Silver interface equilibrium

Fig. 10.1 Silver in contact with Ag4Rbl5 at equilibrium. The fluxes of Ag across the interface are equal and opposite so that no net current flows.

The interface structure for non-blocking interfaces is similar to that for related blocking interfaces. Thus the distribution of charge at the C/ Ag4Rbl5 interface will be similar to that at the Ag/Ag4Rbl5 interface. The major difference is that there is one particular interfacial potential difference at which the silver electrode is in equilibrium with Ag ions in the bulk electrolyte phase. At this value of A, there is a particular charge on the electrolyte balanced by an equal and opposite charge — on the metal. At any potential different from value of q different... 

Fig. II.9.9a Schematic presentation of anodic and cathodic partial current densities dashed lines), net current densities thin full line) and the position of the equilibrium electrode potential Eq for a silver electrode and a glass electrode (one glass-solution interface). Subscript gl indicates the surface of a glass membrane and sol indicates the electrolyte solution...

In certain experimental configurations, reference electrodes of this type, i.e., with an internal compartment, may be difficult to implement. In such cases a pseudo-reference might need to be used. For instance, it may be a metal wire (silver, platinum, etc.) or indeed the Ag,AgCI in direct contact with the electrolytic medium. In these examples the interface between the pseudo-reference and the electrolyte studied is generally not in thermodynamic equilibrium , in contrast to the case of the interface in usual reference systems which have a suitable internal solution. However, thanks to the use of a potentiostat , no current flows in the electrode, and therefore it is correct to assume that its open-circuit potential remains constant in the course of the experiment. This hypothesis has to be checked in each experimental situation. Moreover, the value of this open-circuit potential is most of the time not known in precise terms. Using a pseudoreference therefore requires that the potential shift of this electrode be determined, e.g., by implementing a reference compound at the end of the experiment... 

At open circuit the interface with the silver electrode is In thermodynamic equilibrium. The interface with the zinc electrode has a mixed potential, which is defined by adding the zinc oxidation and proton reduction currents, with the latter reaction characterised as being very slow at the zinc electrode. The polarities of the electrodes in open-circuit conditions are defined by the experimentally measured potentials of each electrode vs a saturated calomel reference electrode. 

We consider competitive chemisorption of anions and cations at a fixed array of adsorption sites on an electrode surface in terms of a voltage-dependent ideal thermodynamic equilibrium. For simplicity, we consider systems with sufficiently high electrolyte conductivity that all the interface voltage drop is across a single layer of chemisorbed ions at the polarized electrode surface. This seems a reasonable initial assumption for most conductive electrode materials and for the fused and solid silver halides cited above. 

The transport number can also be obtained by measuring the open circuit potentials of a system under a gradient in chemical potential, e.g. a metal being oxidized to an oxidation product. This method was first described theoretically by C. Wagner and he applied the technique to silver sulfide being formed by reaction of silver and sulfur [6]. The potential is measured between the metal M and the outer surface of the compound, MX, coexisting with the gas phase (X2) Local equilibrium must be established at the interfaces between M-MX and MX-X25 i e. a diffusion controlled reaction must exist for the method to work. 

It should be noted that the equilibrium conditions, Eqs. (2) and (6b), have the same form whether there is adsorbed surface charge or not. The magnitudes of (0) and Vc(0), however, will differ in these two cases because of the difference in the charge neutrality conditions, Eqs. (4) and (6a). An important consequence of the above consideration is that in the presence of adsorbed surface charge and constant x potential the space charge potentials Vs(0) and Fc(0) will not be zero at Cf/O), and in fact will not be zero at the same value of silver ion concentration in solution. The definition of isoelectric point therefore becomes ambiguous and it is necessary to define isoelectric point separately for each side of the interface corresponding to values of silver ion concentration in solution where F (0) = 0 or Fc(0) = 0. If only K (0) is measured, as in the electrophoresis experiment, the isoelectric point determined for F (0) = 0 does not ensure that Kc(0) is zero at the same time. 

The properties of the interface at which the formation of oxide ions occurs have been of special interest [6, 7, 28—35]. While solid electrocatalysts, Pt [28, 29, 31, 32] and C [30], were studied mainly, a molten silver cathode was employed in another type of zirconia-electrolyte fuel cell developed [34,35] at the General Electric Research and Development Center in Schenectady. Since the hindrance of the electrochemical steps of the O2 reduction at the cathode surface is small [28, 32] on platinum around 1000 °C, it is hard to elucidate the reaction mechanism beyond the net reaction 1. Analysis [33] of the potential distribution curves inside Zro 9Yo 2 02.i in contact with two platinum electrodes showed at 1380°C that the electronic hole contribution to the conductivity in the bulk of the specimen depended upon as would be expected from the equilibrium of reaction 15. The partial oxygen pressure had values between 10 and 10 atm. However, if the production of oxide ions is assumed to occur at the cathode solely by reaction 15, the rate of production is much lower than the rate of loss at the anode. A cathodic reaction of the type... 

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