Dangerous reactions mass effects

The usual experimental criterion for diffusion control involves an evaluation of the rate of reaction as a function of particle size. At a sufficiently small particle size, the measured rate of reaction will become independent of particle size. The reaction rate can then be safely assumed to be independent of intraparticle mass transfer effects. At the other extreme, if the observed rate is inversely proportional to particle size, the reaction is strongly influenced by intraparticle diffusion. For a reaction whose rate is inhibited by the presence of products, there is an attendant danger of misinterpreting experimental results obtained for different particle sizes when a differential reactor is used, because, under these conditions, the effectiveness factor is sensitive to changes in the partial pressure of product. 

Considerations of this kind do not apply to labeling with deuterium, which is available cheaply as the almost pure isotope and is not dangerous. Deuterium-labeled compounds are, for instance, used in analysis, in studies of reaction mechanism, for investigation of isotope effects, and for research on the metabolism of biochemically important substances. They are of particular value for the determination of the structure of chemical substances both by means of chemical reactions and by means of mass spectroscopy or IR or NMR spectroscopy. Moreover, the perdeuterated compounds are important solvents for use in these physical measurements. 

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