The heat of dissociation of H5+ derived from measurements of ion mobilities

Abstract

The mobility of hydrogen ions in hydrogen at 240, 274, and 295 K has been measured over the E/N range 0.5–10 Td and at gas number densities in the range 8×1016–5×1018 cm−3 using a variable length drift tube of the four gauze type. The absolute error and statistical scatter in the mobility values are estimated to be <±0.5% and <±0.15%, respectively. The equilibrium constant for the reactions H3++H2+H2⇄H5++H2 was obtained by fitting to the variation of the measured mobility with gas number density at low E/N values. Checks were made to ensure the reactions were in equilibrium in the drift space. The derived equilibrium constants and the heat of dissociation 250 meV were found to agree with those of Elford and Milloy and to disagree with those of previous workers. Possible causes for the disagreement are discussed. The zero field reduced mobility of a number of impurity ions in H2 at 293 K was measured. The values were as follows (in cm2 s−1 V−1), CH5+ (11.8±0.1), H3O+ (10.5±0.1), N2H+ (12.1±0.1), and H5O2+ (11.0±0.1).