328 Van Name and Hill — Solution of Metals 



shows: In ferric sulphate the ratio Cd : Ag, as would be 

 expected, is lower in 5 molar than in () - 2f> molar acid (1*42 and 

 2 - iS respectively). In chromic acid, on the contrary, the cor- 

 responding ratios are 2 - 19 and 1*64, thus showing that the 

 higher acidity h;is here failed to bring silver into closer agree- 

 ment with the other metals. 



We must not overlook the fact that from the standpoint of 

 the diffusion theory alone there is no reason for regarding this 

 particular value as anomalous. It becomes so only when con- 

 sidered in the light of that theory combined with our assump- 

 tion concerning the effect of acidity upon the velocity of the 

 chemical stage of the reaction. Whether this result is to be 

 regarded as an exception to the rule assumed, or is, rather, to 

 be ascribed to the effect of some unknown disturbing factor, is 

 a question which can hardly be settled without more experi- 

 mental evidence. 



The Influence of Adsorption. 



In discussing our experimental results we have thus far 

 taken no account of the possible influence of adsorption effects. 

 A supplementary hypothesis based on adsorption will now be 

 briefly considered. It does not alter or supersede any of our 

 previous conclusions, but is offered in order to show that a con- 

 ception of the mechanism of heterogeneous reactions based on 

 adsorption is not incompatible with the diffusion theory, but 

 rather supplements it. 



Our solutions contained in all cases free acid, usually in 

 relatively large proportion, amounting in some instances to 

 several hundred times the concentration of the oxidizing agent 

 itself. Let us assume that the metal is covered by an adsorbed 

 layer of molecules of sulphuric acid. Similar relations are to 

 be understood for the hydrochloric acid used in the experi- 

 ments with ferric chloride. Following the point of view sug- 

 gested by Langmuir,* we may imagine these acid molecules 

 to form a layer, one molecule deep, in chemical combination 

 with the outer layer of atoms of the metal, which by virtue of 

 their position possess residual combining power. Since the 

 hydrogen of the acid molecules will be virtually competing 

 with the metal for the negative radical of the acid, this hydro- 

 gen will be rather loosely combined, and will undergo oxida- 

 tion very readily. 



The fraction of the whole number of what Langmuir calls 

 " elementary spaces " so covered by adsorbed molecules will 

 depend upon their concentration in the solution and upon the 



* Jour. Am. Chem. Soc, xxxviii, 1147, 1916. 



