Vapor pressure of liquid cesium by a Knudsen-effusion radiotracer technique

Abstract

The vapor pressure of liquid cesium has been measured over the temperature range 327 to 459 K using a Knudsen-effusion radiotracer technique. The vapor pressure over this temperature range is ( R cal th K −1 mol −1 ) ln( p atm ) = −(16493±522) ( K T )+(16.02±1.38) The third-law enthalpy for the reaction Cs(c) = Cs(g) obtained from the present results is ΔH o(298.15 K) = (18.44 ± 0.36) kcal th mol −1. The second-law result is ΔH o(298.15 K) = (17.31 ± 0.52) kcal th mol −1. Standard thermodynamic functions for Cs 2(g) are computed using a spectroscopically derived CsCs bond distance. The entropy of Cs 2(g) is computed to be S o(298.15 K) = 68.053 cal th K −1 mol −1, 0.2 cal th K −1 mol −1 higher than the previously reported value. Literature vapor pressures above 500 K are re-evaluated and compared with the present and literature low-temperature vapor pressures. On the basis of this evaluation, the vaporization enthalpy is chosen to be ΔH o(298.15 K) = (18.33±0.36) kcal th mol −1.