Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

78 EXPERIMENTAL METHODS FOR SLOW REACTIONSsolved this way. For example, in the mixed pyrolysis and photolysis of CH,CHOand CD,CDO, the relative amounts of CD,, CH4CD3H and CH3D proved thatthe major part of the reaction occurs via a free radical mechar~ism’~~. Extensiveuse has been made of this technique in vacuum uv photolyses and radiolyses,particularly for producing evidence for “snap-out” primary processesz6’. Monoisotopicmercury sensitisation of the decomposition of HC1 has produced evidenceof the extent of HgCl formation in the primary process (51)‘”Hg( ,PI) + HC1+ HgCl+ He (51)An isotope effect can also occur in the rate of a chemical reactionz6’. Thus, isotopicsubstitution has a pronounced effect on the falling-off of the rate coefficient forthe decomposition of ethyl chloride and also produces evidence for a four-centredtransition state’ 62.4.2.6 Gaseous titrationsConventional titrations may be used for various gases263. The apparatus consistsof a copper RV fitted with an automatic stirrer and pressure transducer.NH3/HCI has been titrated this way with a precision of 3 %. Ethylene may beused to titrate F,, and C12/F2, ClF/F, and ClF3/F, mixtures. CH4/C2H6 mixturescan be titrated with F, and alkane/alkene mixtures with Cl,.A more spectacular gaseous titration technique is used for the estimation of theconcentrations of various atomic species. This involves the use of a fast flow dischargeapparatus264, and the spectroscopic estimation of the extent of variouschemiluminescent reactions. Hydrogen265 and nitrogen atoms266 may both betitrated with NO, and oxygen atoms with or NO2649267 the reactions beingH+NO+M + HNO*+MN+NO +N,*+OO+NOz +NO+OfO+NO +NO,*In reaction (59, the NO concentration is constant since it is regenerated via reaction(54). These titrations may be used to measure the concentration of these atomsin other reactions such as (56)265H.+NO, + .OH+NO (56)