418 Wells — Chromophore Grouping of Atoms 



Another instance where such a grouping may be 

 regarded as the cause of a very deep color is in Walden's 4 

 two triple bromides 



KFe"Fe'" 2 Br 9 .3H 2 and RbFe"Fe"' 2 Br 9 .3H 2 0. 



These salts contain ferrous and ferric atoms and are 

 described as being dark green and quite opaque. It is 

 the writer's recollection of them, which has been con- 

 firmed by a private communication from Professor 

 Walden, that the green color appeared to be merely a 

 luster and that the crystals seemed to be absolutely 

 opaque. 



It appears, further, that the cuprous and cupric atoms 

 act as a chromophore, for when a hydrochloric acid solu- 

 tion of cuprous chloride, which is colorless when fully 

 reduced, for instance by heating with copper wire, is 

 mixed with the yellow or green solution of cupric chloride 

 in hydrochloric acid a very dark brown solution results, 

 which if fairly concentrated is opaque and practically 

 black. Little attention is paid to this dark colored com- 

 pound in the chemical reference-books, but in one of 

 them 5 it is stated that a triple salt, 6NaCl.Cu 2 Cl 2 .2CuCl2, 

 has been prepared from it by Sievert. The composition 

 of this appears to be uncertain, since it was obtained only 

 in the form of an immiscible oil upon adding alcohol and 

 ether to a solution of the salts, and since when a larger 

 proportion of sodium chloride was used in its prepara- 

 tion a product was obtained containing twice as much of 

 that salt as is shown by the above formula. Perhaps the 

 dark hydrochloric acid solution that has been mentioned 

 contains a hydrogen-cuprous-cupric triple chloride, but 

 it may be a double salt. 



Another case to which the chromophore grouping may 

 be applied is that of Prussian blue and the similar blue 

 products containing both ferrous and ferric atoms. The 

 salt KFe"'Fe"(CN) 6 is recognized, so that the chromo- 

 phore effect may be regarded as taking place in a triple 

 salt in this case. Such formulas as Fe'" 4 Fe" 3 (CN) 18 and 

 Fe" 3 Fe'" 2 (CN) 12 are frequently given to Prussian blue 

 and Turnbull's blue, but that they are pure double salts 



4 This Journal, 48, 283, 1894. . 



5 Dammer, Handbuch anorgan. Chem., II., 2, 671. 



