Electrode Potential of Tantalum

This can be determined only in solutions of fluorides, since tantalum shows valve action in all other electrolytes, and even in the case of fluorides there is some reason to believe that oxide formation takes place, vitiating the results. The potential at the electrode

Tantalum (passive)/tantalum pentafluoride (0.006 mole.),

is + 1.537 volt; H = zero. Passive tantalum is obtained by anodic polarisation; the corresponding figure for the active material is +0.165 volt. The potential obviously depends on the state of the metal; after being rubbed with emery, tantalum gives a low energy potential which slowly returns to the normal figure. The cathodic overvoltage in normal sulphuric acid is 0.39 to 0.50 volt.