D. D. VAN SLYKE 



D is the logarithm of l/T, T being the fraction of Ught transmitted by 

 the substance measured. 



The value of k for a given colored solute varies with the wave 

 length of light. Hence Beer's law holds exactly only for monochromatic 

 light; and the accuracy with which the formula applies to a given 

 photometer depends partly on how narrow a spectral band can be 

 given by the analyzing device interposed between the source of light 

 and the solution, a limitation under which the Duboscq colorimeter 

 does not suffer. Some solutes do not exactly follow Beer's law, even 

 with the narrowest spectral bands. Most solutions, however, do follow 

 the law over the concentration ranges used for analysis, and with the 

 spectral bands provided by the instruments now available for routine 

 analytical work. Solutions for which Beer's law does not hold can be 

 analyzed by using empirical calculation curves of optical density vs. 

 concentration. 



The photometer has several advantages over the Duboscq 

 colorimeter. The validity of equation (1) makes it possible to measure 

 the concentration of one colored solute in the presence of others, since 

 the total optical density is additive: 



D = CiAx + C,/h. . . (2) 



Hence, if the medium in which the concentration of a solute is to be 

 measured is itself colored or turbid, the increase in optical density due 

 to the presence of the specific solute can be used as a measure of its 

 concentration. Correction for nonspecific color is much less simple 

 in a Duboscq colorimeter. 



Because the k value for each solute changes with the wave length, 

 it is possible to determine two colored solutes in the same solution by 

 measuring the optical densities at two different wave lengths. Two 

 densities. Da and Z)j, are thus measured: 



Da = C,/k^ + C^/k^ (2a) 



D, = C,/k, + C2A4 (2b) 



If hi, kz, ks, and ki are known, Ci and C2 can be calculated by simul- 

 taneous equations from the observed Da and i)j. 



The concentration of a turbid suspension can be estimated from 

 its optical density, in the same way as the concentration of a colored 

 solute. 



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