Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

314 DETECTION AND ESTIMATION OF INTERMEDIATESfor atom and radical detection, suggests its use in experiments designed to followquantitatively the decay of active species under non-stationary conditions. Theapproach would be similar to that used in optical spectroscopy combined withflash photolysis, but might possess added advantages in sensitivity and absolutecalibration; and, further, species not showing optical absorption in convenientspectral regions might be studied. Research using flash photolytic radical productioncoupled with EPR spectrometry has been started recent1ylg3, and the techniqueis likely to be a valuable new tool in the field of reaction kinetics.5. ‘Chemical’ methodsThis section deals with methods for the detection or estimation of intermediateswhich depend upon the addition of foreign substances to the reaction system. Bydefinition, such techniques cannot but perturb the system under examination, andthe results obtained must be modified where necessary if they are to apply to theunperturbed case.Experimental confirmation of free radical participation in chemical reactionswas first obtained by Paneth and HofeditzIg4 by the ‘chemical’ method of mirrorremoval. This most familiar technique has been used subsequently in many studiesof free radicals. The apparatus consisted of a Aow tube operated at pressures ofabout 1-2 torr. In the original experiments of Paneth and Hofeditzlg4 the reactantwas lead tetramethyl, and the carrier gas was specially purified hydrogen. A leadmirror was deposited in the downstream part of the tube by local application ofheat. If now the tube was heated upstream from the mirror, the first mirrorgradually disappeared, suggesting that methyl radicals, produced pyrolyticallyfrom lead tetramethyl, removed the first mirror by the reaction4CH3 + Pb + Pb(CH3)4With sufficiently high flow rates (16 metres sec-’) the radicals may be made tosurvive for up to 30 cm in the tube. It was shown195 that the lifetime of methyl (orethyl) radicals is more or lessindependent of the nature of the carrier gas, and thatthe rate of decay of the radicals is not affected by the presence of iron or platinum(i.e. the mirror removal cannot be ascribed to atomic hydrogen rather than thealkyl radicals).There are two main ways in which the mirror technique has been applied. In thefirst, the concentration of radicals is determined by following the rate of mirrorremoval by one of a number of methods. These include determination of mirroropacity, change in radioactivity from a radioactive mirror, measurement of electricalconductivity, and so on. In the other method, the products formed by theremoval of the mirrors are characterized, thus giving an indication of both thenature and concentration of the radicals concerned. Thus Paneth and Hofedit~’~~