Electrode potentials ionic discharge

In the example just studied, the electrolysis of HC1 solution, the ions that transport the current (H+ and Cl-) are also the ones that are discharged at the electrodes. In other cases, however, the main ionic transporters of current may not be of the same species as the ions that are discharged. An excellent example is the electrolysis of CuS04 solution between platinum electrodes. A one molal CuS04 solution is quite acid so that the positive current transporters are both Cu2+ and H+ ions. The main negative transporter is the S04 ions. The solution contains, however, a small concentration of OH- ions. In order to determine which ions will be discharged at the electrodes, it is necessary to consider standard electrode potentials of the concerned species ... 

Since the point of bubble evolution represents a more or less indefinite rate of discharge of hydrogen and hydroxyl ions, recent work on overvoltage has been devoted almost exclusively to measurements made at definite c.d. s it is then possible to obtain a more precise comparison of the potentials, in excess of the reversible value, which must be applied to different electrodes in order to obtain the same rate of ionic discharge in each case. The details of the methods of measurement and a discussion of the results will be given after the general problem of the mechanism of electrode processes has been considered. 

When an aqueous ionic solution is electrolysed using inert electrodes, there is usually only one product obtained at each electrode. The ease of discharge of cations at the cathode is related to their electrode potentials. Figure 20.28 shows some half-reactions and their electrode potentials. 

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

A Method Based on Ionic Bombardment. In this procedure, the specimen is placed in a discharge tube, made into a cathode, and is then bombarded with mercury ions. The ionized gas, drawn from the metal by the high potential between the electrodes, is collected and subsequently analyzed by conventional procedures. 

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