CHAPTER IV. 

 COBALT SALTS. 



Review of previous work. Cobalt chloride and bromide in glycerol. Effect of 

 rise in temperature on aqueous solutions of cobalt chloride, cobalt nitrate, cobalt 

 sulphate, cobalt acetate, and cobalt sulphocyanate. Effect of rise in tempera- 

 ture on the absorption spectra of aqueous solutions of mixtures of cobalt 

 chloride and aluminium chloride, and of cobalt chloride and calcium chloride. 

 Conductivity data. Summary. 



REVIEW OF PREVIOUS WORK. 



Some of the most beautiful color-changes known are those shown by 

 solutions of cobalt and copper salts. For example, aqueous solutions of 

 cobalt chloride are purplish red in color. When a concentrated aqueous 

 solution of cobalt chloride is heated, or treated with hydrochloric acid, 

 aluminium chloride, or calcium chloride, its color changes to blue, the change 

 being quite sudden under certain conditions. On the other hand, the addi- 

 tion of zinc and mercury chlorides produces the reverse effect, changing 

 the blue solution into a red one. Similar changes result when cobalt salts 

 dissolved in other solvents are treated in the same way as aqueous solutions. 

 A very considerable amount of work has been done by various workers and 

 in general different theories have been offered to explain the results. 



Russel, 1 Potilitzin, 2 Lescoeur, 3 Etard, 4 and others favored the view 

 that these color-changes were due to the formation of different compounds 

 of the cobalt salt with the solvent. The hexahydrate CoCl 2 .6H 2 is red, 

 and when heated it is changed into the reddish lilac dihydrate CoCl 2 .2H 2 0. 

 When the latter compound is heated it is transformed into the dark-violet 

 monohydrate, CoCl 2 .H 2 0. The anhydrous salt is blue and is formed at 

 about 140 C. Etard snowed that the solubility curve changed in direction 

 at the same temperature that the red solution becomes blue. Charpy 6 

 showed the same to be true for the vapor-tension temperature curve. 



Engel, 8 Donnan and Bassett, 7 Donnan, 8 Moore, 8 and others do not 

 believe that these color-changes are solvation effects at all. Engel considers 

 that double haloid salts may be formed. He points out the fact that the 

 blue anhydrous cobalt chloride becomes red when sufficiently cooled, and 

 that the red anhydrous sulphate becomes violet when heated. Donnan 

 and Bassett consider that the blue color of cobalt salts is due to the forma- 

 tion of complex anions containing cobalt. By boiling-point determinations 



1 Proc. Roy. Soc, 32, 258 (1881); Chem. News, 59, 93 (1889). 



2 Bull. Soc. Chem. (3), 6, 264 (1891); Ber. d. deutsch. chem. Gesell., 17, 276 (1884). 



3 Ann. Chim. Phys. (6), 19, 547 (1890). 

 Compt. rend., 113, 699 (1891). 

 'Ibid., 113, 794 (1901). 



Bull. Soc. Chim. (3), 5, 460 (1901). 

 1 Journ. Chem. Soc, 81, 942 (1902). 



8 Versuche iiber der Beziehung zwischen der elektrolytischen Dissoziation und der Licht- 

 absorption in Losungen. Zeit. phys. Chem., 19,465-488 (1896); 53, 317-320(1905). 

 ' Phys. Rev., 23, No. 4, p. 321, 357 (1906). 



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