SPECTROPHOTOMETRIC TITRATIONS 15 



that of a maximum of absorption, since here will occur the greatest 

 absolute change of density with concentration. 



In a similar manner the concentration of an absorbing substance 

 may be determined in the presence of a second substance if a wave- 

 length can be found where the second substance has a very small 

 absorption relative to that to be determined. This will, however, 

 rarely be the case where both absorb in the visible part of the spec- 

 trum. Since, however, the extinction coefficients of such substances 

 are additiv^e at all wavelengths, we can write: 



e' = e'sp^ Ca + e'sp^ Cj, (for wavelength X') 



e" = e"sp^ Ca + e"sp, C, (for wavelength X") 



from which, if the specific extinction coefiicients of the two substances 

 a and b at the wavelengths X' and X" are known, and are sufficiently 

 different, the concentrations Ca and Cj, can be calculated. 



Likewise, by measurement at n different wavelengths at which the 

 specific or molar extinction coefficients are known, the concentrations 

 of n absorbing substances simultaneously present may be determined. 



A simplification of this procedure may often be found. If the two 

 substances constitute an equilibrium mixture, and the total concen- 

 tration C is known, measurement at one wavelength will suffice. 

 In this case: 



_ — ^ ~ ^fe , A C), _ (g — e 



C fa ~ «6 C ta — th 



It is sometimes possible to reduce the number of absorbing compo- 

 nents in a solution by chemical transformations. It may be possible 

 to eliminate one component completely, when the remainder may be 

 more easily determined, or one substance present may be converted 

 into one of the others. In the latter case the sum of two components 

 is found in terms of a single component. If a number of different 

 transformations is possible in a complex solution, it may even be 

 possible to determine all components by means of several single- or 

 two-component investigations, instead of the more complex investi- 

 gation involving all components. Good examples of the application 

 of methods of this type are found in the work of Lemberg and col- 

 laborators {1701). 



3.1.4. Spectrophotometric Titrations. For the investigation of 

 equilibria in hematin systems, the change of one substance into 



