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Occlusion of Hydrogen by Cobalt

Occlusion of Hydrogen by Cobalt to an extent dependent upon a variety of factors. Chief amongst these are:

Temperature

Cobalt slowly occludes hydrogen in the cold, but at its temperature of reduction, namely, 400° to 500° C., its occluding power is negligible. At some intermediate temperature occlusion progresses at a maximum rate. Cobalt obtained by reduction in hydrogen but cooled in nitrogen occludes an inappreciable quantity of hydrogen.

Length of Exposure

The amount of occluded hydrogen tends towards a maximum with increasing length of exposure of the metal to that gas.

Physical Condition of the Metal

The more finely divided the metal the greater is its power of occluding hydrogen. This probably explains many of the apparently anomalous results detailed in the literature on the subject. Thus, for example, cobalt reduced from the bromide does not possess the property of occluding hydrogen to any important extent. The metal obtained, on the other hand, by reduction from its oxides contains varying amounts of the gas. Reduced at 400° C., Troost and Hautefeuille found it to contain approximately 100 times its volume of hydrogen, with which it readily parted upon heating to 200° C. in vacuo. The explanation appears to be that the volatility of cobalt bromide allows the metal to be deposited, upon reduction, in a more compact form than that obtained from the oxide, so that its occluding power is proportionately reduced.
Neumann and Streintz observed that repeated oxidation and reduction tends to diminish the power of cobalt to occlude hydrogen. As Baxter has pointed out, this is probably due to the fact that these operations tend to render the cobalt increasingly compact. The presence of impurities does not appear to affect very materially the occluding powers of cobalt.

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