Molecules in flames

The mode of operation of homogeneous or heterogeneously initiated reactions cannot be excluded in all processes under discussion. The reaction temperatures are typically between 623 K and 873 K, well within the range of pyrolysis temperatures [12]. There is ample evidence of complex and selective homogeneous reactions of unfunctionalized hydrocarbon molecules in flame chemistry [13-15]. 

Two general mechanisms are usually advanced to explain ionization of molecules in flames direct ionization by thermoionization, photoionization, or chemiionization and indirect ionization by charge transfer with other ions. The assessment of both mechanisms requires knowledge of the ionization potential of molecules. In the following discussion, computations developed in the Appendix are used to estimate approximate ionization potentials of polynuclear aromatic hydrocarbons. 

The relatively high concentration of foreign molecules in flame AAS results in a Lorentzian broadening line value similar to that of the Doppler broadening value. The frequency shift may be considerable and lead to a... 

W. Ernst, Doppler-free polarization spectroscopy of diatomic molecules in flame reactions. Opt. Commun. 44, 159 (1983)... 

Independent of the sequential or simultaneous detection capability, a very interesting aspect for the application of HR-CS AAS using existing instrumentation is certainly the possibility to determine elements via their diatomic molecules in flames as well as in graphite furnaces. The examples for P via PO and S via CS, which have been presented in Sections 8.1.6 and 8.1.7 may be only the beginning of various similar investigations. Especially non-metals, such as the halogens, will surely be at the center of interest. 

A second mechanism of heat transport is illustrated by a pot of water set to boil on a stove - hotter water closest to the flame will rise to mix with cooler water near the top of the pot. Convection involves the bodily movement of the more energetic molecules in a liquid or gas. The third way, that heat energy can be transferred from one body to another, is by radiation this is the way that the sun warms the earth. The radiation flows from the sun to the earth, where some of it is absorbed, heating the surface. 

Ionisation detectors. An important characteristic of the common carrier gases is that they behave as perfect insulators at normal temperatures and pressures. The increased conductivity due to the presence of a few charged molecules in the effluent from the column thus provides the high sensitivity which is a feature of the ionisation based detectors. Ionisation detectors in current use include the flame ionisation detector (FID), thermionic ionisation detector (TID), photoionisation detector (PID) and electron capture detector (ECD) each, of course, employing a different method to generate an ion current. The two most widely used ionisation detectors are, however, the FID and ECD and these are described below. 

FIGURE H.3 When methane burns, it forms carbon dioxide and water. The blue color is due to the presence of C2 molecules in the flame. If the oxygen supply is inadequate, these carbon molecules can stick togelher and form soot, thereby producing a smoky flame. Note that one carbon dioxide molecule and two water molecules are produced for each methane molecule that is consumed. The two hydrogen atoms in each water molecule do not necessarily come from the same methane molecule the illustration depicts the overall outcome, not the specific outcome of the reaction of one molecule. The excess oxygen remains unreacted. 

Low energy ion-molecule reactions have been studied in flames at temperatures between 1000° and 4000 °K. and pressures of 1 to 760 torr. Reactions of ions derived from hydrocarbons have been most widely investigated, and mechanisms developed account for most of the ions observed mass spectrometrically. Rate constants of many of the reactions can be determined. Emphasis is on the use of flames as media in which reaction rate coefficients can be measured. Flames provide environments in which reactions of such species as metallic and halide additive ions may also be studied many interpretations of these studies, however, are at present speculative. Brief indications of the production, recombination, and diffusion of ions in flames are also provided. 

Most information concerning ion-molecule reactions in flames has been obtained from mass spectrometric measurements, but some inferences have been drawn from results of other types of experiments... 

We shall confine ourselves largely to a discussion of past and current experimental work on ion-molecule reactions in flames much of the interpretation must, in the light of our present knowledge, remain highly speculative. Brief indications of the origins and decay mechanisms of ion concentrations are also included. 

Ion-Molecule Reactions in Nonhydrocarbon Flames. Ion-molecule reactions which play important parts in flame ionization phe-... 

Sugden and Schofield (33) suggest that this reaction (with a rate constant 40-8 cm.3 molecule-1 sec.-1) can account for the boost in ionization of sodium observed when strontium salts are supplied to flames containing sodium. There is evidence (24, 33, 36) which strongly suggests that equilibrium ionization of strontium in flames is rapidly established via... 

Table VI. Rate Constants for Ion-Molecule Reactions in Flames...

Commercially available flame retardants include chlorine- and bromine-containing compounds, phosphate esters, and chloroalkyl phosphates. Recent entry into the market place is a blend of an aromatic bromine compound and a phosphate ester (DE-60F Special) for use in flexible polyurethane foam (8). This paper describes the use of a brominated aromatic phosphate ester, where the bromine and phosphorus are in the same molecule, in high temperature thermoplastic applications. 

The N atoms could form NO, in part at least, by reactions (8.50) and (8.51), and the CN could yield NO by oxygen or oxygen atom attack. It is well known that CH exists in flames and indeed, as stated in Chapter 4, is the molecule that gives the deep violet color to a Bunsen flame. 

The early patent disclosures have claimed the application of a wide spectrum of gas-evolving ingredients and phosphorus-based organic molecules as flame retarding additives in the electrolytes. Pyrocarbonates and phosphate esters were typical examples of such compounds. The former have a strong tendency to release CO2, which hopefully could serve as both flame suppressant and SEI formation additive, while the latter represent the major candidates that have been well-known to the polymer material and fireproofing industries.The electrochemical properties of these flame retardants in lithium ion environments were not described in these disclosures, but a close correlation was established between the low flammability and low reactivity toward metallic lithium electrodes for some of these compounds. Further research published later confirmed that any reduction of flammability almost always leads to an improvement in thermal stability on a graphitic anode or metal oxide cathode. 

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