CHAPTER III. 



EFFECT OF DILUTION ON THE ABSORPTION OF LIGHT 



BY SOLUTIONS. 



The question as to the effect of dilution on the power of solutions to 

 absorb light is an old one. This question became especially prominent at 

 the time the theory of electrolytic dissociation was proposed. In dilute 

 solutions of electrolytes there are practically only ions present, very few 

 molecules existing as such. All of the properties of such solutions are the 

 properties of the ions contained in them. Therefore, the power of these 

 solutions to absorb light must be due to the ions present in them. This was 

 the reasoning in vogue and the conclusion drawn. It was at the same time 

 freely recognized that molecules in solution have the power to absorb light. 

 This was shown by the fact that solutions of non-electrolytes, or completely 

 unionized substances, are often colored ; and color in solution means selective 

 absorption of light. 



The result of the conclusion drawn from the theory of electrolytic disso- 

 ciation was that an enormous amount of work was done on the absorption 

 spectra of dilute solutions of both electrolytes and non-electrolytes. Ostwald 

 carried out an elaborate investigation on the relation between color and dis- 

 sociation, and published the work under the title "Uber die Farbe der 

 Ionen." 1 A large number of salts were brought within the scope of this 

 investigation salts of an acid having a colored anion, with colorless cations, 

 This is illustrated by the various permanganates, hydrogen, sodium, ammo- 

 nium, magnesium, zinc, cadmium, etc. Ostwald showed that these salts of 

 any given acid had essential^ the same spectra. In a similar manner, he 

 studied salts of fluorescein, eosin, iodoeosin, rosolic acid, diazoresorcinol, etc. 

 Ostwald then reversed the process and compared the salts of a given colored 

 base with colorless acids, thus studying the salts of p-rosaniline with acetic, 

 chloric, benzoic, hydrochloric, nitric, butyric, salicylic, lactic, etc., acids 

 and finding practically the same absorption spectra for all of these salts. 



From the standpoint from which he undertook his investigation, Ostwald 

 may be said to have solved the problem of the role of ions in the absorption 

 of light, as far as that could be done with the prism spectroscope. 



The problem that we studied was of a different nature. It had to do with 

 the absorption spectra of ions relative to that of the molecules from which 

 they were formed. Some earlier work of Jones and Anderson 2 had shown 

 that if molecules have different action on light from ions, the difference is so 

 slight that there would be no hope of detecting it by ordinary means, even with 

 a grating spectroscope. This problem was attacked in the following manner : 



1 Zeit, phya. Chem., 9, 579 (1892). 2 Cam. Inst, Wash. Pub. No. 110. 



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