SPECTROSCOPY AND SPECTROPHOTOMETRY 11 



to the left, the other the long waves (red). Means are provided for 

 moving the two spectra longitudinally, so that an absorption band 

 produced by insertion of an absorbing substance in the entering light 

 beam may be made to coincide in the two spectra. The amount of 

 movement necessary to produce such coincidence is a linear fimction 

 of the wavelength of the band. The micrometer screw controlling 

 the motion is calibrated, and wavelengths may be read with accuracy 

 to about one angstrom unit. By taking a large number of readings 

 on the same solution, greater accuracy is attainable (937). 



Since in the Hartridge spectroscope the whole width of the absorp- 

 tion band is made to coincide in the two spectra, the wavelength 

 measured is that of the center of the band, not necessarily that of 

 maximum absorption. Consequently small deviations are often 

 observed between the band positions measured by this means and 

 by the spectrophotometer. The apparent position of an asymmetric 

 band depends on the concentration, and the minimum concentration 

 allowing an exact reading of the band position should be used. 



3. ABSORPTION SPECTROPHOTOMETRY 



3.1. Theoretical 



3.1.1. The Theory of Light Absorption. When a parallel beam 

 of monochromatic light passes through a homogeneous absorbing 

 medium, the intensity of the beam diminishes exponentially with the 

 thickness of the medium. If /o is the intensity of the incident beam, 

 I that of the emergent beam, and / the thickness of the medium, we 

 have the relationship: 



logio h/I = el 



where e is called the extinction coefficient. The ratio I/Io is called 

 the transmittance, its reciprocal the opacity, and the function 

 d — logio /o// is called the density. Hence: 



d = d 



If in addition the absorbing medium is a solution in which the 

 dissolved molecules are independent of one another and are equally 

 influenced at all concentrations by the molecules of a colorless solvent, 

 the density is a linear function of the concentration of the solute. 

 This is Beer's law. We may now write: 



d = ispCl 



