Gas flame

For substances that are gases or are very volatile at ambient temperatures, it is relatively easy to introduce them into the flame. Gases and vapors are discussed in Part A (Chapter 15). Solids are more difficult to handle and are discussed in Part C (Chapter 17). 

It must be appreciated that at high temperatures platinum permits the flame gases to diffuse through it, and this may cause the reduction of some substances not otherwise affected. Hence if a covered crucible is heated by a gas flame there is a reducing atmosphere in the crucible in an open crucible diffusion into the air is so rapid that this effect is not appreciable. Thus if iron(III) oxide is heated in a covered crucible, it is partly reduced to metallic iron, which alloys with the platinum sodium sulphate is similarly partly reduced to the sulphide. It is, advisable, therefore, in the ignition of iron compounds or sulphates to place the crucible in a slanting position with free access of air. 

Apart from the interferences which may arise from other elements present in the substance to be analysed, some interference may arise from the emission band spectra produced by molecules or molecular fragments present in the flame gases in particular, band spectra due to hydroxyl and cyanogen radicals arise in many flames. Although in AAS these flame signals are modulated (Section 21.9), in practice care should be taken to select an absorption line which does not correspond with the wavelengths due to any molecular bands because of the excessive noise produced by the latter this leads to decreased sensitivity and to poor precision of analysis. 

The detector can be made to respond to phosphorus compounds only by earthing the jet, which is at a negative potential for simultaneous nitrogen and phosphorus detection, and altering the flow rates of the flame gases. If... 

There are many different ways to obtain information from either AAS or ICP alone however when combined with MS, the mass spectrometer becomes the detector. Flame gases are taken into a mass spectrometer through a port into a low-pressure compartment and then transferred, once the pressure is low enough, to the mass spectrometer (see Figure 15.4).The interface between the AAS or ICP and the MS has been a problem in the past, but these problems have largely been overcome [8,9],... 

Haynes el al. [14] have shown that when small amounts of pyridine are added to a premixed, rich ( = 1.68 T = 2030 K) ethylene-air flame, the amount of NO increases with little decay of NO in the post-flame gases. However, when larger amounts of pyridine are added, significant decay of NO is observed after the reaction zone. When increasingly higher amounts of pyridine are added, high concentrations of NO leave the reaction zone, but this concentration drops appreciably in the post-flame gases to a value characteristic of the flame, but well above the calculated equilibrium value. Actual experimental results are shown in Fig. 8.9. 

Using laser fluorescence measurements on fuel-rich H2/02/N2 flames seeded with H2S, Muller et al. [43] determined the concentrations of SH, S2, SO, S02, and OH in the post-flame gases. From their results and an evaluation of rate constants, they postulated that the flame chemistry of sulfur under rich conditions could be described by the eight fast bimolecular reactions and the two three-body recombination reactions given in Table 8.4. 

The reaction probabilities for O and OH with soot particles have been measured by Roth and co-workers in a series of shock tube experiments [58-60], They have found that both radicals react with soot particles with a collision efficiency of between 0.10 and 0.20. In contrast, the reaction probability with 02 is at least an order of magnitude lower [55], Of course, at lower temperatures and sufficiently lean mixtures, soot oxidation by radical species becomes small and oxidation by 02 is important (though slow). Consequently, soot that passes through or avoids the primary reaction zone of a flame (e.g., due to local flame quenching) may experience oxidation from 02 in the post-flame gases. Analysis of soot oxidation rates in flames [54-57] has supported the approximate value of the OH collision efficiency determined by Roth and co-workers. 

This applies solely to mercury as it is the only analyte that has an appreciable atomic vapour pressure at room temperature. The 253.7 nm line is usually used for mercury atomic absorption, but the transition is spin forbidden, and relatively insensitive. The 184.9 nm line is potentially 20-40 times more sensitive, but at this wavelength most flame gases and the atmosphere absorb strongly. Thus, flame methods for mercury are not noted for their sensitivity (typical flame defection limits are in the range 1-0.1 pg ml-i). If chemical reduction is employed, mercury can be brought into the vapour phase without the need to use a flame, and defection limits are dramatically improved. 

CP. Fenimore. Studies of Fuel-Nitrogen in Rich Flame Gases. Proc. Combust. Inst., 17 661,1979. 

At higher pressures, the composition limit appears to be experimentally independent of the dimensions of the equipment and has been widely considered to be a property of an adiabatically propagating mixture (Bl). This type of limit has been referred to as a fundamental limit. The demonstration of the existence of such a limit is an exceedingly difficult task. Since all flames radiate some of their thermal energy, it is impossible to stabilize a flame without losses to the surroundings. However, most flame gases are very poor radiators, and, since the residence time of the gases in the reaction zone of a flame is quite small, flames have been observed which come quite close to the adiabatic flame temperature (F14). 

The condensation of solids from flame gases is suggested as a means of preparing finely divided solids. The difficulty of designing a suitable process is discussed in a review of the problem of nucleation and condensation processes in flow systems (C7). The closely allied production of tiny hollow spheres of alumina from the combustion of aluminum is a fascinating, if not necessarily useful, process (S9, F10). 

Beta values represent the fraction of free atoms present in the hot flame gases of the flame indicated. These values have been taken from various sources and were either experimentally measured or calculated from thermodynamic data using the assumption of local thermodynamic equilibrium in the flame. These values do not have very good agreement within each element however, the values do provide an indication of the probable sensitivity of the particular flame. 

Gas chromatography-atomic absorption (AA) has gained popularity because the interfacing is quite simple. In its crudest form, the effluent from the GC column is directly connected to the nebulization chamber of the AA. Here, the effluent is allowed to be swept into the flame by the oxidant and flame gases. There have been several recent reviews of the technique [127,128]. 

In summary the recently developed fields of CARS and laser induced saturation fluorescence spectroscopy offer considerable potential as diagnostic techniques for combustion systems. The techniques are complimentary. CARS has its best application for relatively high concentration flame gases and for tenperature measurement. The fluorescence technique is well suited for low concentration measurements of atoms and radicals and flame transients. 

In Table IV, we see that established techniques for velocity measurement allow us to determine the average momentum flux, average velocity, turbulent intensities, and shear stress. Next on the list, to complete the flow field description, is the fluctuation mass flux, and first on the combustion field list is the temperature and major species densities of the flame gases. 

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