The measurement of high proton affinities for a variety of metallic compounds: a new approach by flame-ion mass spectrometry

Abstract

A small amount (≤ 10 −6 mol fraction) of four alkaline earth metals, tin and yttrium were introduced into five, premixed, fuel-rich, H 2–O 2–N 2 flames at atmospheric pressure in the temperature range 1820–2400 K. Aqueous salt solutions of the metals were sprayed into the premixed flame gas as an aerosol using an atomizer technique. Ions in a flame were observed by sampling flame gas through a nozzle into a mass spectrometer. The concentrations of the major neutral metallic species present in the flame were calculated from thermodynamic data currently available. The principal metallic ions observed were AOH + (A = Mg, Ca, Sr, Ba, Sn) and A(OH) 2 + (A = Y), formed initially by proton transfer to AO and OAOH from H 3O +, a natural flame ion. Except for Mg, the ions were also produced by chemi-ionization processes. By adjusting the concentration(s) of the salt solution in the atomizer, it was found that a pair of ions could be brought into equilibrium within the time scale of the flame; the pairs included H 3O + with a metal ion or two metallic ions. Because water is a major product of combustion, a very large difference in proton affinity PA 0(AO) − PA 0(H 2O) ≤ 490 kJ mol −1 (117 kcal mol −1) could be attempted for the proton transfer equilibrium. Using PA 0(H 2O) = 691.0 kJ mol −1 (165.2 kcal mol −1) as a reference base to anchor the proton affinity scale, ion ratio measurements led to proton affinity PA 0 values of 766, 912, 1004, 1184, 1201, and 1222 kJ mol −1 (183, 218, 240, 283, 287, and 292 kcal mol −1) corrected to 298 K for OYOH, SnO, MgO, CaO, SrO, and BaO, respectively; of these, only the value for OYOH has not been reported previously. If it is assumed that the neutral thermodynamic data are correct (although some appear to be in error), the uncertainties in the PA results reported here are ± 21 kJ mol −1 (5 kcal mol −1). The realization that these equilibria can be achieved in flames provides a new approach to consolidate and build the high end of the proton affinity ladder, primarily of metallic species which are not accessible at lower temperatures.