Permeability to Hydrogen by Palladium

Hydrogen readily diffuses through palladium at temperatures ranging from 240° C. upwards. Graham illustrated this by means of a palladium tube composed of metal 1 mm. in thickness, the tube measuring 115 mm. in length and 12 mm. in internal diameter. It was closed by plates of platinum soldered at both ends, one of the plates being perforated by a long narrow tube of platinum, by which the cavity of the palladium tube could be exhausted of air. The tube remained air-tight when exhausted, both at the ordinary temperature, at 260° C., and at a temperature verging on low redness, the external gas being air. When, however, the external gas was hydrogen, although no gas appeared to pass through at 100° C. hydrogen began to appear at 240° C., and more rapidly at 265° C., namely, at the rate of 327 c.c. per square metre per minute. At just short of redness the rate increased to 423 c.c.

With coal-gas as external atmosphere Graham found that the penetration began at about the same temperature, but the penetrating gas appeared to be perfectly pure hydrogen, and contained no trace of hydrocarbons. This was confirmed by Ramsay. Clearly it should be possible to test the percentage of hydrogen in coal-gas by this means, if the other constituents, as appears to be the case, are unable to pass through. The apparatus, however, will not retain its activity for very long; it is necessary to wash the coal gas with a solution of permanganate in order to free it from sulphur compounds, since otherwise the palladium becomes coated with an impermeable layer of sulphide. Cyanogen, ether vapour, and marsh gas appear quite unable to pass through palladium.

Ramsay found that in all cases the partial pressure of the hydrogen which has diffused from outside into the interior of a palladium tube is lower than the pressure of the surrounding hydrogen no matter what inert gas is present within; although with nitrogen the discrepancy is at its maximum.

In order to explain the passage of hydrogen through palladium, Ramsay suggested that the gaseous molecule is split or dissociated, although he did not make it clear how such a change would facilitate the diffusion. The following year Hoitsema came to precisely the same conclusion for temperatures above 100° C., and this was supported by Winkelmann, who found that the quantity of hydrogen gas passing through a palladium septum does not diminish proportionally to the pressure, but that on the assumption that the hydrogen dissociates, and that the quantity of gas diffusing is proportional to the pressure of the dissociated molecules, an expression can be obtained which gives with reasonable accuracy the relation between the pressure in the apparatus and the quantity of gas diffusing.

The pressure-time curves, representing the diffusion of hydrogen through palladium at temperatures ranging from 100° to 300° C. under pressures of 700 to 100 mm. consist of two portions, which, it is concluded, correspond to the existence of two allotropic modifications of palladium. Generally speaking the rate of diffusion is proportional to the pressure of the gas. Below 100 mm. diffusion takes place more slowly, and the foregoing proportionality ceases to exist. The rate of diffusion under these conditions is apparently not related to any simple function of the pressure.