168 HYDRATES IN AQUEOUS SOLUTION. 



easily produces the blue color. Ostwald concludes that the blue color is 

 due to the driving back of the dissociation of the cobalt chloride, the anhy- 

 drous salt being blue. 



In a recent paper, Donnan and Bassett* take the view that none of the 

 theories thus far proposed is satisfactory. They make the suggestion that 

 the blue color may be due to the presence of complex cobalt anions. They 

 point out that the blue alcoholic solutions when cooled down to 79 become 

 red, and conclude that this can not be due to hydration. The question 

 arises as to whether it may not be due to alcoholation. 



The action of salts of zinc, mercury, cadmium, antimony, and tin, in turn- 

 ing blue solutions of cobalt chloride red, is explained as being due to the 

 fact that these metals have a greater tendency to form complex negative 

 ions than cobalt, and that they therefore break up the complex ions formed 

 by the cobalt. This explanation seems decidedly forced. Evidence of 

 a very direct kind should certainly be furnished, that the above metals do 

 have a very great tendency to form complex anions. In some of the above 

 cases this is certainly not obvious. 



Hartleyf calls attention to certain inaccuracies in the above paper. It 

 is not at all certain that when hydrochloric acid is added to a solution of 

 cobalt chloride, the blue color is due to the same cause as when an aqueous 

 solution is heated. Indeed, the absorption spectra in the two cases are 

 quite different. Hartley points out further that the color of hot aqueous 

 solutions was not supposed to be due to the anhydrous cobalt chloride, 

 but to a dihydrate. Again, the spectra of a solution of cobalt chloride sat- 

 urated at 20, and taken at the temperatures 23, 33, 43, 53, 73, and 

 93, are all different from the spectrum obtained from the anhydrous salt 

 in absolute alcohol, and are also different from the cobalt chloride to which 

 hydrochloric acid had been added. 



Hartley also criticizes certain other points in the paper by Donnan and 

 Bassett, but it would lead us too far to discuss these in detail. 



Quite recently, Lewis| has called attention to the importance of these 

 color changes for the theory of hydrates in solution. 



Jones and Bassett have discussed the reactions described by Lewis, and 

 Jones || has supplemented them by a number of other cases which bear 

 directly upon the hydrate question. 



We shall now take up the work that we have carried out on the problem 

 in hand. 



*Journ. Chem. Soc. (Lond.), 81, 942 (1902). 

 t/Wd., 83, 401 (1903). 

 fZtschr. phys. Chem., 52, 224 (1905). 

 Amer. Chem. Journ., 34, 291 (1905). 

 HJourn. de Chim. phys., 3, 455 (1905). 



