INTRODUCTION AND METHODS 



If Dk is small, squared and higher terms can be ignored and the value 

 of the series is then given approximately by the first term, 2-303 Da 

 (see straight dashed line tangential to the origin of the curve in 

 Fig. 1.8). Thus in solutions of low optical density {Dx > 0-05) the 

 fraction of Hght absorbed is nearly a direct measure of the optical 

 density and hence, also, of a;., the absorption coefficient. Conse- 

 quently the absorption spectrum of such a solution (see for example 

 dotted curve in Fig. 1.6) gives the wavelength variation of the absorp- 

 tion coefficient. This characteristic function, approximated by 

 absorption spectra only when the absorption is low, is simply and 

 directly represented when density is plotted against wavelength (see, 

 for example. Fig. 1 .7). 



The functions obtained by plotting optical density against wave- 

 length are usually called 'absorption' spectra: 'density' spectra 

 would have been a better name. The term absorption spectra is more 

 appropriate to the functions shown in Fig. 1.6, viz. the plots of Hght 

 absorbed vs. wavelength. The use of the name absorption spectra for 

 what are really density spectra sometimes leads to confusion in the 

 literature and, unfortunately, the practice is firmly estabhshed. 



MEASUREMENT OF DENSITY SPECTRA 



In the definition of optical density as log^o (/inc/Arans), Zinc is the 

 intensity of Hght entering the front surface of the medium, and 

 /trans that leaving the back surface. It is not possible to measure 

 /inc and /trans directly because of reflections which, in the case of 

 liquids confined in an optical ceU, take place at two vessel/air inter- 

 faces and at two vessel/Hquid interfaces. 



This difficulty can be overcome in the following way. A second 

 (reference) optical cell, identical with that containing the solution 

 under test, is filled with solvent alone. The two cells are placed 

 successively in the monochromatic light beam and the intensities of 

 light measured which pass through them. It can be shown (taking 

 first order reflections into account, but neglecting those of the second 

 and higher orders) that D^, the density of the solute is given by, 



Ds and Dr are the optical densities of the solution and solvent 

 respectively, and Ir and Is are the intensities of Hght which leave the 



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