ULTRAVIOLET ABiSOllPTION SPECTRA 



181 



of the light wave in a dielectric medium (Bayliss, 1950; Hartmann and 

 Schlafcr, 1950). 



As indicated, absorption bands are widened in polar solvents because of 

 increased molecular interaction with conseciuent perturbation of the elec- 

 tronic configurations (Fig. 5-10). 



The total absorption intensity, the je dv, can also vary with the solvent 

 and would be expected, in general, to increase moderately with increasing 

 solvent refractive index (Chako, 1934) owing to augmentation of the 



220 230 240 250 260 270 280 290 300 320 340 

 WAVE LENGTH, m/( 



Fig. 5-11. The absorption spectrum of cytosine at three different vahies of pH 

 (« = 8.0G X lO'D). I,pH 1.2;II, pH6.0;III, pHl2.7. (Scott, unpublished data, 1951.) 



exciting electric field by the field of the induced dipoles in the medium. 

 However, this expectation is frequently not fulfilled for unknown reasons 

 (Jacobs and Piatt, 1948). 



pH VALUE 



In aqueous solutions of substances containing dissociable groups, the 

 pH of the solution will usually have a marked effect on the absorption 

 spectrum. Ionization of any such group, resulting in gain or loss of 

 charge, will certainly alter the basic electronic configuration of the mole- 

 cule and thereby the spectral distribution of absorption. An example is 

 the absorption of a solution of cytosine at various pH values (Fig. 5-11), 



