COLORIMETRY-SPECTROPHOTOMETRY 



125 



possible causes, such as scattering. In addition, at these maxima a dif- 

 ference of one or two milHmicrons in the setting of the wavelength 

 on the spectrophotometer makes relatively little difference in the amount 

 of light absorbed. If, alternatively, the wavelengths were set on a steep 

 portion of the absorption curve, then one or two millimicrons could 

 make considerable difference in the absorption. For routine measure- 

 ments of concentration it is common practice to make up a standard 

 curve from freshly prepared solutions of known concentrations. A curve 

 such as that shown in Fig. 9-7 is drawn, from which the concentration 

 of unknown solutions can be determined. If suitable care is taken in the 

 measurements of the standard solution, it is possible to calculate the 

 specific or molar absorption coefficient from the slope of the curve. If 

 these values are known with precision, then it is not necessary to draw 

 the curve. One merely measures the light absorption, uses the proper 

 absorption coefficient, and calculates the concentration of the solution. 



u 



c 

 o 

 Xi 



k- 

 o 



< 



600 650 



Wavelength (m/i) 



Fig. 9-10. Absorption spectra of a pair of hypothetical pigments, and the 

 spectrum to be expected from a solution containing equal concentrations of 

 the two pigments. 



