N U M B E R 6 IN A S E R I E S How can Electron Paramagnetic Resonance (EPR) provide a chemist with information about free radical reaction mechanisms? First, he can detect transient free radicals in the course of a reaction. He can then identify the free radical by obtaining information concerning the molecular environment of the unpaired electron. In addition, he can make a quantitative measurement of the free radical concentration; and finally, measure this concentration as a function of time.
QUANTITATIVE M E A S U R E M E N T OF FREE RADICALS Numbers 1 through 5 of this series illustrated the effectiveness of EPR in detecting and identifying free radical intermediates in dynamic reactions. This example, No. 6, will illustrate an equally important aspect of EPR—quantitative measurements of free radical concentrations.
FIG.1
FIG. 2
NITROSYL DISULFONATE
CONCENTRATION OF NITROSYL DISULFONATE Sample # 1
Sample #2
10"'
5.0x10"'
10"'
Optical absorption
7.8x10"'
3.9x10"'
7.7xl0"3
lodometric Titration
7.3x10"'
3.8x10"'
7.6xl0":1
Method c i Determination Weighing Pure Material
Sample # 3
The ability of EPR to measure free radical concentrations is inherent in the resonance phenomenon since the intensity of the absorption is directly proportional to the number of free radicals contributing to the absorption. Since most research EPR Spectrometers present the first derivative of the absorption, the intensity of the absorption must be determined by a double integration {jT/f (x) dx3) of the first derivative or by a first moment determination
(Ji:xf(x)dx).
FIG. 3
Absolute concentration measurements of unknowns can be made by comparison of the intensity of a known free radical solution with the intensity of the unknown. This example illustrates one common standard that is used for determination of aqueous solutions of free radicals. The potassium salt of nitrosyl disulfonate when dissolved in water at pH 8.0 yields a characteristic three line EPR spectrum as shown in Fig. 1. The concentration is verified in two ways; optically from the known extinction coefficient (£545=20.8) and by an iodometric titration. Figure 2 tabulates the results of the standardizations while Figure 3 is a comparison of the first moment of the EPR signals to the known concentrations. 30
45
60
75
CONCENTRATION MxlO"
Detection, identification, and quantitative measurement of free radicals are not the only results obtainable from EPR spectra. It is also possible to measure the rate of free radical formation for studies of complete reaction kinetics. Varian EPR Spectrometer systems and accessories are designed for a wide range of applications in the fields of chemistry, biology, medicine, and physics. For additional information about the example above or other examples in this series, please write: Instrument Division. In Europe.· Varian A. G., Zug, Switzerland.
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