Hydrogen occluded in palladium conductivity of metal
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Palladium possesses to a remarkable extent the power of absorbing or occluding hydrogen, and numerous researches have been carried out by different investigators with a view to determining the condition of the occluded gas, and the influence it has upon the properties of palladium.
The absorption of hydrogen by palladium foil is readily shown by passing an electric current through acidulated water, using a platinum anode, but a plate of palladium, just previously heated to redness, as cathode. By using a narrow, vertical glass cell an image of the apparatus may be thrown on to a screen. Oxygen gas is evolved from the anode, but no gas evolution appears at the cathode, until the palladium has become saturated with gas, after which point hydrogen is evolved.
The first detailed researches on the subject are those of Graham, who experimented with thin palladium foil. He observed that palladium which has been ignited in vacuo absorbs large quantities of hydrogen at ordinary temperatures, rapidly yielding a large portion of the gas up again upon being placed in a vacuum, and slowly yielding it when exposed to air. The gas is rapidly and almost completely evolved in vacuo at 100° C. In experiments in which the foil was heated in hydrogen and allowed to cool in the gas, the metal absorbed 643 times its own volume of hydrogen, whilst in later experiments palladium wires absorbed over 900 times their volume of hydrogen. The occluded hydrogen was termed by Graham hydrogenium.