3916
J. JOHNSON, W. SILVA,AND D. CUBICCIOTTI
The Vapor Pressure and Enthalpy of Vaporization of Molten Bismuth Chloride to the Critical Point'
by J. W. Johnson, W. J. Silva, and Daniel Cubicciotti Stanford Research Institute, Menlo Park, California 04026
(Received June 25, 1965)
The vapor pressure of molten bismuth chloride has been measured by an inverted capillary technique from 721'K. and 1.2 atm. to 1175'K. and 110.6 atm. The data are fitted by the equation log Patm= 5.2349 - 3725.7/T°K., with an average deviation of 0.7%. A critical pressure of 118.1 f 4 atm. is predicted from these data. The enthalpy of vaporization of molten bismuth chloride has been calculated from 900 to 117OOK. and near the normal boiling point; values a t intermediate temperatures have been interpolated. The deviation of bismuth chloride vapor from the ideal gas relation has been found to begin just above the normal boiling point (714'K.).
Introduction The physical properties of electrically conducting molten salts have not been investigated to any great extent above their normal boiling points. The electrical conduction of molten bismuth chloride has been measured by Grantham and Yosim2 to 625". I n a previous paper3 we reported the critical temperature and coexistence curve for bismuth chloride liquid and vapor. This paper reports measurements on the vapor pressure of bismuth chloride from the normal boiling point to the critical temperature.
Experimental Section The vapor pressure measurements were made using an inverted capillary technique modified to use smallbore tubing that could withstand high pressures. The apparatus, described in detail el~ewhere,~ was made of quartz and consisted of a straight tube, 3-mm. bore X 6-mm. 0.d. X 35-cm. length. The top end was flared for connection by a Bridgman seal to the pressurizing system; the bottom was sealed shut 2 cm. above the end to provide a thermocouple housing. An inverted capillary, 1-mm. bore X 2-mm. 0.d. X 4cm. length, had a length of platinum wire sealed in the top portion to increase the density and ensure that the open end of the capillary rested on the bottom of the tube. A schematic diagram of tube and furnace assembly is shown in Figure 1. The furnace, 33 cm. long with a 15-cm. controlledThe Journal of Physical Chembtry
heating zone, had two diametrically opposed slits 5 cm. long X 0.6 cm. wide so that the bottom of the tube containing the BiCL and the inverted capillary tube, as well as the thermocouple bead, were viewed by transmitted light. The BiC4 was distilled a t 300" under a stream of oxygen into the tube containing the inverted capillary until a liquid height of 8 cm. was obtained. The salt was then cooled to room temperature under oxygen, removed from the distillation apparatus, and attached t o the pressurizing system. The pressurizing system consisted of two Heise Bourdon tube gauges with ranges of 0-250 p.s.i. in 0.25-lb. graduations and 0-3000 p.s.i. in 2-lb. graduations, a compressed argon tank, and a connection to a vacuum line. After attachment to the pressurizing system, the whole assembly was evacuated to remove air and oxygen and then repressurized with argon to approximately 150 p.s.i. The temperature was slowly raised until vigorous bubbling occurred at the base of the capillary; this served t o purge the capillary of (1) This work was made possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract No. A T (04-3)-106. (2) L. F. Grantham and S. J. Yosim, J . Phys. Chem., 67, 2506
(1963). (3) J. W.Johnson and D. Cubicciotti, ibid., 68, 2235 (1964). (4) W.J. Silva, J. W. Johnson, and D. Cubicciotti, Rev. Sci. Instr., in press.
VAPORPRESSURE AND ENTHALPY OF VAPORIZATION OF MOLTEN BiC13
3917
TO
PRESSU~RIZING SYSTEM
I
I
.FURNACE
OUARTZ TUBE 6mm O D 3mml D
0.5
PLATINUM WEIGHT INVERTED OUARTZ CAPILLARY
-
VAPOR SPACE
0
08
PmmOD ImmlD
VIEWING SLIT
I.o
0.9
1.1 IOOO/T
I.3
1.2
I .4
Figure 2. Vapor pressure of molten bismuth chloride. 0.5
0.4
Figure 1. Boiling point apparatus.
0.3
f 0.2
entrapped oxygen. The pressure was then increased, and the rise of the liquid into the capillary was observed with a telescope. I n operation the pressure was set to a selected value, and the temperature was raised slowly to the point where the vapor pressure was slightly less than the applied pressure. The system pressure was slowly reduced until the liquid level in the capillary dropped to the bottom and occasional bubbling occurred. The applied pressure and the e.m.f. of the thermocouple were recorded. The pressure was then increased, and the same procedure was followed at another temperature. There was a vertical temperature gradient in the tube because of the slits, and therefore the reading of the thermocouple had to be corrected. This was accomplished by inserting a platinum-platinum10% rhodium thermocouple into the empty tube and comparing the reading of the two couples. This procedure established a calibration curve for the assembly. The system thermocouple and calibrating thermocouple had been compared with a standard platinumplatinum-lO% rhodium thermocouple calibrated by the National Bureau of Standards. The validity of this method was determined by measuring the vapor pressure of CC1, from the boiling point to the critical point2 with an average deviation of less than 1% from published values.
-t
n
-g
0.I 0 -0.1 -0.2
-
THIS WORK EXTRAPOLATION
--I
1.30
l
l
l
l
l
l
l
l
l
l
1.40
I.35
l
l
l
1.45
1000 / T'K
Figure 3.
Comparison of bismuth chloride data.
Results and Discussion Measurements of the vapor pressure of molten bismuth chloride were made from 721 to 1175'K. over a pressure range of 1.18 to 110.6 atm. The experimental data are presented in Table I. The temperatures recorded are the corrected values obtained from the calibration curve run on the empty tube. These corrections ranged from 3 to 5 " , the larger corrections being required at the higher temperatures. The observed pressures up to 17 atm. are corrected by the addition of 0.04 atm. for the static head of bismuth chloride in the tube. Above this pressure, where the 0-3OOO-p.s.i. gauge was used, the pressures are recorded to the nearest 0.1 atm., and the correction was not applied. Volume 69,Number 11 November 1966
J. JOHNSON, W. SILVA,AND D. CUBICCIOTTI
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Table I : Vapor Pressures of Molten Bismuth Chloride Run ----Obsd.-no. T, OK.
P,atm.
---Celcd.-P, atm. T ,O K .
2 2 1 2 1
721.6 761.0 781.5 806.8 836.7
1.18 2.16 3.02 4.11 6.20
1.18 2.18 2.94 4.14 6.06
2 1 1 1 1
858.9 880.7 906.4 909.7 929.7
7.83 10.15 13.37 13.80 16.99
7.89 10.10 13.32 13.78 16.89
2 952.8 1 979.0 2 1006.2 1 1027.2 2 1044.1
21.3 26.9 36.1 40.4 46.3
1 2 2 1 2
1080.7 1087.5 1112.9 1132.3 1145.6
2 1156.4 2 1175.5 1178 (T,)
% press. dev.
T0&d Tobad
721.6 0 760.3 $1.0 783.5 - 2 . 6 805.2 $0.7 838.6 -2.2
0 -0.7 $2.0 -0.6 $1.9
858.2 881.1 906.8 909.8 930.3
$0.8
-0.7 $0.4 +0.4 $0.1 $0.6
21.1 26.9 34.1 40.5 46.4
953.7 979.1 1013.1 1026.8 1043.8
-0.1 -5.5" $0.2 +0.2
+0.9 $0. 1 $5.9" -0.4 -0.3
59.2 64.6 77.6 86.0 95.8
61.3 64.4 77.1 88.0 96.1
1076.0 1087.9 1113.8 1128.9 1145.1
$3.5" -0.3 -0.6 +2.3 $0.3
-4.7" $0.4 $0.9 -3.4 -0.5
102.7 110.6
103.0 116.2
1155.9 $ 0 . 3 1167.5 $5.1"
-0.5 -8.0"
.. .
118.1
... Av. dev.
-0.5 -0.4 -0.1 -0.1
0
...
...
0.7%
0.7%
' Values omitted in equation and deviation calculations.
Two runs were made using different samples of bismuth chloride. It was found that the data could be fitted by the relation log Patrn = 5.2349
- 3725.7/T
(1)
with an over-all average deviation in pressure of 1.2% for the 22 experimental points. If the three indicated values, obviously discordant, are ignored, the average deviation is 0.7%. The inverse calculation, ie., calculating the temperature by eq. 1 which corresponds to the observed pressure, is also listed in Table I. The average deviation is well within the error limits on the temperature which is estimated at *2" except a t the highest temperatures where the limit is probably nearer 4". Figure 2 presents the data of Table I in graphical form. The solid circles represent the experimental values obtained in run 1; the open circles, values in run 2; and the line is calculated from eq. 1. The pressures observed in run 1 are slightly higher at low temperatures and lower a t high temperatures than those calculated from eq. 1 but not significantly so. The Journal of P h y a h l Chemistry
The shaded area represents the uncertainty of temperature and pressure at the critical point. The critical temperature has been reported in a previous paper3 as 1178 5°K. Using the critical temperature and the error limits set on the critical temperature, the critical pressure is calculated from eq. 1 to be 118.1 f 4 atm. As a check on the validity of applying the temperature correction curve obtained in air to the sam'e tube filled with BiC13, the applied pressure was set at 135 atm., and the critical transition was observed to occur in the capillary tube at a corrected temperature of 1178.3"K. The agreement with the reported critical temperature is better than might be expected from our estimate of the accuracy of the temperature measurement. Data available in the literature on the vapor pressure of BiCI3 overlap the present data only at the lower range of our measurements. Figure 3 presents the available data on a greatly expanded scale over that of Figure 2. The solid line is calculated from eq. 1, and the dashed portion represents an extrapolation of the present data from 720 to 690°K. Above 720°K. the data of Evnevich and Sukhodskii5 and Illaier6 agree well with the present data. Below 720°K. the data of Maier fall on the linear extrapolation of the present data, while those of Evnevich and Sukhodskii and of Keneshea, Wilson, and Cubicciotti' fall below the linear extrapolation. The data of Tarasenkov and Afinogenovs are substantially lower than any of the other data available. The normal boiling point of BiCL lies outside the range of the present data, but an extrapolated temperature of 712°K. is obtained; this compares with 713.3,5 712.3,6and 714°K.9 An examination of BiC13 after completion of the run and cooling to room temperature gave no indication of darkening due to thermal dissociation of the salt at the high temperatures. The color of BiC13 is very sensitive to the presence of excess bismuth metal both in the liquid and solid states. This was taken as evidence that BiC13 is thermally stable at the critical temperature since the apparatus was such that any
*
(5) E. V. Evnevich and V. A. Sukhodskii, J . Russ. Phys. Chem. SOC., 61, 1503 (1929).
(6) C. G. Maier, U.S. Department of the Interior, Bureau of Mines, Technical Paper No. 360, 1925. (7) F. J. Keneshea, W. Wilson, and D. Cubicciotti, J . Phys. Chem., 64, 827 (1960). (8) D. H. Tarasenkov and B. P. Afinogenov, Zh. Fiz. Khim., 9, 889 (1937). (9) K. K. Kelley, U. S. Department of the Interior, Bureau of Mines, Bulletin No. 383, U. S. Government Printing Office, Washington, D. C..1935.
VAPORPRESSURE AND ENTHALPY OF VAPORIZATION OF MOLTEN BiC&
TEMP - O K
Figure 4.
Heat of vaporization of bismuth chloride.
chlorine produced could escape from the heated zone. The bismuth would presumably have remained in solution and resulted in a darkened melt. The enthalpy of vaporization of bismuth chloride may be calculated from the Clapeyron equation, which can be written
where V gand VI are the orthobaric volumes of vapor and liquid, respectively, dp/dT is the rate of change of vapor pressure with respect to temperature; the numerical factor converts the value to kilocalories per mole. The orthobaric volumes were taken from the measurements of ref. 3 and dp/dT from eq. 1. Figure 4 shows the variation of the enthalpy of vaporization as a function of temperature. The solid line below the normal boiling point (712-714°K.) is the enthalpy of vaporization calculated from Kelley's e q ~ a t i o nwhich ,~ was derived from the vapor pressure data of ref. 5 and 6. The triangles result from the use of the ideal gas relation for the vapor volume with dp/dT from eq. 1 and show the vapor begins to depart from ideality in the neighborhood of the normal boiling point. The circles represent the calculation of the volume change on vaporization from the Guggenheim relations for the orthobaric densities of bismuth chloriden3 The vapor density of bismuth chloride has not
3919
been determined between 714 and 900"K., and, as was pointed out earlier,3 the Guggenheim relation predicts a vapor density of zero at 766"K., which would result in an infinite volume change on vaporization. The value for the enthalpy of vaporization calculated at 850°K. using the Guggenheim relation is much too high, owing to a high value for the volume change. The enthalpy of vaporization cannot be calculated in the range of 714-900°K. owing to a lack of knowledge of the vapor density or a means of calculating it with any degree of reliability. The dashed line serves to bridge this temperature range and provides an estimate of the enthalpy of vaporization. There is no theoretical reason for making it a smooth curve, and, indeed, if association into polymers occurs in this range, the curvature may be greater than that shown. Table I1 presents the enthalpy of vaporization of bismuth chloride calculated and interpolated over the range from 700°K. to the critical temperature of 1178°K.
Table I1 : Enthalpy of Vaporization of Bismuth Chloride T,OK.
700 714 750 800 850 900 950 1000 1050 1100 1120 1140 1160 1170 1178 (T,)
P, a h .
0.82 1.03 1.85 3.78 7.11 12.45 20.6 32.3 48.6 70.4 81.0 92.6 105.5 112.3 (118.1)
dP/dT, atm./OK.
A5'~apv
AHvap,
co./mole
koal./mole
0.0143 0.0175 0.0282 0.0507 0.0844 0.1319 0.1958 0.2771 0.3782 0.4997 0.5538 0.6114 0.6726 0.7038
73,000" 58,000"
...
(0)
17.7 17.5 (16.8)b ( 16.2)b (15. 6)b 14.9 14.1 13.2 12.1 10.3 9.3 7.9 5.9 4.4 (0)
...
... ... 5,200 3,130 1,960 1 260 775 617 468 313 218
a V , calculated from ideal gas relationship. polated from curve.
Yalues inter-
Volunte 69, Number 11 November 1966
