Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

24 EXPERIMENTAL METHODS FOR SLOW REACTIONStained at a given temperature to provide a constant vapour pressure (liquid), ora constant flow obtained by means of a needle valve (gas), passes through theRV and condensable species are trapped at E, F and G. The flow rate is controlledby capillaries at A and D and measured by manometers such as C at the entry andexit of the RV. The latter measurement also allows the average pressure in theRV to be measured. Non-condensable products are passed into the storage vesselH by diffusion or Topler pumps. If a carrier gas is used, such as in the toluenecarrier techniquea2, A now becomes the carrier gas. The reactant is fed into thecarrier gas stream from systems I (liquid reactant) or I1 (gaseous reactant). In thecase of the liquids, the bulbs W and A are weighed before and after the experimentto determine the amount used in a run. The flow rate of the reactant is determinedby a capillary at the exit point and the temperature of the bath surrounding W.System I1 is used for a gaseous reactant stored in the bulb Z of known volume suchthat the amount of reactant used may be determined from the change in pressureof the manometer Y. System 111 is used for a reactant of low volatility such thatit is picked up from the U-tube, held at different temperatures, by the carrier gas.A flow system has been used to determine the explosion limit at different flow ratesand pressures83. The mixture in the gas burette G (Fig. 16) is passed into the RVat a rate and pressure determined by the taps V, and V, and measured by themanometer M. The levels of the water in the U of the gas burette are kept the sameby automatic adjustment of C. At the required flow rate and pressure the temperatureof the furnace is raised until explosion occurs.Circulating system. In some cases a circulating system may be useda4. The car-cFig. 16. Flow method for determining explosion limits. From ref. 22c.