Hydrogen evolution at mercury

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

It is known that the presence of an organic base lowers the overpotential for hydrogen evolution at mercury therefore it appears reasonable that the presence of an amine should alter the course of the reduction of the coumarin. The mechanism for the hydrogen evolution reaction required the formation of a quaternary radical ... 

Table 3. Characteristics of the Process of Hydrogen Evolution at Mercury for Low Current Densities...

The temperature dependence of the rate of a barrierless discharge of hydrogen ions allows us to find some important parameters of the process of hydrogen evolution at mercury[130]. 

Adsorption of surface-active substances is attended by changes in EDL structure and in the value of the / -potential. Hence, the effects described in Section 14.2 will arise in addition. When surface-active cations [NR] are added to an acidic solution, the / -potential of the mercury electrode will move in the positive direction and cathodic hydrogen evolution at the mercury, according to Eq. (14.16), will slow down (Fig. 14.6, curve 2). When I ions are added, the reaction rate, to the contrary, will increase (curve 3), owing to the negative shift of / -potential. These effects disappear at potentiafs where the ions above become desorbed (at vafues of pofarization of less than 0.6 V in the case of [NR]4 and at values of polarization of over 0.9 V in the case of I ). 

FIGURE 14.6 Influence of surface-active ions [N(C4H9)4]+ (curve 2) and I (curve 3) on the polarization curve for hydrogen evolution at a mercury electrode in acidic solutions (curve 1 is for the base electrolyte). 

FIGURE 15.3 pH dependence of potential (1) and polarization (2) in cathodic hydrogen evolution at a mercury electrode (lOmA/cm ), and the pH dependence of equilibrium potential of the hydrogen electrode (3). 

Fig. 5.39 Tafel plot of hydrogen evolution at a mercury cathode in 0.15 m HC1, 3.2 m KI electrolyte at 25°C. (According to L. I. Krishtalik)...

The Kolbe reaction is earned out in an undivided cell with closely spaced platinum electrodes. Early examples used a concentrated, up to 50 %, aqueous solution of an alkali metal salt of the carboxylic acid and the solution became strongly alkaline due to hydrogen evolution at the cathode. Ingenious cells were devised with a renewing mercury cathode, which allowed removal of alkali metal amalgam. These experimental conditions have been replaced by the use of a solution of the carboxylic acid in methanol partially neutralised by sodium methoxide or trieth-... 

In heterogeneous electro catalysis, the catalyst is immobilized on the electrode surface, or the electrode itself plays the role of a catalyst. Catalytic effects of various electrode materials on the hydrogen evolution reaction are typical examples of heterogeneous electro -catalysis [iii]. Further examples are electrode mechanisms involving hydrogen evolution at a mercury electrode catalyzed by adsorbed organic bases, microparti-... 

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 when they are employed. 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... 

For hydrogen evolution at various metals, the Tafel slope b is typically about 0.12 V, whereas Jo varies greatly from one metal to another, being about 10" A/cm for platinum and 10 A/cm for mercury in acid solutions. 

In order to suppress or inhibit zinc corrosion and hydrogen evolution at zinc anode, a number of additives are selected and used for the anode. Because of its high over potential to the hydrogen evolution reaction, mercury is an effective gassing suppresser and used to be widely employed in alkaline Zn/Mn02... 

Studies of the hydrogen-tritium separation factor as a function of potential on mercury have led to the interesting conclusion that proton tunneling contributes significantly to the over-all rate of hydrogen evolution at overpotentials up to about 1.1 volts (76). 

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. 

TABLE 4.4.4 Effect of Metallic Impurities on Hydrogen Evolution at the Cathode of a Mercury Cell Operated at 4 kA and at 80°C... 

The amalgam process needs a closed loop of anolyte in which only salt and nearly no water is converted (see Fig. 2), contrary to the diaphragm process, where all introduced brine leaves the anode compartment via the diaphragm (see Fig. 1). NaCl has to be delivered by solid salt in the brine saturation. First, the anolyte outlet must be dechlorinated by acidification and vacuum application. It is important for the brine purification to remove aU heavy metals, e.g., vanadium, that can decrease the hydrogen overpotential at mercury. Otherwise hydrogen evolution becomes possible, in worst case an explosion together with chlorine can happen. 

Figure 63.8.4 Formation and characterization of the Hg/Pt hemispherical submarine UME. (a) The Pt submarine electrode in phosphate buffer (pH = 7) as it approached the HMDE while poised at —1.1 V vs. Hg/Hg2S04. Upon contact with the HMDE, a hemispherical mercury layer is deposited onto the Pt UME. (b) Hydrogen evolution at Pt and Hg/Pt submarine UME in phosphate buffer, (c) Voltammogram of the 10 M T1(I) at the Hg/Pt submarine electrode in phosphate buffer. Reprinted with permission from reference (2). Copyright the American Chemical Society.

During an investigation of the temperature dependence of the rate of hydrogen evolution at a liquid mercury-gallium alloy (containing 2.1 at. % Hg), very high values of the preexponential factor were observed in acidic as well as alkaline solutions. At least in acidic solutions, the experimental value (log k = 7.6) exceeded any reasonable theoretical estimates of the upper limits of the preexponential factor by several orders of magnitude[262]. Hence, it was naturally assumed that the experimental value was considerably distorted due to the dependence of the composition of the alloy surface on temperature. This assumption was verified with the help of the kinetic isotope effect. 

---