120 



COLORIMETRY-SPECTROPHOTOMETRY 



important colorless compounds (proteins, nucleic acids, vitamins, hor- 

 mones) absorb strongly in the ultraviolet. 



Infrared spectrophotometry requires special detectors— often lead sul- 

 fide— but an otherwise similar optical system. Absorption in the infrared 

 depends upon different principles than absorption of visible and ul- 

 traviolet radiation. Infrared quanta are generally too small to bring about 

 transitions in electronic energy levels. Instead, the quantum size corre- 

 sponds to the energy differences of the various vibrational states of certain 

 chemical bonds. Most important of these are the heteroatomic bonds, 

 such as O— H, C— H, and N— H. Any molecule which possesses such 

 bonds will absorb infrared radiation at a series of wavelengths. Since the 

 actual collection of absorption bands depends upon the number and kinds 

 of bonds, the infrared spectrum is a characteristic property of a certain 

 molecular structure. 



I 2 3 



Concentration 



Fig, 9-6. Effect of concentration of a colored 

 solution upon the transmittancy. 



Measurements of concentration 



We can observe visually that the more concentrated a solution of a 

 colored material, the more hght is absorbed and the more intense its 

 color. More formally, however, it is possible to express these relationships 

 by means of equations. Imagine a spectrophotometer which is emitting a 

 beam of monochromatic light. We place a container of the solvent in 

 the light path and measure the output of the photocell, or use this 

 amount of light to set the instrument at "100 per cent light transmission." 

 In effect, we are setting the instrument so that it appears that all of the 

 light passes through the pure solvent. With the colored solution in the 



