36 ABSORPTION SPECTRA OF SOLUTIONS. 



plex hydrates break down into simpler ones. Also with increase in dilu- 

 tion more and more complex hydrates are formed, which would also cause 

 the band to disappear. 



The green band appears in all aqueous solutions, although with vari- 

 ous intensities and apparently with somewhat different positions. The 

 change in position is inappreciable in the case of dilute solutions, and 

 with concentrated solutions it depends entirely upon whether the band 

 widens symmetrically or not. In general, it widens perhaps a trifle more 

 towards the violet, especially at first. A very slight amount of general 

 absorption will shift the apparent center of an unsymmetrical band, and 

 this is perhaps the explanation of the slight variation in the position of 

 the center of this band in concentrated solutions of different salts. 



The intensity of the band as indicated by its width on the photo- 

 graphic plate is more interesting. For if it is due to the cobalt cation as 

 such, it ought to have a greater intensity in solutions which are strongly 

 dissociated than in slightly dissociated solutions, concentration and depth 

 of layer being constant. 



The spectrograms of this chapter show the following: For solutions which 

 have a concentration of 2 normal or more, the salts arranged in the order 

 of increasing intensity of the green band are, nitrate, bromide, chloride, 

 sulphocyanate. For dilute solutions (concentrations of about 0.1 or 0.2 

 normal) the order is, bromide, chloride, nitrate, sulphate, acetate, sulpho- 

 cyanate. Arranged in the order of increasing dissociation the salts would 

 be acetate, sulphate, sulphocyanate, nitrate, chloride, bromide, which is just 

 the opposite of the order of increasing intensity of the green band, if we 

 leave out the sulphocyanate. It is very evident, therefore, that something 

 besides the cobalt cation must play a part in the production of this band. 



Plate 21 shows that when the concentration of the sulphate is varied 

 from 0.65 to 0.06 the width of the band does not vary, provided the light 

 is made to pass through such depths of the solution that the product of 

 concentration and depth remains constant; it follows, therefore, that in 

 this case the absorption is simply proportional to the number of cobalt 

 atoms in the solution, and independent of whether these exist as ions, 

 combined with SO 4 , as molecules, or as parts of the various aggregates 

 or hydrates that we may assume to exist in the solution. The same is 

 approximately true for the nitrate solutions, although in this case there 

 is a slight narrowing of the band, indicating that the absorbing power of 

 the various "absorbers" is not the same. In general, the simplest expla- 

 nation of the green band is to assume, as we have just done, that the cobalt 

 atom, no matter what it is combined with, has the power of absorbing 

 green light, the intensity of the absorption depending, however, upon the 

 nature of the combination. 



In the red, aqueous solutions show little or no absorption unless they 

 are very concentrated, or are at a high temperature, or have relatively 

 large amounts of such substances as HC1, CaCl,, or A1C1 3 , etc., added 

 to them. Whether the absorption produced by these different methods 

 is the same or not for any given salt can not yet be answered definitely. 

 We have already pointed out (see page 23) that the absorption of a solu- 



