Absolute radical concentration measurements

Absolute Radical Concentration Measurements and Modeling of Low Pressure Flames... 

EPR methods that allow a more direct determination of kv have been developed. These enable absolute radical concentrations to be determined as a function of conversion. With especially sensitive instrumentation, this can be done by direct measurement/57 160 An alternative method, applicable at high conversions, involves trapping the propagating species in a frozen matrix361 362 by rapid cooling of the sample to liquid nitrogen temperatures. 

There are only a few observations of specific reactions of the OF radical and even fewer direct measurements of rate constants. This lack of data is partly due to the difficulties associated with radical detection. Several radical reactions were assumed in reaction mechanisms, particularly in those involving OFg chemistry. Based on these mechanisms some rate constants were indirectly determined. Mass spectrometric detection enabled direct determination of some rate constants. The rapid exothermic reaction of OF with NO to give NO2 and F (reaction enthalpy AH298 = -239 kJ/mol [26]) was used to obtain absolute radical concentrations [1 to 3]. 

In summaty, HOz and ROz radical concentrations are substantially greater than those of OH, typically by several orders of magnitude. There are several different approaches to measuring these peroxy radicals, and the results from these are in overall agreement as to the magnitude of the concentrations and their diurnal variation. However, there have not been a significant number of intercomparison studies of these methods, so evaluation of the absolute accuracies will require further work. 

Wang et al. (1058) have recently measured OH radical concentrations in a simulated smog chamber by the laser induced fluorescence of OH. The OH concentrations in the chamber range from 0.5 to 1.5 x I07 molec cm"3. In view of the difficulties involved in the absolute determination of OH radicals at such low levels, the uncertainty must be larger than 50%. Table VIII-1A summarizes the ambient concentrations of reactive species and their rate constants with hydrocarbons and NO in polluted air. 

Although one can estimate changes in radical concentration with considerable accuracy, absolute concentrations are very difficult to measure precisely. This arises, at least in part, because in order to achieve high sensitivity, the first or even the second derivative of the... 

If the split 395 nm band were due to the monomer, the band shape should be independent of concentration. Spectra can be measured over a concentration range of about 1000 using a VV-cell . The maxima are matched in height to simplify the comparisons. Absolutely no concentration dependence of the 395 nm band shape was obs ed for solutions of the l-methyl-4-carbo-t-butoxypyridinyl radical in both CH3CN and 2-MTHF over the range from 10 M to 10" M. 

Fig. 8. (a) LIF signal intensity versus laser energy from the excited CH B-X fli(12) rind the excited CN B-X Pi,2(10) lines, (b) Calibration of the detection system by N2 Rayleigh scattering. Right side Schematic description of the Rayleigh scattering calibration procedure for the measurement of absolute CH radical concentration NcH Details are described in the text. 

Temperatures of around 1000 K are the upper limit for conventional flash photolysis experiments, higher temperatures require specially designed apparatus or shock tubes. There have been three shock tube studies of reaction (32). Glanzer et al. [62] determined k22 at 1350 K, between 1 and 25 atm, initiated by the rapid thermal decomposition of azomethane with the methyl radical concentrations being monitored by UV absorption. Direct measurements of the absorption coefficient at 1400 K were used to determine absolute methyl radical concentrations. Similar measurements were performed by Hwang et al. [63]... 

It is apparent from the chemical shifts (g-values), the hyperfine coupling constants (A-values), and the linewidths that the free radicals and vanadyl species are in very similar environments in both samples. It was not possible to obtain meaningful values for the absolute numbers of spins per gram for either species, but estimates of the relative concentrations obtained by measuring peak heights indicate that the vanadyl and free-radical concentrations do not differ significantly between the two asphaltenes. It thus appears that heat treatment of Cold Lake asphaltenes to 320°C does not alter the nature or abundance of paramagnetic centers. 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

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. 

This fact precluded the use of a simple titration reaction for the calibration of BrO absorption intensities in terms of absolute concentrations, and led to small concentrations of radicals in the flow system. An improved version of the absorption system proved satisfactory, however, for measurements of absorption intensities down to 0-02%. Absolute BrO concentrations were obtained by measurement of the amounts of O3 consumed in the reaction, Br -1- O3 -> BrO -1- Og. Br atoms were detected in the products of decay of BrO, and measurement of Br by absorption of Brg showed that a simple scheme was followed... 

The area of the absorption spectrum does not yield an absolute value for radical concentration. This must be obtained by calibration with a sample containing a known concentration of radicals using standardized conditions of measurement. A sample of pitch/KCl provided by Varian was used in the present study, and this sample was calibrated with a measured concentration of recrystallized diphenylpicrylhydrazine (DPPH) in benzene. 

When radical A- reacts to form product radical B- with an appropriate rate constant, the absolute concentrations of each radical can be determined in a steady-state ESR experiment. This ratio and a measured or calculated rate of destruction of A- and B- by diffusion-controlled radical-radical reactions can be used to calculate the rate constant for formation of B-from A-. 

Detection of Radical Anion by ESR Spectroscopy. The ESR measurements of the rate of free radical formation by electron transfer from fluorene to nitroaromatics were obtained by use of the flow system and U-type mixing cells described previously (18, 20). Concentrations were estimated by comparison of the total area of overmodulated first-derivative spectra with solutions of diphenylpicrylhydrazyl under identical solvent and instrumental conditions. Relative concentrations within a given experiment are considered accurate to within a few per cent, while absolute concentrations are considered to be accurate to 30%. 

If Ja has been determined and the concentration of X is known it is now possible to determine kt 5 [Rt-]. The classical problem confronting the kineticist now arises how can one obtain a useful quantitative measurement of the rate coefficient, kx s, so that it will be of value either to theoreticians or to persons interested in applying kinetic data to complex systems It must be remembered that, if Rt- radicals react sufficiently rapidly with X so that they may be considered to disappear right at the place where they are formed, it might be possible to use electron spin resonance, or in certain rare cases absorption spectroscopy, to determine [R ]. Suffice it to say that some measurement other than a purely kinetic one must be used if k13 is to be obtained with the system in question. If one value of k15 is known, values for other gases can be obtained since relative rate coefficients are easier to determine than absolute ones. 

Concentration Profiles. The relative fluorescence intensity profiles for OH, S2, SH, SO, and SO2 were converted to absolute number densities according to the method already outlined. Resulting concentration profiles for a rich, sulfur bearing flame are exhibited in Figure 17. H-atom densities were calculated from the measured OH concentrations and H2 and H2O equilibrium values for each flame according to Equation 6. Similar balanced radical reactions were used to calculate H2S and S concentrations 6). Although sulfur was added as H2S to this hydrogen rich flame, the dominant sulfur product at early times in the post flame gas is S02 ... 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

From ESR spectra, the relative concentration of the radical products of reactions (99) and (100) was determined. When both reactions are sufficiently rapid, the concentration ratio is equal to k1/k2, i.e. to the relative reactivity of the monomer CH2=CHX with Me3CO radicals. In the same solvent, measurements with various monomers yield relative initiation rates [132] according to reaction (94). By a combination of this procedure with an absolute method (e.g. with inhibitors), for which the most favourable conditions can be selected, the accuracy of the determination of the kinetics and mechanism of initiation can be significantly enhanced. 

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