The formation of SO5− by gas phase ion–molecule reactions

Abstract

A flow tube apparatus was used to investigate the formation of SO5− ions by gas phase ion–molecule reactions. The reactions studied in an N2 buffer gas at 2.5 hPa pressure and room temperature (298 K) included SO2 and O2 reactions with O2−, O3−, CO3−, SO2−, and SO3− as well as their hydrates. Reaction rate constants were measured and the major product channels were identified for most reactions. The free energy changes for the hydration reactions of SO3−, SO4−, and SO5− were derived from equilibrium constant measurements. The present investigations clearly show that SO5− ions are formed in the gas phase by the association of O2 to SO3− and by the switching reaction of SO3−H2O with O2. An effective binary rate constant of 2.0×10−12 cm3 s−1 was measured for the association reaction at 2.5 hPa N2 and the rate constant of the switching reaction was 5.0×10−11 cm3 s−1. Also the reaction of O3−H2O with SO2 probably yields SO5− by a switching process having a rate constant of 1.8×10−9 cm3 s−1. The heat of formation of SO5− was estimated to be less than −715 kJ/mol. The present results have implications to the negative ion chemistry of the atmosphere and are important for measurements of atmospheric SO2 concentrations by chemical ionization mass spectrometry.