The stabilities of the gas-phase ions Li+·H2O, Li+·(H2O)2 and Li+·CO as measured by mass-spectrometric sampling of fuel-rich flames of C2H2 + O2

Abstract

Fuel-rich flat flames of C₂H₂ + O₂ with CO₂ as diluent have been burnt at 1 atm; such flames constituted plug flow reactors without walls and had temperatures of 2020–2500 K. Each flame was continuously sampled directly into a mass spectrometer; this revealed the formation and presence of the ions: Li⁺, Li⁺·H₂O, Li⁺·(H₂O)₂ and Li⁺·CO, when trace amounts of lithium were added to the gas supplies of these flames. A detailed examination showed that equilibria like: Li⁺ + H₂O = Li⁺·H₂Oare set up extremely rapidly in a flame. Furthermore, when such a flame at 1 atm is sampled into the mass spectrometer at a much lower pressure, the sample is cooled, with the result that these equilibria shift position to those for lower temperatures. By varying the diameter of the sampling orifice it proved possible to measure the equilibrium constant of (4) for conditions at the throat of the sampling hole, i.e., where the local Mach number is unity and the temperature and pressure are lower than in the flame. This has led to equilibrium constants being measured for (4) (and also for the formation of other cluster ions) at a variety of temperatures. The results are summarised in terms of values of ΔH and ΔS for the monohydration of Li⁺ and Li⁺·H₂O; the values are compared with those measured in flames for nine other gas-phase ions. All the ΔS are very much as expected, but the ΔH do show interesting variations. Measured values of ΔH and ΔS for the formation of Li⁺·CO from gaseous Li⁺ and CO are also reported.