The vapor pressures of the isotopic acetylenes: C2H2, C2HD, and C2D2, have been investigated by mercury manometry. Samples of relatively pure C2HD were prepared for this purpose by lowhyphen;temperature gas chromatography. The vapor pressure measurements were made from 0.5 to 800 mm pressure (125ndash;190deg;K) with a precision of sim;0.01 mm. The vapor pressure isotope effects were normal with the isotope effect for C2HD being slightly larger than the rule of the mean value. The data for C2D2showed a sharp change in the temperature dependence at 147.4deg;K which has been interpreted as a result of a solidndash;solid phase transition in C2D2. The experimental results have been considered in the light of the statistical theory of isotope effects in condensed systems. A model calculation made within the framework of this theory is in agreement with the experimental data.
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