( 166 ) 



Chemistry. — "On coJorimetry and a cohrimetric method for 

 determininy the dissociation constant of acids'' By Mr. F. H. 

 EiJDMAN Jr. (Communicated by Prof. S. Hoogewerff). 



On cohrimetry. 



During the last few years I have been obliged to undertake a 

 large number of colorimetric determinations, which had to be made 

 as accurately as possible. 



The impossibility of making really accurate colorimetric deter- 

 minations without taking a number of precautions, made Knecht ^) 

 utterly reject this method of working. As Knecht's method (titration 

 of the colouring matters by means of titanous chloride) is not appli- 

 cable in all cases, it Avas thought that an effort to improve the colori- 

 metric method, would not be undesirable. 



Principle of the colorimetric method. 



Starting from the supposition that on diluting a solution of a 

 colouring matter, neither the amount, nor the nature of the colouring 

 matter iwesent, undei^goes a change, the prijiciple of the colorimetric 

 method is as a rule indicated as follows : 



If we examine in transmitted light two solutions, containing the 

 same colouring matter, the concentrations ivill be inversely proportional 

 to the heights of the layers of the same colour. 



This formulation will be found in Ostwald, Handbuch fur Physiko- 

 Chemische Messungen ^) and in Heermann, Ooloristische und Textil- 

 chemische Untersuchungen '). 



The first supposition cannot at all be accepted as being generally 

 correct; in fact, in the practice of colorimetry the circumstances, in 

 which it is correct, occur but rarely. 



In future those solutions of colouring matters, where these suppositions 

 are permissible and which may, therefore, be determined colimetrically 

 without precautionary measures, will be styled directly measurable. 



If the colouring matters under examination are not electrolytes, 

 their nature and amount will suffer no change by dilution. Such 

 colouring matters are, therefore, directly measurable. 



But with acid, or basic colours, or their salts the case is different, 

 as these can but rarely be determined directly. The cause may be 

 found sometimes in the electrolytic dissociation, in other cases in a 



1) Journal of the Society of Dyers & Golorists 1904. p. 242. 



2) Ibid. p. 179. 



3) Ibid. p. G3. 



