GASES IN METALS 183 



is proportional to the square root of its partial pressure.^' This 

 square root relationship (which may well be called Sieverts' law) 

 emphasizes the difference between the solution phenomena of gases in 

 metals and gases in aqueous liquids, since, in the latter, solubility is 

 proportional to the partial pressure of the gas (Henry's law). There 

 are, however, some gas-metal systems in which solubility does not 

 follow Sieverts' law. It is well known that the solubility of hydrogen 

 in palladium is not proportional to the square root of the gas pressure 

 and this is true also of hydrogen in cerium, thallium, zirconium, 

 titanium, tantalum, and vanadium. These exceptions are the same 

 as those mentioned in the section above on the effect of temperature. 

 These metals are noteworthy for their comparatively great absorption 

 of hydrogen and for their compound formation with it. 



Sieverts' law is usually explained by application of the Nernst 

 distribution law, which states that there is a constant ratio between 

 the concentrations of a given molecular species distributed between 

 two phases of a system in equilibrium. Considering, first, molecular 

 oxygen dissolved in a liquid, let Po, denote its partial pressure in the 

 gas phase and Co, its concentration in the liquid. The distribution 

 law is then 



Po, = kCo,. (1) 



The concentration here is directly proportional to the partial pressure, 

 a fulfillment of Henry's law which is a special case of the distribution 

 law. This adequately describes the solubility of most gases in aqueous 

 liquids. 



Considering, now, the solubility of gases in metals, and assuming 

 that molecular gas is dissociated into atoms at the surface of the 

 metal before it is dissolved, let Po^ denote the partial pressure of 

 molecular oxygen, Po the concentration of atomic gas at the metal 

 surface, and Co the concentration of atomic gas in the metal. Then, 

 according to the law of mass action, 



Po, = UPoY. (2) 



Applying the distribution law to the equilibrium between atomic 

 oxygen at the metal surface and atomic oxygen dissolved gives 



Po = ktCo. (3) 



Combining equations (2) and (3) gives 



Po, = k,{kiCny = KCo'. (4) 



" Loc. cit. 



