lJ^-2 



UADIATION mOLOGY 



iiulicatiiifi; the olTccts of ionization of the amino sroup (i)K' = 4.()0) and 

 of the enolic group (pK = 12.16) (Levenc and Bass. I'.Kil). 



CONCENTRATION 



As was mentioned in the discussion of Beer's law, in concentrated solu- 

 tions the association of solute molecules may cause modification of their 

 absorption spectrum. This elTect may give rise to a nonlinear relation 

 between the optical density of such solutions at certain wave lengths and 



Fig. 5-12. Variation of the absorption spectrum of pinacyanol chlorides in water at 

 20.0°C with concentration. I, 4.44 X 10"^ M; II, \.••^'^ X 10"^ M ; III, 4.44 X 10"^ M; 

 IV, l.:^3 X 10-'' .1/; V, 4.44 X 10-« M . (Scheibe, 1938; reproduced from Kolloid- 

 Zeitschrift. ) 



the solute concentration, as is observed with the Nessler test for ammonia 

 (Hawk etal, 1947). 



In more extreme cases, extensive molecular association, possibly involv- 

 ing electron transfer through intermolecular hydration, may cause the 

 development of entirely new absorption bands. An example of this is the 

 "mesophase" J band (Fig. 5-12) of the cyanine dyes (Sheppard, 1942). 



TEMPERATURE 



The temperature of an absorbing substance significantly affects its 

 absorption spectrum by controlling the statistical distribution of mole- 

 cules among various vibrational energy states associated with the lower 

 energy electronic state and by influencing the velocity of Brownian 



