If absorption of gas occurs with formation of a solid solution, then, as the system consists of two phases, solution—vapour, it is bivariant. At constant temperature, therefore, the pressure will still vary with the concentration of the gaseous component in the solid phase. This is represented by the curve AB in Fig. 47. When, however, the pressure has reached a certain value, combination can take place; and since there are now three phases present, the system is
univariant, and at constant temperature the pressure is constant, as shown by the line BC.
III. Absorption of gas occurs, but at a certain concentration the solid solution can separate into two immiscible solid solutions.
We have seen, in Chapter VI., that two liquids can form two immiscible solutions, and the same has also been found true of solid solutions, as we shall presently learn more fully. If, now, two immiscible solutions are formed, then the system will become univariant, and at constant temperature the pc-curve will be a straight line, as in the case of the formation of a compound (cf. p. [86]). The behaviour of this system will, therefore, also be represented diagrammatically by Fig. 47.
Palladium and Hydrogen.—The phenomenon of the absorption of hydrogen by palladium, to which Graham gave the name "occlusion," is one that has claimed the attention of several investigators. Although Graham was not of opinion that a compound is formed, but rather that the gas undergoes very great condensation, acts as a quasi-metal (to which he gave the name hydrogenium), and forms a homogeneous alloy with the palladium, later investigations, especially those of Troost and Hautefeuille,[[259]] pointed to the formation of a definite chemical compound, having the formula Pd2H. This conclusion has, however, not been confirmed by subsequent investigation.[[260]]
Roozeboom and Hoitsema[[261]] sought to arrive at a final decision as to the nature of the phenomenon by an investigation of the equilibrium between hydrogen and palladium on the basis of the Phase Rule classification given above. If a compound is formed, diminution of volume would cause no increase of pressure, but only an increase in the amount of the compound.
As this is the only case of gas absorption which has been
accurately studied from this point of view, a brief account of the results obtained will be given here, although these are not so clear and free from ambiguity as one would desire.
The scientists just mentioned investigated the variation of the pressure of hydrogen with the amount absorbed by the metal at different temperatures, and a few of their results, typical of all, are represented graphically in Fig. 48; the curves indicating the variation of the gas pressure with the concentration of the hydrogen in the palladium at the temperatures 120°, 170°, and 200°. As can be seen, the curve consists of three parts, an ascending portion which passes gradually and continuously into an almost horizontal but slightly ascending middle part, which in turn passes without break into a second rapidly ascending curve. This, as Fig. 48 indicates, is the general form of the curve; but the length of the middle portion varies with the temperature, being shorter at higher than at lower temperatures.