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The measurement of the vapour pressure of mercury in the intermediate pressure range Dauphinee, Thomas McCaul

Abstract

An apparatus for determination of vapour pressures of metals in the pressure range 5 x 10⁻³ to 10² mm. Hg. by the streaming or transpiration method is described. The carrier gas is circulated, and purified, in a closed system. Saturation is achieved by condensing out an excess of the metallic vapour. Very large condensing surface and increased flow are used, with provision for checking the effects of flow, initial quantity of metallic vapour, total pressure and temperature gradients. The quantity of metal carried over is determined by titration. Results of measurements of the vapour pressure of mercury from 5 x 10⁻³ mm. to 60 mm. (30°C. to 240°C.) are given. Low temperature values are significantly above previous determinations, but are in complete agreement with the vapour pressure formula proposed by Ditchbum and Gilmour in 1941. log p. = 10.3735 – 3308/T = 0.8 log T Values given by the International Critical Tables are shown to be inaccurate below 140°C. and equations based on free energy calculations are shown to be inconsistent and unsuitable for extrapolation to low pressures. The Ditchburn and Gilmour formula is recommended for all temperatures between 140°C. and -38°C. (the freezing point). The error at the lower temperature is probably less than 4%.

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