102 ABSORPTION SPECTRA OF SOLUTIONS. 



Another region of absorption is in the green, near A 5200, for most 

 solutions. This band, which is the most characteristic one of cobalt solu- 

 tions, was ascribed by Ostwald to the cobalt ion. That the molecules also 

 absorb in this region, and in fact have a greater absorbing power than the 

 ion. has been abundantly shown in Chapter II. Whether the simple theory 

 of dissociation is able to account for the observed deviations from Beer's 

 law for this band is not known, but is improbable. The question is now 

 being investigated in this laboratory, and a definite answer will probably 

 be given in the near future. 



The absorption band at X 3300 in the spectrum of the aqueous solution 

 of cobalt chloride, since it disappears with dilution even when molecules 

 are kept constant, can not be due to the cobalt chloride molecules; but we 

 found good reasons for thinking that it is due to some hydrate of these 

 molecules which is formed in solutions of moderate concentration even at 

 ordinary temperature. The two bands in the same region, which appear 

 in the alcoholic solutions of the same salt, behave so much like the band 

 in the aqueous solution that they are undoubtedly due to some relatively 

 simple alcoholate. 



The bands in the red region of the spectrum of solutions of cobalt salts 

 we concluded were due to very simple solvates, such as are formed only 

 in the most concentrated aqueous solutions, or in such solutions of moder- 

 ate concentration, but at very high temperatures. Donnan and Bassett 

 assumed that these bands are due to some complex anions, such as CoCl 2 .Cl 

 or CoCl 2 .Cl 2 , which would then be the same in aqueous and non-aqueous 

 solutions. There are a great many objections to this explanation. In the 

 first place, such complexes ought to obey the usual rule for aggregates, 

 that is, they ought to break down with rise in temperature, whereas the 

 change in the spectrum demands the opposite. In the second place, accord- 

 ing to this theory, the bands ought probably to be the same in aqueous as 

 in non-aqueous solutions, which we have found is not the case. On the 

 theory of solvates, however, everything is perfectly clear. The difference in the 

 structure of the group of bands with different solvents is what we should 

 expect, and the appearance of the bands with rise in temperature of aque- 

 ous solutions, or with the addition of large quantities of a dehydrating 

 agent, is simply due to the formation of the required simple hydrates 

 under these conditions. 



The bands of solutions of nickel salts are all of the same type as the 

 green cobalt band, and hence must be studied spectrophotometrically. 

 The change in the ultra-violet band with addition of dehydrating agents, 

 however, suggests that here also hydrates play an important part. An- 

 hydrous nickel chloride could not be dissolved in the non-aqueous solvents 

 used, hence the work was of necessity limited to aqueous solutions. 



With the exception of copper chloride in acetone, which has a band 

 at ^ 4700, all copper solutions show only two regions of absorption, one 

 in the ultra-violet and one in the red. The ultra-violet band, since it 

 narrows rapidly with dilution even when molecules are kept constant, 

 can not be accounted for by the simple theory of dissociation. And as it 

 widens rapidly with rise in temperature, we must conclude that it is due 



