Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

126 EXPERIMENTAL METHODS FOR FAST REACTIONSparameter associated with the shock process. For practical reasons this propertyis generally the shock velocity, and this is one of the two functions of the observationpoints indicated in Fig. 3. The temperature rise at the walls of the tubeis much less than in the body of the shock wave (if it were not, it would obviouslybe rather an unrealistic approximation to regard the gas as a non-conductor of heat),and hence it would be very dificult to estimate the temperature of the bulk of thegas directly. What is often done is to use the comparatively small temperature riseat the walls as an indication of the passage of the shock front. The time taken forthe shock wave to pass between sets of sensors placed known distances apart ismeasured. These devices may be small areas of platinum, of which the electricalconductivity changes with temperature, deposited on plates fixed flush with the wall.The temperature, density, etc. are then calculated. (Where possible, the samplecomprises at least 90 % argon, the remainder being the substance under investigation.Provided that the heat of reaction is not very large, the density and temperaturein the shock front are then determined by the argon.)A feature of shock waves not yet considered is that there is inevitably a low pressureor “rarefaction” wave produced at the diaphragm at the same time as theshock wave. This moves initially in the opposite direction from the shock wave butis reflected by the back wall of the tube, and so eventually follows the main shockwave down the tube. Relative to laboratory coordinates this rarefaction wave travelswith the local velocity of sound in the gas. This is considerably less than that of theshock wave because of the substantially lower temperature, but superimposed on itis the flow motion of the driver gas towards the low-pressure region. This has theresult that the rarefaction wave tends to catch up with the shock wave. Becauseof the simplifications it allows, it is convenient to make the measurements on theshocked gas before the rarefaction arrives. This consideration is an important onein deciding on the relative positions of the diaphragm and observation points, andon the relative lengths of the high- and low-pressure areas2’. For a reason consideredbelow, measurements are also sometimes made after the shock wave has beenreflected from the front wall, but before the rarefaction wave has arrived. Such asituation is only used where absolutely necessary because it is now felt that the shockfront is significantly distorted on reflection.In addition to the shock wave velocity it is necessary, in a system where the densitydoes not uniquely determine the composition (and this includes all but the verysimplest chemical systems of interest), to measure some concentration function inorder to follow the reaction. This is one of the greatest experimental difficultiesassociated with the method, since the changes occur so rapidly. Where possible,the concentration change is followed spectrophotometrically. This concentrationmonitoring is the second function of the observation points in Fig. 3. Especiallyfor species with line spectra, the small optical density change, coupled with the fastresponse-time necessary, excludes the use of a conventional spectrophotometer.An example of a detection system which has been used for the hydrogen/oxygen