CHAPTER 



9 



Colorimetry-Spectrophotometry 



The colorimetric procedure, one of the most common methods used in 

 analytical chemistry today, finds application in biology also. The method 

 depends upon those physical principles which are related to the color 

 of various substances. In its simplest form, colorimetry measures the 

 amount of a material by measuring the intensity of its color. The greater 

 the concentration, the more highly colored the solution. An extension and 

 refinement of the technique is commonly given the term spectrophotome- 

 try. Spectrophotometry also can be used to determine concentrations, but 

 has the added advantage that it can be used to identify materials and 

 measure rates of reactions. Any material that has color, or more properly, 

 any material that absorbs radiant energy in the visible region, in the ul- 

 traviolet, or in the infrared, is adaptable to measurement by this pro- 

 cedure. 



General considerations 



Measurement of the color of materials depends upon the nature of 

 light itself. The wave nature of light was described in Chapter 8. 

 The conception of light as wave motion, however, does not entirely 

 and adequately describe its behavior. Although seemingly incongruous 

 at first, it is now relatively easy to accept a different nature for light. 

 It can be shown mathematically that when it interacts with matter, light 

 behaves as if it were made up of corpuscles or packets of energy. 

 These packets of energy are commonly known as quanta (singular: 



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