Hydroxyl radical competition kinetics method

Onstein P, Stefan MI, Bolton JR (1999) Competition Kinetics Method for the Determination of Rate Constants for the Reaction of Hydroxyl Radicals with Organic Pollutants Using the UV/H2O2 Advanced Oxidation Technology The Rate Constants for the tert-Butyl Formate Ester and 2,4-Di-nitrophenol,/. Adv. Oxid. Technol. 4, No. 2 231-236. 

In principle, EPR spectrometry is well suited as a method to monitor kinetic events however, in practice, the time required to tune the spectrometer, and its intrinsically low sensitivity compared to fluorescence or light-absorption spectrometry, affect its competitiveness. Relatively slow reactions on the timescale of minutes, such as the decomposition of the DMPO-superoxide adduct and the subsequent formation of the hydroxyl radical adduct (cf. Pou et al. 1989) are readily followed, either as the first-order disappearance of the DMPO/ OOH signal... 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Addition of materials capable of reacting with electrons reduces the lifetime of enabling kinetics of its fast reactions to be determined. The powerful oxidant OH (the hydroxyl radical) has only a weak absorption in the ultraviolet, and its reactivity is best measured by a competition method based on its very fast oxidation of thiocyanate ion CNS to yield the intensely absorbing (CNS) ion (Xmax 472 nm). Addition of a second substrate X will provide competition for OH, and the intensity of the absorption of (CNS)2 will be systematically reduced as [X] is increased, enabling a rate constant to be derived. 

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