T H E VAPOR PRESSURE OF SILICOK TETRAFLUORIDE' B Y WI?*TON I. PATNODE APZD JACOB PAPISH
During an investigation of the occurrence of germanium in silicate minerals* it was observed that minerals which were fluoriferous contained considerably more germanium than their associates which were free from fluorine. This applies to specimens of topaz, tourmaline and lepidolite when compared with cleavelandite, beryl and cassiterite. An assumption was made, that preceding the pneumatolytic reactions involved in the formation of the fluorine-bearing minerals, the germanium and some of the silicon may have been ejected as the volatile tetrafluorides, which in turn may have formed constant-boiling mixtures. It was deemed desirable to find out whether such mixtures can be obtained under laboratory conditions. The results were inconclusive because it was learned upon experimentation that within the pressure ranges for which t'he apparatus was designed the system GeF4-SiFt is practically immiscible. I n connection with this work, however, it was necessary to investigate the vapor pressure-temperature relations of silicon tetrafluoride. A search of the literature on this subject yielded scant information. The gas was first liquefied by Faraday: who found that a t a temperature of -105.5' and under a pressure of nine atmospheres, the gas condensed to a clear, limpid liquid. Olschewsky4 found that the substance sublimed a t a temperature of - 102' under a pressure of one atmosphere. Moissan5 gave -97' as the sublimation temperature a t a pressure of one atmosphere, and -77' at two atmospheres as the triple point. Ruff and Albert6 gave --go as the sublimation temperature under one atmosphere of pressure. I n view of the scantiness and conflicting nature of the information it was decided to undertake the investigation of the vapor pressure-temperature relations of silicon tetrafluoride by working with the pure gas. Silicon tetrafluoride was prepared by treating an intimate mixture of calcium fluoride and an excess of silica with concentrated sulphuric acid. The evolved gas was passed through U-tubes containing glass wool, anhydrous calcium chloride, and phosphorus pentoxide, in the order named. It was then condensed in a trap immersed in liquid air. This trap was attached to the apparatus' by means of a glass slipjoint. The gas was purified by fractional 1 The investigation upon which this article is baaed was supported by a grant from the, Heckscher Foundation for the Advancement of Research, established by August Heckscher at Cornel1 University. 2 See Pagish: Econ. Geology, 24, 470 (1929). SFaraday: Phil. Trans., 13514, I j j (184j). Olschewsky: Monatshelft, 5, 128 (1884). 6 Moissan: Compt. rend., 139, 7 1 1 (1904). 8 Ruff and Albert: Ber 138, 5.3 (190,s). 7 This apparatus was very similar to that described by Laubengayer and Corey: J. Phys. Chem., 30, 1043 (1926). ~
THE V.4POR PRESSURE O F S I L I C O S TETRAFLUORIDE
I495
FIG. I
TABLE I Vapor Pressure-Temperature Relations of Silicon Tetrafluoride Temperature degrees C .
Pressure in mm.
-12j.1
22
-121.5
42
-115.8
68
-113.7
93
-112.7
I22
-110.7
162
-108.7
198
-107.7 -105, j
2 13
-104
335
280
Temperature degrees C.
Pressure in mm.
-90.5 -89.3 -88.3 -87.7 -86.1 -84.6
1317 1415
-84.0 -82.5 -82 .o
-99.8 -97.7 -95.5
63 8
-81 . o - 79 .o -78.3 -77.7
7;s
-i6.3
-94.3
1887
_--
-93.4 -91.5
I001
-75
0
-101.8
SI5
I221
IjII I 600
1698 1906 2077
2198
2304 2428 2670
1 3 . 8
2789 2889 3110 3160
5
3238
W I N T O S I. PATSODE A S D J h C O B PAPISH
I496
distillation under pressure as recommended by Germann and Booth.8 Several distillations were made, discarding the first and last fractions. The vapor density of esch of the middle fractions mas determined by direct weighing. A constant value of 4.680 grams per liter was found for the middle fractions of the last two distillations, and this value is well in agreement with that obtained by Germann and Booth.* Determinations of the vapor pressure-temperature relations of silicon tetrafluoride were made by the method described by Dennis, Corey and h l 0 0 r e . ~ The high pressure needed to balance the manometer was supplied from a tank of compressed nitrogen, and t,he apparatus was calibrated with the aid of purified germanium tetrafluoride.’O A typical set of data for silicon tetrafluoride is recorded in Table I. I n order to determine the triple point, the logarithms of the pressures were plotted against the reciprocals of the absolute temperatures. The graph thus obtained consisted of two straight lines, one of which represented the solid-vapor range and the other the liquid-vapor range. These lines intersected at a point corresponding t o -90’ and 1318 mm. The equations of the curves calculated from these data are:- For the solid-vapor curve, log p = - 1346.2 X I / T 10.48. For the liquid-vapor curve, log p = -97 j.0 X
I/T
+ 8.453.
+
The vapor pressure-temperature curve (see Fig. I ) was drawn with the aid of data obtained from t,he logarithmic curve. The boiling point, or more correctly, the sublimation temperature at one atmosphere as derived from this curve is -95.7’. Cornell L’wai,ci.,slty j
lo
Germann and Booth: J. Phys. Chem.. 21, 81 (1917). Dennis, Corey and Moore: J. Am. Chem. Soc., 46, 663 (1924:. See Dennis and Laubengayer: Z. physik. Chem., 130, 520 (192;).
