COLORIMETER FOR MONOCHROMATIC LIGHT 149 



areas of the apertures of the slits S\ and S 2 when an intensity match for 

 a given wavelength is produced on the photographic plate. The quanti- 

 ties of radiation transmitted by the apertures and incident on the 

 absorption cells C\ and C 2 are Ail and A 2 Io, respectively. After trans- 

 mission through the absorption cells, both these quantities are reduced 

 to Id. The matched intensities for a given wavelength are given in 

 the Bunsen-Roscoe notation 



I d = Ai/ l(T* ld = A 2 J 1(T*** 

 where d is the length of the matched absorption cells. Then 



logio ~r = (k 2 - h)d 

 Ai 



The screw regulating the size of the slit S 2 , if calibrated to read log 

 (A 2 /Ai) for a fixed value of A\ and d, will give the optical density of 

 the absorbing substance with reference to that of the comparison liquid 

 (the solvent) for a given wavelength. If the extinction coefficient of 

 the solvent is known, the extinction coefficient of the solution may be 

 obtained. 



Colorimeter for Monochromatic Light 



Colorimetric methods of analysis consist of treating a solution of a 

 substance whose absorption characteristics are desired with a reagent 

 so as to produce a color which is proportional in intensity to the amount 

 of the substance present in the solution. If Beer's law is applicable, 

 then the concentration must be directly proportional to the logarithm 

 of the transmitted light intensity (Fig. IV-11). In the usual colori- 

 metric analysis it is desirable to determine the amount of colored material 

 in the sample as compared with a standard. The amount of material 

 present is measured by the amount of light absorbed. Accurate meas- 

 urements can be made only by a spectrophotometric determination at 

 that wavelength which corresponds with the maximum of its absorption 

 band. A very good first approximation can be attained if one uses a 

 narrow band of wavelengths as a source coinciding with the maximum 

 absorption in the absorption band of the colored solution. Figure 

 IV-12 shows a simple way in which this analysis may be accomplished. 



The basic design developed by Moll [1919] uses a nearly monochro- 

 matic illumination obtained by means of a direct-vision spectroscope or 

 Amici prism. A straight horizontal-filament lamp L is placed in the 

 focal plane of an achromatic objective 0\. The beam of parallel light 

 passes through an Amici prism train from which it emerges as a con- 



