1412
ROGERL. JARRYAND HENRYC. MILLER
Vol. GO
THE LIQUID DENSITY, VAPOR PRESSURE AND CRITICAL TEMPERATURE AND PRESSURE OF NITROGEN TRIFLUORIDE1a2 BY ROGERL. JARRYAND HENRYC. MILLER Pennsylvania Salt Manufacturing Co., Research and Development Department, Whitemarsh Research Laboratories, Wyndmoor, Pa. Received April 97, 1866
The liquid density of nitrogen trifluoride has been measured over the temperature range 78 to 170°K. These data have been fitted to the equation d(g.lcm.9) = 2.103 - 3.294 X 10-3 T - 4.675 X 10-6 T*. The estimated uncertainty in the measured values is *O.l%. Vapor pressures have been measured to the critical point and are represented to a precision of zk0.17 atm. by the equation loglo P(atm.) = 4.27264 - 613.330 T-1. The vapor pressures to one atmosphere have been measured to a higher order of precision and the data have been fitted to the Antoine equation: log,, Ptmm.) = 6.77966 501.913/T - 15.37. The normal boiling point calculated from the Antoine equation is 144.10"K. Temperature was known in all cases to f0.05'. The critical temperature is 233.90 & O.lO°K., the critical pressure is 44.72 j=0.17 atm. acetone. The disappearance and reappearance of the meIntroduction niscus was noted by several observers and an average value As part of a program at this Laboratory on the of temperature used. physical properties of fluorine compounds, various Temperature was measured by means of a capsule type properties of nitrogen trifluoride have been meas- platinum resistance thermometer, or by means of thermocouples which had been compared with the thermometer. ured. The temperature range over which the The thermometer had been compared with the National liquid density had been previously measured by Bureau of Standards temperature scale. During the Ruffa has been considerably extended by this study. equilibrium periods of 15-20 min. during which measureVapor pressures were checked against the data of ments were taken the temperature drift in the metal system the dilatometer were no more than 0.002'/min. and Pierce and Pace4and extended to the critical point. or 0.004' /min. , respectively. The high vapor pressures and the critical constants Material.-The nitrogen trifluoride was prepared by have not appeared in the literature heretofore. the electrolysis of molten ammonium bifluoride as described
Apparatus and Procedure .-The density was measured between 97 and 170°K. using an all-metal apparatus, described in a previous publication.6 This system allowed measurements t o be made to 5 atm. pressure. For the lower temperature measurements a dilatometer comprising a glass bulb of about 4 cm.3 volume connecting to the gas system by means of 2 mm. i.d. capillary tubing was used. Ten cm. of this capillary tubing immediately above the bulb was calibrated along with the bulb using mercury. The bulb of the dilatometer was enclosed in a copper block and a second block placed about the capillary tubing above the calibrated section. This upper block was equipped with a heat leak which dipped into the refrigerant used, liquid nitrogen. The entire assembly was enclosed within a vacuum shield. By means of heaters on the two blocks and the heat leak the temperature of the blocks could be adjusted so as to have a minimum temperature differential over the calibrated 10 cm. of the capillary. By this means measurements could be made over the full length of the calibrated section. The nitrogen trifluoride was condensed into the measuring devices from calibrated volumes in an air thermostat. Since the range of determinations in the two devices overlapped, and the dilatometer required several fillings, the precision of filling by the volumation method could be readily checked. Vapor pressures were measured in the metal system in which the density device had been replaced by a nickel container connected by means of l/g in. nickel tubing to the senaing unit. Pressures to two atmospheres were read on a mercury manometer by means of a cathetometer to a precision of 3Z0.05 mm. Pressures to the critical point were taken on a 1000 p.s.i.g. Bourdon gage with an accuracy of k0.2570 of the total scale. Critical temperature was measured on samples sealed in 2 mm. i.d. capillary tubing. This tube was placed directly into a bath of cold acetone; temperature was controlled by a heater immersed in the bath and/or the addition of cold (1) Presented at the Delaware Valley Regional Meeting, Philadelphia, February 16, 1956. (2) This paper represents the results of one phase of research carried out under Contract No. 18(600)-761, supported by the United States Air Force through the Air Force O 5 c e of Scientific Research of the Air Research and Development Command. (3) 0. Ruff, 2. anoro. alloem. Chem., 197, 273 (1931). (4) L. Pierce and E. L. Pace, J. Chem. Phys., 23, 551 (1956). (5) R. L. Jarry and H. C. Miller, J . Am. Chem. Soc., 7 8 , 1052 (1956).
by 0. Ruff and co-workers.6 A nickel anode was used to prevent the formation of carbon tetrafluoride. The crude gas resulting from the electrolysis was purified by low temperature filtration and distillation using a still set-up and controls as described by Booth and Jarry.' As a check of the purity, both infrared spectra and the freezing point were determined. The infrared scans were compared with the data of Wilson and Polo* and later of Pace and Pierce.@ The spectra found for the purified material corresponded to the later work with its inclusion of two peaks, at 7.8 and 6.45 p which appear to be part of the nitrogen trifluoride spectra and were not reported by Wilson and Polo. The freezing point was determined on two samples and gave an average value of 66.49'K. as compared to the value of 66.37'K. given by Pierce and Pace.10 Preliminary toxicity studies were made on the crude nitrogen trifluoride (approx. 90% NFs, the remainder probably N20). These tests indicated a moderate degree of hazard. The main toxic effect of this co'mpound appears to be irritation of the lung tissue comparable to the nitrogen oxides, but re uiring higher concentrations, and recovery from non-letha? exposure seems to be rather complete. Irritation of the external mucous membranes cannot be used as a warning sign as this appears at concentrations (4000 p.p.m.) affecting the lungs.
Results and Discussion The data for the liquid density are given in Table I, and have been fitted to the equation d = 2.103 - 3.294 X 10-3 T - 4.675 X 10-8 T 2 (1) where d is the density in g./cm.a at the absolute temperature T. Deviations between the experimental and values calculated using eq. 1are given in column 3 of Table I. A comparison with the previous data of Ruff3 shows good agreement, with his data being generally 0 . 2 4 3 % lower. The precision of these measurements is estimated to be ~ O . l ~ o The . volume values for the glass bulb were corrected using the (6) 0. Ruff, J. Fischer and F. Lutz, 2. anorg. alloem. Chem., 172, 417 (1928). (7) H. S. Booth and R. L. Jarry, J. Am. Chem. Soc., 71, 971 (1949). (8) M. K. Wilson and 9. P. Polo, J. Chem. Phys., 20, 1716 (1952). (9) E. L. Pace and L. Pierce, ibid., 98, 1248 (1955). (IO) L. Pierce and E. L. Pace, iMd., 22, 1271 (1954).
DETERMINATION OF VAPOR PRESSURES OF MOLTEN SALTS
Oct., 1956
TABLE I1
TABLE I
THEDENSITY OF LIQUIDNITROGEN TRIFLUORIDE BETWEEN 78 AND 170°K.(0%. = 273.16%) Dev. Temp., OK.
Dendty, g./cm.*
obsd.
-
oalcd.
1413
Temp., OK.
Density, g./om.u
Dev. obsd. oalcd.
-
119.53 1.643 0.001 78.05 1.819 .OOO 124.50 1.621 79.10 1.814 .001 .001 129.42 1.600 79.44 1.812 131.06" 1.593 .002 81.68 1.802 -.001 134.23 1.580 .003 .OOO 87.23 1.780 135.35" 1.574 .003 87.66 1.775 -.003 139.16 1.558 .004 .001 92.22 1.760 .002 94.27 1.748 --.002 139.94' 1.552 144.00 1.536 .004 97.01 1.740 .001 99.45 1.727 --.002 144.66" 1.530 .002 149.34" 1.508 .001 .OOO 101.79 1.720 153.93" 1.484 --.001 104.07 1.708 -.001 158.53" 1.461 --.003 109.91 1.683 -.002 114.79 1.663 .OOO 164.16" 1.434 -.002 119.47 1.643 .001 169.54 1.406 -.004 Indicates the measurements made in the metal appara0.002 .001
THEVAPORPRESSURE OF NITROGEN TRIFLUORIDE (0°C. = 273.16"K.) Temp.. OK.
89.33 94.14 98.99 103.87 108.58 113.70 118.49 123.16 127.61 131.95 136.08 140.17 144.18
Pressure, mm.
1.00 2.59 5.92 12.84 24.79 47.39 81.64 132.83 203.16 297.96 418.31 573.08 764.26
Dev. obsd. calcd. mm.
-
0.01
.03 - .07
.Ol
- .05 .04
- .09 .Ol
- .02
- .13 - .14 +.39 +.21
(I
tus.
expansion coefficient data for Pyrex glass of Buffington and Latimer." Temperature was known to f 0.05". The vapor pressure results are given in Table IT. The data to one atmosphere were fitted to the Antoine equation by the method described by C. B. Willingham, et aZ.,12and the following equation derived The normal boiling point calculated using eq. 2 is 144.1OoK., in good agreement with that of Pierce and Pace14144.15"K. The vapor pressure data to the critical point were fitted by the method of least squares to the equation 613.33 T '
loglo P(atm.) = 4.27264- __
(3)
Deviations between the experimental and calcu( 1 1 ) Buffington and Latimer, J. Am. Chem. Soc., 48, 2305 (1926). ( 1 2 ) C. B. Willingham, et aE., J. Research NatE. BUT.Standards, 8 6 , 219 (1945).
Temp., OK.
148.97 153.80 158.67 163.52 168.40 173.36 178.27 183.16 188.03 193.15 198.06 203.14 208.02 217.94 222.96 227.81 232.78
Pressure, atm.
Dev. obsd. oalcd. atm.
-
1.388 0.043 1.884 .043 .004 2.558 .049 3.374 .055 4.326 .122 5.551 6.912 - .lo6 .149 8.544 .129 10.380 12.490 - ,009 15.006 ,011 17.796 - .126 20.925 - .172 28.272 - .464 32.830 - .326 37.863 - .187 .076 43.510
+
lated values for these two equations are given in column three of Table 11. Temperature was known in these measurements to 0.05'. The heat of vaporization at the normal boiling point calculated using eq. 2 and a gas imperfection of -3.78%, derived from the Berthelot equation, is 2769 cal./mole. This value is the same as that obtained experimentally by Pierce and Pace.4 The critical temperature was determined to be 233.90 & 0.10"K. From this value the critical pressure was calculated using eq. 3 to be 44.72 0.17 atm. Acknowledgments.-The authors wish to express their thanks to Professor J. J. Fritz, of the Pennsylvania State University, for his advice in this work; to Miss Ruth Kossate, of the Pennsylvania Salt Manufacturing Company, for performing the infrared analyses and their interpretation; and to Mr. Gordon E. Webb, of the Pennsylvania Salt Manufacturing Company, for his excellent assistance.
*
A BOILING POINT METHOD FOR DETERMINATION OF VAPOR PRESSURES OF MOLTEN SALTS BYJ. L. BARTON AND H. BLOOM Auckland University College, Auckland, New Zealand Received May I, 1966
a
A method has been developed for determining vapor pressure of fused salts by measuring their boiling points over a ran e of measured constant pressures. Superheating was prevented by passing a slow stream of nitrogen bubbles through t e melt and the temperature of the boiling liquid was measured directly. The reliability of the method has been demonstrated for NaC1, KCl, CdC12, and PbC12. The heats of vaporization and normal boiling points of the four salts have been calculated.
In only very few cases are the vapor pressures of molten salts known to a reasonable degree of accuracy.l Many methods have hitherto been used for such determinations, but the results of different investigators vary considerably. The (1) K. K. Kelley, Bureau of Mines Bull. No. 383 (1935).
purpose of this investigation was to develop a reliable method for the measurement of vapor pressure of fused salts and t o establish accurate vapor pressure data for pure NaC1, KC1, CdClz and PbClz prior to an investigation of binary mixtures of these salts.
