Two methods were used to measure the vapor pressures of zirconium tetrachloride and hafnium tetrachloride over the range from 10 to 10,000 mm Hg. The lower pressures (10-1000 mm) were measured in a nickel diaphragm apparatus1 which has been used successfully on other compounds by this laboratory and others2 recently. The higher pressures were obtained using a modification of the capillary "bridge" apparatus which was used by the U. S. Bureau of Mines3 in the extensive investi gation of the vapor pressures of common metal chlorides.The charge materials were prepared from oxides purchased from De Rewal International Metals Company. The ZrO2 contained less than 0.1 per cent HfO2 and the HiO2 was 98.7 per cent pure.l The oxides were purified by a mandelic acid precipitation4 to remove impurities such as iron, titanium, and calcium.The vapor pressure curves of both salts are shown on the accompanying graph. The equations for the curves are: zrci4 10g P = -5'Ír00 + 11.76Hfc14 log P = -sïooThe melting points were found to be 437°C for ZrCl4 and 434°C for HfCl4.By extrapolation, the vapor pressure of ZrCl4 at its melting point is 14,500 mm, and the vapor pressure of HfCl4 at the same temperature is 25,000 mm.The data of this report are in close agreement with and appreciably extend the range of the data of the Ames Laboratory.5The investigation of the high pressure range is being continued and it is hoped that it will be possible to obtain data above the melting points in the near future. In any event, this brief memo randum will be followed by a complete technical report which will be issued before January 1, 1950.

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