Amalgam concentration

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

If the zinc amalgam concentration is high, both Cu2 -and Pb2+-ions are simultaneously cemented from the very beginning and the mixed potential lies near the equilibrium potential of the zinc amalgam. 

The choice of the amalgam concentration is free, but one always obtains the same standard potential for the condition aMn+ = aMa or approximately for [M"+ ]solution = [m]amalgara this is the standard reduction potential that is usually mentioned in tables e.g.,... 

Now, if we assume throughout the entire concentration step a current yield of 100% and no perceptible alteration of the bulk concentration, in view of its function as an indicator in the stripping step the electrode area cannot be taken as large, Faraday s law can be applied to calculate the final amalgam concentration according to... 

A number of detailed thermodynamic comparisons of half-cells containing alkali metal and alkali metal amalgams are available. For example, Cogley and Butler examined cell potentials as a function of amalgam concentration for the cell shown below [22]. 

Dropping indium and thallium amalgam electrodes [41] were used to determine kinetic parameters of Zn(II) reduction as a function of the amalgam composition. The formal potentials were shifted to more negative values with increasing thallium and indium amalgam concentrations. 

Rare earth chlorides, anhydrous, 28 Rare earth metals, 18 amalgams, concentration of, 17 Rhenium, metallic, 175 by reduction of ammonium per-rhenate, 177... 

The difference in concentration which causes the potential difference within the cell is a result of either the difference in concentration of the electromotively active substance in the electrodes (with gas and amalgam concentration cells), or of the different concentration of solutions surrouding the electrodes (with electrolyte concentration cells). As will be seen later electrolyte concentration cells must be adjusted in special way in order to exclude liquid junction or diffusion potential. 

Concentration Cells with a Single Electrolyte Amalgam Concentration Cells.—In the concentration cells already described the e.m.p. is a result of the difference of activity or chemical potential, i.e., partial molal free energy, of the electrolyte in the two solutions it is possible, however, to obtain concentration cells with only one solution, but the activities of the element with respect to which the ions in the solution are reversible are different in the two electrodes. A simple method of realizing such a cell is to employ two amalgams of a base metal at different concentrations as electrodes and a solution of a salt of the metal as electrolyte thus... 

In the general case of an amalgam concentration cell in which the valence of the metal is z and there are m atoms in the molecule, the equation for the e.m.f. becomes... 

The reversible potential of the sodium amalgam electrode, considering an amalgam concentration of 0.2 wt% sodium and the already mentioned sodium concentration, amounts to —1.78 V (the difference to the Na/Na+-standard potential —2.71 V is due to the fact that in the case of amalgam the discharged sodium ions must not be incorporated into a metallic structure). 

E is positive when the zinc activity in the left electrode is higher than that in the right electrode. Study of these systems shows that the amalgam phase behaves as a non-ideal solution. Thus, activity coefficients for amalgam solutions can be determined using cells such as (9.5.17). In these experiments, the amalgam concentration is changed from very dilute, for which the activity coefficient can be assumed to be unity, to more concentrated. 

D7.7 Electrode combinations that produce identical cell compartments with differing concentrations only (electrolyte concentration cells) have a cell potential dependence upon the liquid junction potential and the concentration difference. If the cell has identical compartments with either gaseous or amalgam electrodes (electrode concentration cell), the cell potential will depend upon the gas pressure differences or the amalgam concentration differences but will not have a liquid junction potential. Other electrode combinations produce cells for which the cell potential depends upon the half-reaction reduction potentials. 

FIGURE 4.4.11. Equilibrium potentials of sodium amalgam vs NaCl concentration at various amalgam concentrations at 80°C [3]. (Courtesy of Marcel Dekker, Inc.)... 

FIGURE 4.9.6. NTU of the amalgam decomposition tower as a function of the amalgam concentration and of the operation factor under the condition of yj = 0 [1]. 

FIGURE 4.9.10. Minimum height vs amalgam concentration in the decomposition tower [I]. 

To attain the uniform amalgam concentration inside the working electrode, the forced convection is stopped at the end of the deposition period and a rest period is started. The stripping step following the rest period requires the uniform concentration distribution of the amalgam in the working electrode. The mean concentration of the accumulated metal, c (mol m ), in the mercury drop of radius ro (m) can be easily... 

Here, is the amalgam concentration after accumulation, rj the radius of the mercury drop. / the thickness of the mercury film on the electrode surface, and y o the current density during the stripping process. 

In his own paper, Frumkin also analyzed the effect of the sodium amalgam concentration on the rate of electrolysis of water, and arrived at the conclusirm that even for physically and chemically uniform surfaces, the processes of anodic metal dissolution and cathodic hydrogen evolution could occur simultaneously at the same potential. This same idea was later used by K. Wagner and W. Traud (1938) in their formulation of the theory of the mixed potential, the cornerstone of modem corrosion theory. 

The e.m.f. of a chemical cell varies with the concentration and, hence, the activity t (q.v.) of the electrolyte solution, the gas pressure for gaseous electrodes, and the amalgam concentration for an amalgam electrode (q.v.). For the cell under consideration, the free energy change, from the van t Hoff isothermt (q.v.), is given by... 

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