128 



COLORIMETRY-SPECTROPHOTOMETRY 

 (^ Photocell 



Fluorescent 

 light 



f 



2 Filter 2 



Filter Cuvette 

 I 

 Fig. 9-11. A simplified diagram of a system for 

 measuring the fluorescence of solutions. 



cells behind the cuvette so that they can measure the light absorption 

 by the sample as well as the fluorescence. 



Fluorescence spectra are characteristic of the fluorescing molecule. The 

 instrument used for measuring a fluorescence spectrum must, of course, 

 be more complicated than that for measuring concentration. The fluores- 

 cent light is collected and resolved by a monochromator. The intensity 

 of this light at various wavelengths is measured photoelectrically. 



Flame photometry 



The atoms or ions of metals characteristically absorb and emit light 

 with sharp-line spectra. The flame photometer is used for determining 

 the concentrations of such metals in solutions. The solution to be ana- 

 lyzed is sprayed into a gas flame where the metal atoms are heated until 

 they glow in their characteristic color. Sodium ions, for example, change 

 the colorless gas flame to a brilliant yellow-orange by emitting a strong 

 double line at about 589 m/tt. The light from the flame is collected and 

 passed through a filter system or monochromator, which passes a band 

 of wavelengths including the bright emission lines of the metal being 

 analyzed. Other wavelengths are removed by the monochromator or 

 filter so that only light emitted by this particular metallic atom is allowed 

 to fall on the light-detecting device. Under otherwise identical condi- 

 tions, a more concentrated solution of the metallic ions produces a 

 brighter light so that measurement of the Hght passing through the 



