Flow atom concentration measurement

The abstraction of H from HOF by atomic F in a discharge-flow apparatus at 296 to 298 K was measured mass spectrometrically by determining F concentrations. In about 1 Torr of He carrier gas with an initial F atom concentration of -1.5 xIO cm , the reaction 

Mahan and Solo studied the reaction in a stirred continuous flow reactor, in which O atoms produced by a microwave discharge through pure O2 or 02-inert gas mixtures reacted with CO. O atom concentrations were measured by titration with NO2. They found that radiation accompanied reaction 0.29 % of the time for the process at 298 °K, and concluded that the reaction was second-order. They proposed the mechanism 

In addition to the use of spatially-resolved concentration measurements for the determination of rate constants for reactions of ground state atoms, the discharge-flow method has been extensively applied to kinetic and spectroscopic studies of chemiluminescent phenomena. In these cases, the flow parameters in the flow tube are of no great importance, as time resolution is not obtained from axial displacements consequently, the total pressures and flow rates, and tube diameters may be varied over wide limits, since it is unnecessary to ensure adherence to the conditions for plug flow. 

Absolute H-atom measurements also were made using the Na/Li method (1(3) in sulfur free flames. An aerosol of an equimolar solution of NaCl and LiCl was added to the central core flow through the nebulizer. Relative intensity measurements were made of the Na 589.0 nm and Li 670.8 nm emission from which the H-atom concentrations were calculated. The H-atom measurements could only be made in the sulfur free flames. Reaction of Na or Li with sulfur species would render the technique inoperative. 

The methods listed above all enable relative concentrations of atoms or radicals to be measured. It is a much more difficult problem to measure absolute magnitudes of atoms and radicals in discharge-flow systems, or indeed in any other systems such as flash photolysis experiments. Two principal methods are used for the derivation of absolute concentrations (a) the combination of spectrometric measurements with calculated transition probabilities or (b) the use of the stoichiometry of rapid titration reactions. Of these methods, (b) is probably the most frequently used at the present time. Emphasis will be given to the possibilities of absolute concentration measurements in the discussion of the methods which follows. 

In the first section of this chapter the methods of production of atoms and the determination of their concentrations in discharge-flow systems are discussed, with particular reference to two important problems. Firstly, the identification of secondary active species which may accompany the primary active species (a ground state atom) in the products of an electric discharge and secondly, a critical discussion of various methods for the measurement of atom concentrations. 

Whilst the rate constant k may thus be deduced from measurement of [A] at different distance (times) along the length of the flow tube, a more satisfactory technique for the measurement of pseudo-first-order rate constants k is the fixed observation point method. In this method, the atom concentrations [A] remaining at a fixed observation point are measured when the same (excess) concentration of reagent [R] is added in turn at each of several inlets along the flow tube. 

Consider an elementary transfer reaction of an atom with a stable molecule, of simple stoichiometry, and sufficiently rapid for at least 99% extent of reaction to occur within the time resolution of a discharge-flow system (1 to 100 ms). Such a reaction constitutes a possible titration reaction for the measurement of atom concentrations if some means of detecting atoms in the system is also available. An atom indicator is sometimes provided conveniently by a chemiluminescent emission associated with the titration reaction. 

Braun and co-workers [32] at the National Bureau of Standards have recently developed a powerful new method of studying H atom reactions. The H atoms are generated from the flash photolysis of an olefin with <1% of photodecomposition and the H atom concentration monitored from the resonance fluorescence by atomic H excited by absorption of Lyman-a 1216 A radiation. So far only one transfer reaction has been studied over a range of temperatures by this method but others are likely to follow. The technique has also been extended to measurements of H atom concentrations in fast flow systems [42]. 

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