Flow systems, kinetic measurements apparatus

A schematic of the apparatus is shown in Figure 1. OH was produced by 248 nm (or 266 nm in some experiments) pulsed laser photolysis of H2O2 and detected by observing fluorescence excited by a pulsed tunable dye laser. Fluorescence was excited in the 0H(a2e+ - X tt) 0-1 band at 282 nm and detected in the O-O and 1-1 bands at 309+5 nm. Kinetic data was obtained by electronically varying the time delay between the photolysis laser and the probe laser. Sulfide concentrations were measured in situ in the slow flow system by UV photometry at 228.8 nm. 

The cell shown schematically in Figure 29.20b permits external generation, followed by EPR detection. The solution can either be recirculated to the electrolysis cell or discarded after observation. Umemoto [36] used a similar apparatus to generate moderately stable radicals coulometrically, followed by stopped-flow measurements of the decay kinetics. Forno [37] used a more elaborate recirculating system with two electrochemical cells in series. The unstable product of the first electrolysis was pumped to the second electrolysis cell, where it was converted to a free radical and thence to the cavity for observation (Sec. VI.A). 

A modification of the conventional flowing afterglow apparatus, in which a drift section is incorporated, is shown schematically in Fig. 6.46i-141 In the so-called flow-drift apparatus reactant ions are produced in the upstream section just as in the conventional afterglow system, but the downstream section, where reactions with neutrals occur, is a drift tube, in which a uniform electric drift field is applied. In the latter section ions can be accelerated from thermal kinetic energies to several electron volts. The two sections of the apparatus are separated by an electronic ion shutter, which makes it possible to admit narrow pulses of ions into the drift region at specified times. This permits measurements of ion-drift velocity and, in... 

More recently ion cyclotron resonance (ICR) mass spectrometric techniques have been applied to proton transfer equilibria measurements [8, 9]. One advantage of this method is the in situ determination of the ion concentrations. The reaction chamber is the resonance cavity in which the microwave absorption by the ions is measured. The method works only at low pressures / <10 torr and has been used only at room temperature. Clustering reactions like (3) are kinetically third order at low pressures since they require a third body collision for stabilization of the exothermic association product of the reaction. Therefore they are too slow for equilibrium determinations by ICR. The flowing afterglow technique which uses a flow rather than a stationary reaction system, and external ion sampling (as in the Alberta apparatus) has been also used with very good success for ion equilibria measurements [10,11]. 

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