CRITICAL TERfPERATllRE AiVD COEXISTEKCE CURVE FOR
BISMUTHCHLORIDE
2235
The Critical Temperature and Coexistence Curve for Bismuth Chloride1
by J. W. Johnson and Daniel Cubicciotti Stunford Reseurch Institute,Menlo Park, California 9.4096
(Receiaed March 12, 1964)
The critical temperature of BiCIa has been found to be 1178 f 5OK. aiid the critical density to be 1.210 i: 0.006 g./cc. The coexistence curve for liquid and vapor has been fitted by 1.268((1178 - T)/1178) + 2.347((1178 - T)’”/1178) the equations: p = 1.210 (+ for liquid phase; - for vapor phase). These equations are of the same form as those developed by Guggenheim for argon, nitrogen, oxygen, methane, and carbon monoxide. It is concluded that bismuth chloride behaves very much like a molecular fluid a t least with respect to the critical parameters.
+
Introduction The critical parameters of iiiaiiy metallic halides which do not exhibit ionic conductance have been The liquid densities of sodium and potassium chlorides from t)he melting point to the normal boiling point have been reported, and estimates of the critical constants of the alkali halides have been made.Zb Recently the electrical conductivity of molten bismuth chloride has been measured3 to 625” and has been Sound to exhibit a maxiniuin in the specific conductance curve near the normal boiling point off 441 A knowledge of the orthobaric densities and critical temperature of bismuth chloride permits a deterrninatioii of the applicability of various relationships developed for the critical parameters of molecular liquids to a molten, ionically conducting salt. O.
Experimental A visual method was used in the deteriniliation of‘ the crit,ical temperature and orthobaric densities of bismuth chloride. The salt was contained in sealed clear fused-quartz tubes which were heated in a furnace containing two vertical slits €or illumination and observation of the samples a t temperature. Bismuth chloride, prepurified by distillation under an atmosphere of hydrogen chloride, was distilled into quartz tubes of various dimensions in a n atmosphere of dry hydrogen chloride and inert gas. The tubes were then evacuated to remove residual gas and sealed off under vacuum. Those tubes intended for use in the critical teniperature determinations were constructed of quartz tubing, 2-mm. bore >c 8-mm. 0.d. and 6 cni. long. These
were filled to approximately one-third the total internal volume with rnolten bismuth chloride at 300”. The tubes used in the determinations of liquid and vapor volumes were 3-mm. bore X 6-mm. 0.d. and 6 cm. long. The amount>of BiClB introduced a t 300” was governed by the requirement that the salt completely fill the tube as a liquid or vapor a t the desired temperature. Additional liquid density measurements were niade by using quartz “floats” of known densities constructed by sealing tungsten wire of various lengths into evacuated quartz tubes, 2-miii. bore X 4-mnz. 0.d. and 2 em. long. These Yloats’’ were sealed into quartz tubes, 5-mm. bore X 7.4-mm. 0.d. aiid 6 em. long, containing enough bismuth chloride so that the “float” would float in the liquid phase. A nickel cylinder I15 cni. long and 4.8 cm. in diameter with a central hole of 1.6 em. diameter drilled to within 1 crn. of the bottom was used to hold the quartz ampoules iu the furnace. This block had two 3-mni. diameter holes drilled parallel to the central hole for the insertion of therniocouples. The block was mounted on a rotating ceramic cylinder in the center of the furnace and could be turned so that the sample tube could be observed through the observation slit in the furnace shell with a telescope mounted on a stand outside the furnace. When observations were ~
~~~
(1) This work was mado possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract No. AT(04-3)-106. (2) (a) K. A. Kobe and R. E. Lynn, Jr.. Chem. Reo., 5 2 , 117 (1953): (b) A. D. Kirshenbaum, J. A. Cahill, P. J. McGonigal, and A. 1’. Grosse, J . Inorg. A‘ucl. Chem., 24, 1287 (1962). (3) L. F. Grantham and 5. J. Yosim, J . Phys. Chem., 67,2506 (1963).
Volume 68, -Vumber 8 August, 1964
J. W. JOHNSON AND DANIEL CUBICCIOTTT
2236
not being made, the block could be rotated to shield the sample from view. The furnace used in this work was 350 cm. long and 200 cni. in diameter with a Mullite center tube having a bore of 5 cin. Radial illuminatioil and observation slits 6 inin. wide and 7.6 cni. long a t an angle of 60" were cut through the furnace insulation and central tube. These slits were covered with 6-mm. thick Pyrex plate-glass mounted on the outside of the furnace to reduce heat loss through convection. The windings of 20-gauge Nichronie wire were arranged parallel to thc longitudinal axis of the central tube and insulated with an 8-em. layer of firebrick. I n operation a piece of clear, fused quartz tubing was inserted in the nickel block and the quartz sample tube placed 011 a ceramic insulator inside this tube. The nickel block was inserted in the furnace and adjusted so that the sample was clearly visible through the slit when illuminated with a tungsten lamp. A control thermocouple and a measuring thermocouple were inserted into the thermocouple wells in the nickel block and then a ceramic cylinder 10 em. long and 5 cni. in diameter was placed on top of the nickel block for insulation. After the assembly was complete the furnace temperature was slowly raised and periodic observations of the sample tube were made by turning 011 the tungsten lanip and rotating the nickel block until the sample tube and contents were visible in the telescope whose objective was mounted 30 cin. from the center of the furnace. The density determinations were carried out either (a) by observing the temperature at which the liquid just filled the tube (and then quickly reducing the furnace temperature to avoid breaking the tube), or (b) by observing the temperature at which the last trace of liquid disappeared in the case of the vapor densities. With practice these temperatures could be determined with a reproducibility of k 3 " or less. After the ternperature was determined, the tube was cooled to room temperature, cleaned, scratched with a file, and its weight in air and water was determined. The tube was then broken a t the file mark; the bismuth chloride was dissolved in 3 N H1\'03; the pieces of empty tube were rinsed in distilled water and were dried. The tube fragments were again weighed in air and water. From these weighings the amount of bismuth chloride and the iiiternal volume of the tube (approximately 0.4 ~ 1 1 1 . ~could ) be found and the density of liquid or vapor a t the recorded temperature calculated. Additional liquid density measurements were made by using quartz "floats" whose density was determined by weighing in air and water. In these cases the temperature a t which the float sank and the temperature at T h e Jouwrial of Physical Chemistry
which it rose again agreed, in some cases, within 2" although in general the spread was about 4". Determination of the critical temperature required continuous observation of the sample over long periods of time, and this exposure of the saniple through the open slits set up temperature gradients in the tube. A correction for this error was made by determining the temperature indicated by a thermocouple placed inside an open sample tube compared to the block temperature. This correction varied depending on the vertical position along the slit and amounted to 4-8" a t the critical temperature.
Results and Discussion In the determination of the critical temperature of bismuth chloride, the liquid-vapor interface was observed through the telescope as the sample was heated. The ineniscus became flat and then disappeared as the critical temperature was approached and exceeded. As the critical temperature was approached from above, a dark band about 2 inin. wide appeared in the tube out of which the meniscus emerged. The temperatures of disappearance and reappearance of the liquidvapor interface were observed three times for each of four sample tubes. As mentioned earlier, a correction of 4-8 O had to be applied to each observed temperature, the individual correction being dependent on the position of the interface in the tube. The corrected ternperatures are listed in Table I. In view of the magnitude of the correction and the effect of the temperature gradient on the liquid-vapor interface, the critical temperature was taken as 905 i 5 " . Table I : Critical Temperature of Bismuth Chloride Tube no.
-Temp., O C . , meniscus--Disappearance Appearance
1
906.2 905,9 906.1
904.1 904.0 904.4
2
903,6 903,9 903.1
901.7 901.7 901.9
3
904.6 904,9 905.0
902.3 902,O 902.6
4
907.1 905,9 906,l
904,9 905,l 905.0
Average
905.2
904.3
Accepted value
905 f 5'
CRITICAL TEMPERATURE AND COEXISTEKCE CURVEFOB BISMCTI-I CHLORIDE
The experimental densities of bismuth chloride liquid and vapor a t v,arious temperatures are presented in Table I1 and the coexistence curve is shown in Fig. 1. The first column in Table I1 indicates the method used to determine the liquid density. The vapor densities were all determined by volume. An uncertainty of & 3 o is assigned to all temperatures listed in Table 11.
2237
These equations were rearranged and the coefficients of the temperature terms generalized to the form
where T is the temperature and T , the critical temperature. The experimentally determined densities given in Table I1 and the critical temperature of 1178°K. were used to determine the values of a, b, and Pcrit which gave the best fit to both liquid and vapor densities. The equations derived are
-
I
w 900E 3
pvap =
1.210
+ 1.2@3( 11781178- T ) -
-
G
2.347(
E a
800-
P
-
w I-
700 600 -
500
1
2 3 DENSITY -g/cc
Figure 1. Bismuth chloride coexistence curve: 0, float calculated measurements; 0 , volume measurements; -, experimental liquid density, ref. 5 ; from eq. 3; experimental gas density (zero); - - -, average of experimental liquid and gas densities.
)
1178 - T 'I3
1178
(3b)
These relations reproduce the experimental data quite well as may be seen in Table I1 and Fig. 1 where the coexistence curve was calculated from these equations. Below about 8OO"K., eq. 3 does not fit experience very well. For the vapor eq. 3b gives negative densities. The actual values are essentially zero (5.4 X 10-3 at the boiling point-714 OK.-and smaller a t lower temperatures). Guggenheim's equation in general gives nega,tive vapor densities below O.55Tc. For the liquid eq. 3a gives values that are also too small. It is of interest to compare the average density line derived from the addition of (3a) and (3b) and dividing by 2 Pliq
+
~ v n p-
2
- 1.210
+ 1 . 2 6 411781178- T ) =
--$
Guggenheim4 has developed empirical equations for the coexistence curves of the inert gases, oxygen, nitrogen, methane, and carbon monoxide, having the form
1.210 4- 1.072 X 10-'(1178 - T ) (4) with that derived from the density equation given by Keneshea and Cubicciotti5 for their measurements of the density of bismuth chloride between the melting point and the normal boiling point. Their data are fitted by pliq
=
4.417 -- 2.2 X 10-'t
(250-450')
(5)
(4) E.A. Guggenheim, J . Chem. Phys., 13, 253 (1945). (5) F.J. Keneshea and D. Cubicciotti, J . Phys. Chem., 62,843 (1958).
Volume 68, Number 8 August, 1964
J. W. JOHKSON ASD DANIEL CURICCIOTTI
2238
Table 11: Orthobaric DenRity Data for Bismuth Chloride L‘iquid phase---------
_ I _
Method
Temp.,
Float Volume Float Float, Volume Float Volume Volume Float Volume Volume Float Volume Float Float Volume Float Volume Float
Obsd.
OK.
797 868 892 930 942 1009 1032 1061 1079 1085 1085 1107 1107 1117 1138 1140 1157 1158 1171
3.239 3.012 2,974 2,864 2 834 2.630 2.536 2 428 2.360 2.303 2.287 2.221 2.188 2,164 2.022 2.017 1.895 1.804 1.648
7--------Vapor
Density, g./ec.---------Calod. Calod. - Obsd.
3,231 3.048 2.971 2,873 2.837 2.621 2.537 2.423 2 344 2.317 2.317 2.206 2 206 2.151 2.031 1.999 1.846 1.835 1.645
Av. dev.
+
x
10-3(1178 - T ) (6)
where T is expressed in OK. This comparison can be seen in Fig 1. The excellent agreement between cq. 4 and 6 is to a certain extent fortuitous since the critical teiiiperature has a n uncertainty of + s o and the critical density a n estimated uncertainty of f0.006 g.i cc. However, the agreement does suggest the average density line for bismuth chloride IS a linear function of the temperature froin t h e meltliig point to the critical point. The applicability of Guggenheim’s relation for molecular fluids to bismuth chloride prompted a coniparisoii of the experimental critical temperature and density with values that would be predicted for a “normal” fluid. Accordingly, the graphical iiiethod of R iedel, as revised by Pitzer6 and eo-workers, was applied to bismuth chloride. The parameters used were the normal boiling point (713O K i ) > the 100-mm. boiling point (6160K7), and the liquid density (3.713 g. lee. a t 603°1
