124 



COLORIMETRY-SPECTROPHOTOMETRY 



These two pigments must be similar in structure because two of the 

 peaks correspond. The third peak in one is completely missing from 

 the other, however, so the molecules cannot be identical. Absorption 

 spectra for many kinds of materials appear in the literature. It is possible 

 to identify materials by comparison with these published curves. 



The qualit)^ of the spectrophotometer affects the absorption spectrum 

 of a solution in a way that is surprising at first. Narrow spectral band 

 widths produce sharper spectra, as can be seen from Fig. 9-9. One curve 

 was determined by using quite broad bands of monochromatic light. 

 Although the instrument may seem to be set at a certain wavelength, 

 what the photocell sees is really that wavelength plus a band on either 

 side. Thus the apparent absorption at this wavelength is really the 

 average absorption over a wider band. If the actual curve happens to 

 be passing through a maximum at this point, the measured absorption 

 (average) is lower than it should be. The maximum absorption seems to 

 be reduced, and minima are increased. 



0.4 r 



0.3 



>» 

 u 

 c 

 o 



■e 0.2 



o 



< 



0. 



Narrow slit 



Brood slit 



• • • • 



± 



500 520 



540 560 580 

 Wavelength (m/i) 



600 620 



Fig. 9-9. Absorption spectra of extract like that shown in Fig. 9-8, 

 measured on two different instruments differing principally in effec- 

 tive spectral band width. (Less concentrated solution than in Fig. 

 9-8); student data. 



In measurement of concentrations of materials it is usual to set the 

 instrument at a point of maximum absorption. At this wavelength the 

 reduction in light intensity caused by the absorption by the pigment 

 is considerably higher than the reduction in light intensity from other 



