Cell, amalgam with liquid junction

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

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. 

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. 

In electrochemical cells without liquid junctions, the two electrodes are in contact with the same electrolyte of uniform concentration. For example, the cell shown in Figure 2.23 is made of a lead electrode and a lead amalgam electrode (lead dissolved in mercury), in contact with an aqueous solution of PbCl2. This cell corresponds to the schematic representation (2.123), where M and M" refer to the metal of the two conductors attached to the voltmeter. 

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