The stabilities of the gas-phase ions CO - 3 and HCO - 3 , together with the kinetics of both their production and removal in O 2 -rich flames of H 2 + O 2 + N 2

Abstract

Six well-defined laminar flat premixed oxygen-rich flames of H 2 + O 2 + N 2 have been burnt with CO 2 added to the burner supplies. In addition, trace quantities of an alkali metal (K or Cs) were present and provided free electrons in the hot gases (temperature ~ 2000 K). The positive ions were found to be K + or Cs + by continuously sampling a flame into a mass spectrometer. The negatively charged species were mainly OH - , O - 2 , CO - 3 , HCO - 3 and the free electron. It is concluded that HCO - 3 is formed from OH- in the forward step of OH - + CO 2 + M ⮂ HCO - 3 + M, (I) where M is any molecule in the flame capable of removing energy from the other two reacting species in the forward step. In fact, well downstream in a flame, reaction (I) is equilibriated and also has a time constant small enough for its equilibrium position to shift, while the sample is cooled on entering the mass spectrometer. Techniques were found to quantify these perturbations of an ion spectrum, which enabled the equilibrium constant of (I) to be deduced as a function of temperature. As a result, values of Δ H ᶱ and Δ S ᶱ for reaction (I) were also obtained. That reaction (I) is equilibrated in these flames and also is perturbed by the process of sampling into a vacuum system enables the rate constants for the forward and reverse steps to be quantified. The ion CO - 3 is at least three times more abundant than HCO - 3 . The reactions HCO - 3 + OH ⮂ CO - 3 + H 2 O (X) are responsible for creating and removing CO - 3 Reaction (X) is also equilibriated well downstream in a flame, but its equilibrium position is normally not shifted during sampling. Thus, it proved possible to measure its equilibrium constant over a range of temperatures, leading to corresponding values of Δ H ᶱ and Δ S ᶱ. That reaction (X) appears not to be shifted on cooling a flame sample while it enters the mass spectrometer, in fact, enables values of the rate coefficients for the forward and reverse steps of (X) to be deduced. In addition, the stabilities of both CO - 3 and HCO - 3 are fully characterized by this study. The situation early in one of these flames, i.e. in or near the reaction zone, seems to be one where steady-state relationships hold, rather than equilibrium being established locally.