Ions in flames

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. 

Negative Ions. Negative ions in flames have been studied less widely than positive ions because they are present in much smaller concentrations typically, n+//i 100. The primary source of negative ions has been suggested (20) to be Reaction 3, comparable to Reaction 4 (see the section on flames with halogen additives). 

Collisional Detachment. Reactions of negative ions in flames not containing hydrocarbons have not been widely studied, although OH -ion formation is important in flames containing high electron concentrations. The rate constant k l of the reaction... 

With the irradiated probe negative, the ionization signal decreased by 10% when the distance between the probes was increased from 2 to 10 mm. However, a decrease in ionization signal by a factor of 5 occurred in the same experiment with the polarity of the probes reversed (see Fig. 1). This observation shows that the current is limited by the sodium ions. Calculations, using literature values (8,9) for the mobility of sodium ions in flames, support this observation. 

Green and Sugden (5), based on the studies of the positive ions in flames by mass spectrometry, prefer the value derived from the proton affinity of water obtained by Tal rose and Frankevitch, rather than the electron impact data. From the approximate equilibrium constant calculations for the reaction HCO (g) + H 0(g) = H 0 (g) + CO(g), Clifton (6) has also found that the heat of... 

Ashton and llayhurst [56] rationalized it in terms of collisional ionization and recombination of electrons with alkali metal ions in flame, for example ... 

Lovelock often simply tried to measure the response of every volatile compound in the laboratory to characterize a detector. This method of inquiry was a Lovelock trademark later adopted by the Wentworth group. In the first half of the twentieth century few reactions of thermal electrons with molecules were studied. Some studies of electrons and ions in flames and electron swarms were conducted during this time. H. S. W. Massey described the work on negative ions in a series of monographs [3]. 

In the 1980s Shuie, D sa, Limero, Gigi Bear, and Bernard White were the doctoral students at Wentworth. The latter two studied negative ions in flames. Master s student Hernandez examined the response of the ECD at short reaction times. Limero studied fluorobenzenes. D sa discovered a fourth temperature region in the ECD for chloro and bromoethylenes. Limero, D sa, and Shuie researched electron capture in the atmospheric pressure chemical ionization system and obtained data for anion complexes using such equipment. Batten became intimately involved with this work. Lee and R. Ranatunga, the doctoral students of Zlatkis, made additional contributions. 

The existence of ions in flames has been known for many years, with Langmuir probes (JO and mass spectrometric sampling (2) used in a number of the early investigations. However, ion number densities were found to be much higher than could be explained by equilibrium thermal ionization at the temperatures occuring in flames. Calcote (3) first suggested the chemiionization mechanism... 

The potassium hydrate K OH2 was detected by Chupka " in a mass spectrometric study of ions emerging from a heated Knudsen cell containing KCl salt, and water as an impurity. The ion clusters Ar OH2, where M = Li, Na, and K, were also mass spectrometrically observed in a study of ions in flames by Hayhurst and Sugden. " The ions were produced by spraying an aqueous solution of the alkali chlorides in a premixed H2, 02,N2 flame at 1 atm. The authors also attempted to measure the ionic equilibria -h H2O = M OH2. However, their data are inconsistent, which is not surprising, considering the complexity of the experimental conditions and arrangement. 

The reverse of reaction (21) can be measured in a nonhydrocarbon flame, e.g., in an H2/O2/N2 flame where equilibrium ionization is approached slowly. From the measured forward ionization rate and the equilibrium constant, the recombination rate coefficient can be obtained. The measurements of recombination coefficients of metal ions in flames at 1 atm have been compiled as recommended values in Table II. Flame studies have... 

TABLE II. Recommended Recombination Coefficients for Metal Ions in Flames ... 

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