28 A STUDY OF THE ABSORPTION SPECTRA. 



and 24 mm. From the spectrograms we see that Beer's law holds within 

 the range of concentrations studied. 



The above spectrograms show that the absorption spectra of potassium 

 ferricyanide and of potassium ferrocyanide are quite different. This differ- 

 ence is shown in three ways: First, for the same concentrations the absorp- 

 tion of potassium ferricyanide is much the greater; second, the limit of the 

 absorption band of the ferricyanide is much sharper; and third, for certain 

 concentrations of the potassium ferricyanide solution, there appears a blue- 

 violet band having its center at about X 4200. The band is entirely absent 

 in the absorption of potassium ferrocyanide. 



Potassium Ferricyanide in Water. 



Potassium ferricyanide, K 3 Fe(CN) 6 , usually exists in the form of dark- 

 red anhydrous monoclinic prisms. It dissolves in water, giving a yellowish 

 solution, which on dilution becomes lemon-yellow in color. According to 

 Locke and Edwards ' an isomeric form of potassium ferricyanide exists as 

 olive-colored crystals having the composition K 3 Fe(CN) 6 ,H 2 0. 



Potassium ferricyanide is but slightly soluble in solvents other than 

 water, and for this reason the present work was limited to aqueous solutions. 



One reason for examining the absorption spectra of potassium ferri- 

 cyanide and potassium ferrocyanide was to find a clue, if possible, to the 

 manner in which these solutions dissociated in dilute aqueous solutions. 

 Most physical chemical investigators have considered that these two salts 

 dissociate according to the following equations: 



K 3 FeC 6 N 6 <= K, K, K, FeC 6 N 8 and 



K 4 FeC 6 N 6 ?=> K, K, K, K, FeC 6 N 6 

 The absorption spectra of these two salts would then be due to one 



or more of the four kinds of absorbers, K 3 FeC 6 N 6 , FeC 6 N 6 , K 4 FeC 6 N 6 , and 



FeC 6 N 8 , and it is quite probable that each one of these absorbers would 

 give rise to a different absorption spectrum. 



According to Jones and Getman, and Jones and Bassett (Hydrates in 

 Aqueous Solution, p. 46, Carnegie Institution of Washington Publication 

 No. 60), it is possible that the dissociation takes place in a different 

 manner. They concluded from conductivity measurements that potassium 

 ferricyanide and potassium ferrocyanide dissociate as follows: 



+ - 



K 3 FeC,N, * K, CN, K, CN, K, FeC 4 N 4 and 



+ - + + + 



K 4 FeC 8 N 6 => K, CN, K, CN, K, CN, K, FeC 3 N 3 

 The absorbers in an aqueous solution in this case would include four 

 groups, K 3 FeC 6 N, FeC 4 N 4> K 4 FeC 6 N, nd FeC 3 N 3 . Thus, according to either 

 theory we get four kinds of absorbers. If we found that we had four 

 different kinds of absorbers in potassium ferricyanide and potassium ferro- 

 cyanide solutions and mapped these spectra, and if we knew the absorption 



1 Amer. Chem. Journ., 21, 193 (1899). 



