Reversible potentials amalgam electrode

The theoretical decomposition voltage of sodium chloride may be calculated as the sum of the reversible oxidation potential of the chlorine electrode and the reversible reduction potential of the amalgam electrode. 

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). 

The preconcentration of trace metals by electrodeposition is an integral part of anodic-stripping voltammetry. The method consists of the preelectrolysis of the stirred solution with a small mercury drop or solid electrode as the cathode (112-114). The metals, which are deposited and dissolve in the mercury, are then stripped from the amalgam after a suitable rest period by a reversal of the electrode potential. The resulting current-polarization curve is characteristic of the metal and its concentration. Concentrations as low as 10 M of metal ions require a preelectrolysis of about 60 min or longer. Other electrodes such as mercury films, platinum, gold, silver, and various forms of carbon have been used (77 ). 

Since metallic sodium is unstable in aqueous solutions, the measurement of the reversible potential for reaction (44) is difficult. This was, however, overcome by measuring the voltage of a cell with sodium metal and amalgam as the electrode in an aprotic electrolyte, to ensure stability of sodium, as ... 

Equation (53) represents the reversible potential of the sodium amalgam electrode at any concentration, with respect to the same electrode at a specific concentration or activity. For example, the potential of the 0.1 w/o Na amalgam at 80°C is ... 

Amalgam electrodes are of value because they allow active metals, e.g. Na and K, to be used as electrodes. Reproducible, reversible potentials can be obtained at very low concentrations of the metal it is best to use dropping electrodes, in which the amalgam drips slowly into the electrolyte, so that the surface is always fresh. The potential of the amalgam is compared with that of the pure metal by using both in a cell in which the electrolyte is dissolved in a solvent which does not attack the pure metal (e.g. ethylamine). Thus, to determine E (K, K), the sum of the standard e.m.f. values of the two cells ... 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

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... 

The basic kinetics of the amalgam, the diaphragm, and the membrane cells are discussed according to Fig. 11, containing reversible electrode potentials and current-potential relationships. 

Amalgam Cell Starting with the chlorine electrode, the reversible standard potential is +1.37 V (all potentials are given versus NHE), due to the sodium concentration of 6 mol L-1 we have to note +1.33 V. The current-overpotential curve represents an unhindered process... 

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