Van Name and Hill — Solution of Metals. 301 



Aet. XXXIV. — On the Rates of Solution of Metals in 

 Ferric Salts and in Chromic Acid; by R. G. Van Name 

 and D. U. Hill. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cclxxxii.] 



Former papers from this laboratory have dealt with the 

 rates of solution of metals in iodine." 51 " The present investiga- 

 tion is an application of the same method to further measure- 

 ments of the rates of solution of metals in oxidizing solutions, 

 and was undertaken on account of the probable bearing of the 

 results upon the so-called Diffusion Theory of heterogeneous 

 reactions. 



It will be recalled that this theory is based on the hypothesis 

 that in a reaction between two phases, let us say a solid and a 

 liquid, the stirring of the liquid is not effective up to the 

 actual boundary surface, but that there remains a narrow zone 

 or layer of liquid, adjacent to the solid, which is so far 

 unaffected by the stirring that the transport of dissolved sub- 

 stances through this layer, to or from the solid, must be 

 brought about essentially by diffusion. From this point of 

 view the observed velocity of a reaction between a solid and a 

 dissolved substance should be the resultant of two consecutive 

 reactions, (a) the diffusion process, and (b) the chemical reac- 

 tion! proper, which occurs at the surface of the solid. The 

 slower of these two reactions will obviously determine the 

 reaction velocity actually observed. 



Many instances have come to light in recent years in which 

 this hypothesis seems to give the best explanation of the facts. 

 As an example the above mentioned work on rates of solution 

 of metals in iodine may be cited, in which it was shown that 

 eight different metals, Ag, Hg, Cu, Ni, Co, Fe, Cd and Zn, all 

 dissolved at the same (equivalent) rate in a solution of iodine 

 in potassium iodide. This fact seems to show that the rate of 

 diffusion of the iodine is here actually the determining factor. 



Such results are, of course, conditioned upon the absence of 

 any interfering secondary effects, which are often encountered 

 in practice and may obscure or wholly conceal the influence of 

 diffusion. The formation of an insoluble coating on the solid 

 is a common type of interference. Other types will be referred 

 to later. As a rule, however, such cases show characteristic 



* Van Name and Edgar, this Journal (4), xxix, 237, 1910 ; Van Name and 

 Bosworth, this Journal (4), xxxii, 207, 1911 ; Van Name and Hill, this 

 Journal (4), xxrvi, 543, 1913. 



fThe term '-chemical reaction" in this connection is used in the broader 

 sense, and may in certain cases include processes not always classed as 

 chemical, such as solution in water, or crystallization from solution. No 

 such cases, however, are included in the present investigation. 



Am. Jour. Sci.— Fourth Series, Vol. XLII, No. 250. — October, 1916. 

 21 



