July, 1956
VAPORPRESSURE AND DIELECTRIC CONSTANT OF DIBORANE
911
VAPOR PRESSURE AND DIELECTRIC CONSTANT OF DIBORANE' BY HENRYE. WIRTHAND EMIEL D. PALMER Department of Chemistry, Syracuse University, Syracuse, N . Y . Received December Y, 1966
The vapor pressure of diborane is represented by the equation loglo pmm= 6.9681 - 674.82/( T - 15.02), (108."K. < T < 150°K.) and log,, pmm= 6.61885 - 583.120/(T - 24.63), (150°K. < T < 181°K.). The boiling point of dtborane is 180.63 f 0.02"K.,and the heat of vaporization a t the boiling point, as calculated from the vapor pressure equation, is 3,413 cal./mole. Two melting points were found for diborane: 108.14 f 0.02"K. and 108.30 f 0.02"K. The first value is probably that of a metastable phase. No hase transitions'were observed in the solid. Diborane is non-polar, and the dielectric constant of the liquid is represented k?y the equation E = 2.3721 - 0.0027653T. and 300"K., and as 15.9 X per degree between 80 and Introduction the calculated capacitance of the empty cell was The vapor pressures of diborane previously pub- 300°K.,7 81.83 ppF. a t 20"K., and 81.85 p p F . a t 80°K. The cell l i ~ h e d are ~ - ~reported with an accuracy of 1%. The capacitance was assumed to vary linearly with femperaturc present work was undertaken to obtain precise val- above 80°K. The dielectric constant of liquid and solid oxygen was ues for the vapor pressure of diborane, and to inves- determined to check the cell calibration and the sensitivity tigate the use of dielectric constant measurements of the dielectric constant method for the detection of phase in establishing phase transitions in the solid state. transitions. The oxygen was prepared by electrolysis between platiExperimental num electrodes of 20% KOH solution. The evolved oxyApparatus .-The dielectric constant-vapor pressure cell (Fig. 1) was constructed from a cylindrical copper block (A) 6.4 em. in diameter by 19 em. long. I n the center of the block a hole 1.07 em. in diameter contained the cell proper. The cell consisted of a dopper cylinder (C) (12.2 em. long, 0.92 em. 0.d. and 0.80 em. i.d.) supported by glass spacers on a copper rod (B) 0.63 cm. in diameter. The wire (D) to the "hot" cylinder passed through insulated terminals (E) and out of the cell. The inner surfaces of the cell were silver plated and polished before assembly. Approximately 6 ml. of liquid was required to fill the cell to the point where addition of more liquid did not change the capacitance. The cell was mounted in an assembly similar to that used in a Nernst type calorimeter including an upper block with shield and an auxiliary block through which all heater and thermocouple leads passed. The assembly was placed in a Collins helium cryostat, enabling measurements to be made at any temperature between 8°K. and room temperature. The sample inlet tube was connected to a gas handling system, to a vacuum system, and to a manometer made of 2.2 em. precision bore tubing maintained at 30 f 0.1". The temperature of the cell was determined with copperconstantan thermocouples6 which had been calibrated against the Ohio State helium thermometer. A Schering type bridge (General Radio Type 716-C) was used to determine capacitance and dissipation factor. Measurements were made in the frequency range 1-200 kilocycles. Calibration of the Capacitance Cell.-The capacitance of the cell was found to be 82.11 ~ M Fa t. room temperature, using benzene ( e = 2.2747 a t 25")6as a standard. The capacitance of a condenser consisting of two coaxial cylinders is given by.the relation
where 1 is the length of the cylinders and r1 and r p are their radii. Therefore the only effect of temperature on the capacitance would be due to the change in length of the cylinders. With the cell used, which is made up of two such simple condensers, the length of the central cylinder is the determining factor. Taking the average linear coefficient of expansion of copper as 12.6 X 10-6 per degree between 0 (1) Presented at the New York meeting of the American Chemical Society, September, 1054. (2) A. Stock and E. Kuss, Eer., 66, 789 (1923). (3) John T.Clarke, E. B. Rifkin and H. L. Johnston, J . A m . Chem. Soc., 76, 781 (1953). (4) E. B. Rifkin, E. C. Kerr and H. L. Johnston, ibid., 76, 785 (1053). ( 5 ) The thermocouples were supplied b y the Cryogenic Laboratory of The Ohio State University through the courtesy of Professor T . R. Rubin. (6) A . R. Brown and E. W. Abraliarnson, J . Chem. Plrya., 19, 1226 (1051).
gen was passed through tubes containing KOH pellets, CuO a t 450", amalgamated Cu turnings, and Mg(C104)Z. It was collected in a trap cooled with liquid NP,and then distilled into the dielectric constant cell which was held at 60°K. About one ml. more oxygen than that required to give a constant capacitance reading was condensed in the cell. On cooling, the oxygen usually first formed a glass,* then abruptly crystallized a t about 40°K. The cell was then warmed to the melting point of oxygen to permit liquid to fill the voids produced when the glass crystallized. After cooling to 10"K., measurements were made as the temperature was increased. Breaks in the dielectric constant-temperature curve were observed a t temperatures corresponding to the two known solid-solid phase transitions of oxygen and a t the melting point. The transition temperatures observed were 01 + P 23.68 f 0.02"K., ,6 - y 43.81 f 0.02"K., and y liquid, 54.38 f 0.02'K., in excellent agreement with those reported by Giauque and Johnston.9 There are large errors in the absolute magnitudes of the dielectric constants obtained as this type of cell could not be completely filled with solid. However, abrupt changes in the dielectric constant were always observed a t temperatures corresponding to the phase transitions. The empirical equation e 1.6751 - 1.698 X 10-8T - 4.00 X 10-'T2 (1) fits the ten observations between 54.4 and 89.9"K. made on liquid oxygen with an average deviation of f 0 . 0 0 0 2 . These values average 0.0062 unit higher than those reported by Werner and Keesomlo on oxygen containing 2.4% nitrogen. After correcting for the error due to the nitrogen, our values are 0.005 unit higher. A sample of tank electrolytic hydrogen was liquefied, distilled, and the middle fraction retained for use. The empirical equation E = 1.2688 3.96 X 10-4T - 1.125 X 10-4T2 (2) fits the eight observations between 13.8 and 20°K. made of the dielectric constant of liquid hydrogen with an average deviation of f0.0003. These values agree with those reported by Werner and Keesomll with an average deviation of f0.0008 unit, but are 0.0025 unit higher than those reported by Guillien.12 It is possible that our values for 6-1 may be high by as much as 1%. Since Werner and Keesom used the same cell for their measurements on both oxygen and hydrogen, and since the oxygen used by them was known to be impure, we -f
+
(7)
M.Wolfke and H. K. Onnes, Leiden Comm., 171b.
(8) W. Wahl, PTOC.Roy. Soc. (London), ASS, 61 (1913).
(9) W. F. Ciauque and H. L. Johnston, J . Am. Chem. Soc., 61, 2300 (1029). (10) W.Werner and W. H. Keesom, Leiden Comm.,178.. (11) W. Werner and W. H.Keesom, abad., 178a. (12) R . Guillien, Reu. sei., 'IT, 575 (1939).
HENRY E. WIRTH,AND EMIEL D. PALMER
912
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,c
i9
Vol. GO
sample (Series 11), only one agreed with the value found previously. The other measunements gave a sharp melting point a t 108.14"K. The indicated purity was 99.95 mole per cent., based on the fraction melted vs. temperature curves. It is believed that diborane can exist in two solid phases, a stable phase melting a t 108.30"1
