Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

20 EXPERIMENTAL METHODS FOR SLOW REACTIONSPipette and compression methods. The reactants are pre-mixed in the burette B(Fig. 12) and admitted to the RV from the pipette P. Pressure is measured as afunction of time, the explosion limit being revealed by the appearance of a flame,a click or a sharp pressure pulse75. A variation of this (Fig. 13) is to pre-mix thereactants in the RV and slowly compress the gases by means of the mercury inthe burette B76.Heating method. Oncs again the reactants, say CO and 0275a, are pre-mixedin the RV but at a low temperature. The temperature is gradually raised when thepressure increases according to Charles' Law. At a critical pressure a blue glow appears.At this point contraction occurs due to reaction. As the temperature israised, the intensity of the glow increases and finally a flash occurs. This is accompaniedby a pressure kick. The pressure limit is taken as that at which the blue glowfirst appears. A variation of this is also used with a flow system (see p. 24).Capillary method. Neither of the above methods is suitable for determining thesecond and third limits since ignition would occur at the first limit. An alternative(of which there are two variations) is to add one reactant to the reaction vessel followedby the next through a capillary until explosion occurs (Fig. 14)76. The maximumand minimum pressures at which ignition no longer occurs give values forthe first and second limits respectively. Increasing the pressure, the minimumpressure for ignition gives the third limit (for the HJO, system). Here H, is addedfirst, otherwise instantaneous explosion occurs; i.e. explosion below the secondlimit. It is avoided when beginning with H,, since for a given stoichiometric mixturetwice the pressure of gas is added compared with 0277. Alternatively, thetime for the pressure pulse or the flash to occur is noted and the pressure limit iscalculated from the known rate of flow of the second component into the RV76b.A variation is to add O2 to the RV, followed by hydrogen, to a pressure above thesecond limit78. The gases are allowed to mix for one minute, and then pumpedthrough one of a series of capillary leaks of different sizes until explosion occurs.Reaction before explosion is negligibly slow.Conventional manometers have usually been used but the ideal pressure-measuringdevice is the transducer provided that heterogeneous reactions due to it areavoided. Adequate precautions have to be taken when carrying out these explosionstudies by encasing the RV in the furnace and possibly putting a shield round thegrid'. This also applies to reactions studied at high pressures. Techniques for thistype of kinetic measurement are amply covered in ref. 22c, pp. 347-358.(d) MixingIf more than one reactant is being used in a system, it is essential that the gasesare well mixed prior to admittance to the RV. This may be achieved by convection,producing a thermal gradient79, by stirring using a soft iron core and rotatingt See ref. 22c, p. 417.