Electromotive Behaviour of Niobium

The electromotive behaviour of niobium is of interest in that it is considerably influenced by cathodic and anodic polarisation. On being immersed in concentrated caustic potash solution or in 10 per cent, ammonium hydroxide, or on being made the cathode in the electrolysis of water, caustic potash, or ammonium chloride for a few minutes, the metal is activated and its potential towards normal potassium chloride increases. On the other hand, on being immersed in concentrated nitric acid, chromic acid, perchloric acid, potassium permanganate, or thiocyanic acid, or on being made the anode in the electrolysis of water, potassium cyanide, chromic acid, or hydrochloric acid, the metal becomes passive and the potential drops to a remarkable extent. Anodically polarised niobium displays valve action to a pronounced degree. A specimen of niobium which was not very compact (the valve action varies with the degree of compactness), when used as the anode in from 1 to 5 per cent, sulphuric acid, gave only a momentary current with an applied E.M.F. of 112 volts. There was a slight evolution of gas, and the electrode became covered with a greenish-yellow or iridescent blue film which was insoluble in the common acids, but was dissolved by hydrofluoric acid or on making the metal the anode in nitric acid. In an electrolyte consisting of a 0.1 per cent, solution of ammonium phosphate, the E.M.F. necessary to overcome the insulating effect of the oxide-gas layer on the metal is no less than 530 volts. Valve action has also been observed with a large number of other electrolytes. In the cases of hydrochloric acid, sodium chloride, nitric acid, sodium nitrate, acetic acid, potassium bromide and potassium iodide, the anode is disintegrated and dissolves with formation of the pentavalent niobium compound, followed by precipitation of niobic acid; hydrofluoric acid acts similarly, but niobic acid is not precipitated.

The valve action displayed by niobium renders it useful in the construction of electrolytic cell "rectifiers." An alternating current does not pass through 10 per cent, sulphuric acid when both electrodes are made of niobium even at a pressure of 120 volts; if one of the electrodes is substituted for platinum a unidirectional current is produced.