General consequences of the current-potential relation

Rather than discuss the mechanisms of electrode reactions in further detail, we will describe some general implications of the current-potential relation. 

A very active metal such as sodium cannot be deposited from aqueous solutions except under special circumstances. The reversible potential for the reduction of iVa is —2.714 V. Even with a lead cathode an enormous current density would be required to bring the cathode below this potential the current efficiency for sodium deposition would be exceedingly small. Sodium can be deposited into mercury, which has a high hydrogen overpotential, if a highly alkaline solution is used. High current densities are required and the current efficiency is very low. Three factors influence the process. 

The alkaline solution, which brings the potential at which hydrogen is deposited closer to the potential for sodium deposition. 

The fact that metallic sodium will dissolve in mercury this brings the sodium deposition potential nearer the hydrogen value and also keeps the sodium that has been deposited from reacting with water. 

It is worthwhile mentioning that charging the lead storage battery would not be possible if it were not f or a high hydrogen overvoltage on the negative plate, which permits the reaction 

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