122 



COLORIMETRY-SPECTROPHOTOMETRY 



these, the specific absorption coefficient (a), tells us the amount of light 

 absorption to be expected from a solution whose concentration is meas- 

 ured in grams per liter. The molar extinction coefficient (e), on the other 

 hand, tells us the amount of light absorption to be expected from a solu- 

 tion whose concentration is expressed in moles per liter. The value h in 

 the equation must be included because the thicker the solution the more 







2 3 



Concentration 



Fig. 9-7. Relationship between absorbancy 

 and concentration. 



light it will absorb. In many of the commonly used instruments the glass 

 tubes or cuvettes have a light path of one centimeter, and h simply drops 

 out of the equation— but should not be forgotten. Further descriptions of 

 the use of these measurements are given in the following section. 



Use of the instruments 



The spectrophotometer is commonly used for the qualitative identifi- 

 cation of materials. Since the amount of light absorbed depends upon 

 the electronic displacements possible within the molecule, the various 

 colors of light absorbed are a strict function of molecular structure. 

 Many metals or their vapors produce sharp-line spectra, but most colored 

 organic compounds are complicated in structure, and the absorption 

 bands are spread out laterally. If we wished to know whether two 

 similar-appearing solutions were actually identical, we could measure the 

 absorption of different colors of light by each. The absorption spectrum 

 is a measurement of the absorption by the colored solution over a range 

 of wavelengths. Wherever the electronic transitions are possible the 

 molecule will absorb light, and where they are impossible relatively 



