COLORIMETRY-SPECTROPHOTOMETRY 



113 



which the unknown fits. Various devices have been developed to faciH- 

 tate this comparison, to reduce stray hght, and otherwise to increase the 

 accuracy of the measurement. For the qualitative identification of mate- 

 rials, by locating the black absorption bands it is possible to use a direct 

 vision spectroscope. This is an art which has not been properly developed 

 in the United States but is much more popular among the European 

 workers, particularly several at the University of Cambridge in England. 



In the United States we are much more Hkely to depend upon in- 

 struments which contain photoelectric cells to measure light intensities. 

 In order to determine the concentration of a colored material, it is neces- 

 sary to measure the amount of light actually absorbed by that material. 

 The greatest response is obtained if measurements are made with light 

 of a color strongly absorbed by the molecules. The typical colorimetric 

 and spectrophotometric instruments isolate a band of wavelengths in the 

 vicinity of this maximum absorption. This may be done by a system of 

 colored glass filters or by the so-called interference filters. For routine 

 measurements these filter systems are extremely convenient and rapid 

 to use. One merely places the pure solvent in the light path and measures 

 the amount of light striking the photocell. This measurement is often 

 used to establish an electrical zero point. The solvent is then replaced 

 by the colored solution and the diminution in light intensity is noted. 

 The concentraton of the solution and the reduction in the light trans- 

 mitted through it are related, as will be described later. Such an instru- 

 ment would be called a colorimeter. 



Spectrophotometers: Other instruments known as spectrophotometers 

 depend upon a monochromator which uses a prism or a diffraction grat- 



Entrance 

 slit 



Lamp 



Dispersed 

 spectrum 



Beam of 

 green light 



Sample 



i) 



Photocell 



Exit slit 



Fig. 9-1. A simplified spectrophotometer, set to allow a beam of green light to 

 pass through the exit slit. The wavelength is adjustable in any of three ways: by 

 rotating the light source through the arc at A, by rotating the prism (B), and by 

 sliding the slit and detector apparatus along the plane at C. The output of the 

 photocell is measured by an appropriate electrical circuit. 



