensed adsorption layer on hydrogen evolution at mercury electrode has been studied by Ponomarev et al. [45]. 

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... 

Note that both lithim amalgams as well as the central Hg(.t,) and Hg2Cl2(s) phases do not appear in the net cell reaction. Consequently we do not need to know the concentrations of " Li and Li in the amalgams. The key to the success of the above quadruple cell is the combination of a hi overvoltage for hydrogen evolution on a mercury surface (ca. IV), together with a very low concentration of lithium in the amalgams (X 10" ). The... 

Shortly after initiation of charge, hydrogen evolution begins on the iron electrode. The considerable hydrogen evolution on charge presumably helps counteract iron passivation in alkaline solution. Mercury additions also have a similar effect, but only in the early formation cycles. 

Table 1.3 contains the approximate exchange current density for the reduction of hydrogen ions on a range of materials. Note that the value for the exchange current density of hydrogen evolution on platinum is approximately 10 A/cm, whereas that on mercury is 10 A/cm. ... 

Table 1 indicates current densities to start formation hydrogen evolution on various metals [17]. Note that such value for mercury is much smaller than to platinum, explaining the use of mercury for the cathodic processes rather than other noble metals. 

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