Theoretical calculations of equilibrium concentrations for species generated in analytically important flames

Abstract

The compositions of several flames at various temperatures and differing oxidant/fuel ratios have been determined theoretically by use of an iterative computer technique based on a thermodynamic model. Systems chosen were: acetylene-nitrous oxide, acetylene-oxygen, acetylene-air, hydrogen-nitrous oxide, hydrogen-oxygen, and hydrogen-air. In view of the importance that acetylene-nitrous oxide flames have in contemporary analytical techniques, this particular system has been examined further when containing a metallic vapor. The calculations indicate that the free-atom fractions for aluminum, molybdenum, silicon, titanium, and tungsten vary with the fuel/oxidant ratio, but that copper and ironfree atom fractions remain relatively invariant with changes in flame stoichiometry.Several methods have been described to estimate the concentrations of various species present in a flame. One approach involves finding a product distribution that will satisfy mass balance criteria and restraints imposed by equilibrium constants for a variety of different reactions (2, 73). A feature of this method is that simplifications may be made by recognizing that some species will be present in very small concentrations, and that the presence of these species is relatively unimportant. However, this method utilizes a set of simultaneous equations to describe the equilibrium processes, and for even moderately complex systems, the set becomes quite formidable. Another method (76), known as the free-energy minimization method, considers all possible gaseous species [condensed species can be handled also ( 7 4 ) ] that might be produced, but in return, eliminates the need for a full knowledge of relative concentrations and therefore avoids the necessity of deciding which species are the major components in the flame. However, even for this method, practical considerations and/or chemical intuition can lead to substantial reductions in computational times. Results of equilibrium calculations by the free-energy minimization method have been reported by Anderson ( 7 ) , and by Taylor and co-workers (3, 4 ) . This paper further extends and amplifies these calculations to flames that are of interest to contemporary analytical chemistry, and includes calculations involving seven selected metal vapors to simulate flame spectroscopy involving added analytes. Experimental A Fortran IV G program was written for an IBM 360/75 computer based on experimental techniques described by White et al. (73, 76) and Oliver et al. ( 7 4 ) . The equilibrium concentrations of the n species that comprise the flame system are given in terms of a set of mole numbers X = XI, x2, x3 . . . xn. The required set X is that one which will conditionally minimiqe the total free energy of 'Deceased. 2To whom correspondence should be addressed.the system subject to overall mass-balance considerations. The total free energy, GfX), can be written as the sum of the chemical potentials, k i , for the species i such that i 1)The chemical potential for species i in an ideal gas mixtur...

Characterization and optimization of flame photometric detection in supercritical fluid chromatography

“…where A, B.....Cp, and X are the molecular and radical species that makeup the flame, their molar heat capacities, and the mole fraction, respectively. For hydrogen-oxygen flames with H2/02 flow rate ratios equal to approximately 0.2, the main components of the flame are water and hydrogen with approximate mole fractions of 0.4 and 0.6, respectively (24). The mole fractions of hydrogen, hydroxyl, and oxygen radicals present under these conditions are 0.04, 0.008, and <0.001, respectively (24).…”

“…For hydrogen-oxygen flames with H2/02 flow rate ratios equal to approximately 0.2, the main components of the flame are water and hydrogen with approximate mole fractions of 0.4 and 0.6, respectively (24). The mole fractions of hydrogen, hydroxyl, and oxygen radicals present under these conditions are 0.04, 0.008, and <0.001, respectively (24). With this information, eq 3 predicts that at optimum gas chromatographic conditions, 20 mL/min helium carrier gas will decrease the thermodynamic flame temperature by 142 K. Whereas, under the same flame conditions flowing carbon dioxide into the flame at a rate of 32 /iL/min from a column at 40 °C and 272 atm, causes a decrease in the thermodynamic temperature of 189 K. From these calculations, the optimum 02/H2 ratio for SFC/FPD would be predicted to be higher than that of GC/FPD since the flame temperature lowers proportionately to increasing hydrogen.…”