Potential Difference A Measurements

For the Volta-potential measurements, the coated electrodes are withdrawn from the electrolyte under potential control. Experimentally this is done in the device shown in Fig. 3.14. The electrode is mounted vertically in a holder that enables it to be moved down (immersion into the electrolyte for electrochemical equilibration) and up (removal from the electrolyte and positioning in front of the Kelvin vibrator). The Atl measurements are conducted in an atmosphere of 

FIGURE 3.14. Experimental device for measuring the Volta-potential difference between an electrode surface emersed from an electrolyte (cell) and a reference surface (vibrator). The inert atmosphere in the stainless steel chamber is water-saturated. The position of the Kelvin vibrator in front of the withdrawn electrode surface is adjusted with a micrometer drive. 

The reference conductor (Kelvin vibrator) for the Atfi measurements should have a constant value to give reproducible Aij/ results (see Fig. 3.13). This condition is difficult to fulfill, because in the water-saturated atmosphere, uncontrolled adsorption processes may take place on its surface. It is therefore necessary to caUbrate the vibrator regularly. This is done by determining A l between the Kelvin vibrator and a stable reference. It turns out that the surface of a polarized electrolyte can act 

FIGURE 3.15. Volta-potential ( ) differences discussed in the text Me is the electrode metal or another conductor in electronic equilibrium with it 5 is the polarized electrolyte poly represents a membrane-type polymer film deposited on the electrode. The subscript ex indicates that the electrode was withdrawn from the electrolyte under potential control. 

In Fig. 3.16 the electric-potential distribution corresponding to the measurement of ( — (lower diagram. Fig. 3.15) is represented. The electrode and the vibrator are assumed to be of the same metal. The sequence of the phases metal, polymer, electrolyte, atmosphere (N2), and reference metal are drawn in vertical slices. The ordinate in this diagram corresponds to the electrical potential. From this diagram we immediately see the relationship 

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