The Calculation of Normal Vapor Pressures from the Data of the Gas Current Method, Particularly in the Case of Iodine

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Abstract
The gas current method of determining the vapor pressure of slightly volatile substances is capable of a high degree of experimental precision and has often been applied. The question—whether the vapor pressure, so determined and subsequently corrected only for the Poynting effect, is accurately equal to the normal vapor pressure of the substance—has not hitherto been the subject of theoretical investigation. A method, dependent on recent developments in the thermodynamic treatment of mixtures of real gases is here developed for calculating the normal vapor pressure from the data of the gas current method. The suggested method requires data at varying total pressure for one inert gas at several temperatures and relatively accurate data at one atmosphere total pressure for two inert gases over the range of temperature. The one-atmosphere data for at least one of the inert gases must be of the precision desired on the normal vapor pressures. The method is applied to the case of the vapor pressure of solid iodine from 0 to 100°C, for which substance data exist which are suitable, though not obtained for the most appropriate choice of experimental conditions. The new corrections (for deviations from the laws of ideal gases) are found to be important numerically in the case of the best measurements. The errors inherent in the various methods of applying the gas current method are briefly discussed. A graphic, empirical, method is found liable to a characteristic source of error.