Vanadium, page-3

Graphite anodes used in caustic-chlorine electrolytic cells, and particularly cells of the so-called mercury type, must be relatively pure in respect to certain trace elements, particularly vanadium and others including chromium, molybdenum and tantalum. These elements, even in minute quantities in a cell in which sodium chloride is electrolyzed for the manufacture of chlorine and caustic, will cause an increase in hydrogen gas being produced.

http://www.google.com/patents/US3035901