1664
J. W. JOHNSON, W. J. SILVA,AND D. CUBICCIOTTI
The Critical Temperature and Coexistence Curve for Aluminum Bromide'
by J. W. Johnson, W. J. Silva, and Daniel Cubicciotti Stanford Research Institute, Menlo Park, California 94026
(Received October 31, 1967)
The critical temperature of aluminum bromide has been determined to be 763 j = 2"K, and the critical density has been found by extrapolation to be 0.8605 0.0023 g/cc. The experimental densities of the coexisting liquid and vapor phases were fitted to a modified Guggenheim relation
*
p ( g / c c ) = 0.8608
+ 0.8968 (76;&
'>
1'6535
762.7 - T 762.7
(
)
0.319
over the temperature range 591 to 760°K. In the last term the upper sign refers to the liquid and the lower to the vapor. Unlike the bismuth halides and mercuric chloride, this salt does not follow the T'Iarelation.
Introduction Previous work in this laboratory has involved the determination of the critical temperatures and coexistence curves of bismuth chloride2 and bromide3 and mercuric ~ h l o r i d e . ~The Guggenheim relation6 has been used to describe the coexistence curves for these salts. Literature data on aluminum bromide6 indicated some modification of the exponents in the Guggenheim relation might be required. This is a report of the determination of the coexistence curve and critical temperature of aluminum bromide by methods employed for the other salts.
Experimental Section Fisher certified reagent aluminum bromide was evacuated overnight at room temperature and then sublimed at 70" through a sintered glass filter into Pyrex ampoules. After approximately one-half of the aluminum bromide had been transferred, the sublimation was stopped and the residue was discarded. The ampoules, each containing about 5 g of the salt, were sealed off under vacuum and stored for future use. Bromine content of this preparation was found to average 89.95%, compared to a theoretical content of 89.88%. The quartz tubes intended for critical temperature and liquid or vapor density measurements were attached to a manifold, one end of which was sealed to a vacuum line. To load these tubes an ampoule containing the salt was opened and inserted into the other end of the manifold; this end was then sealed off and the system was evacuated. The aluminum bromide was sublimed out of the storage ampoule, condensed in the upper portion of the manifold, and the end containing the empty ampoule was sealed off under vacuum. The manifold was then sealed off from the vacuum line and the desired amount of salt was distilled into the individual quartz tubes under its own vapor pressure. The quartz tubes containing the salt were sealed off The Journal of Physical Chemistry
from the manifold and stored until the experimental measurements could be made. The liquid phase densities were all determined by the quartz float method described previously,2 as were some of the vapor densities near the critical temperature. The quartz floats of known density were placed in the tubes prior to attachment to the manifold. The filling was such as to ensure sufficient liquid or vapor volume to support the float entirely within the phase whose density was to be determined. All of these tubes were 5 cm long, but the bore varied depending on the diameter of the float used. Those tubes intended for the determination of the critical temperature were 5 cm long, 4 mm bore by G mm o.d., and were approximately one-third full of molten aluminum bromide at the melting point. In general the bore of the tube was 2 mm larger than the diameter of the float to provide sufficient clearance for vertical movement of the float. Vapor density measurements below 0.3 g/cc (732°K) could not be made by the float method, since it was not possible to construct floats of reasonable size having densities lower than this value. Therefore, vapor density determinations over the range 540 to 745OK (0.017 to 0.370 g/cc) were made by a dilatometer method. In this method the change in the liquid phase volume is measured as the total volume of a system containing a fixed amount of aluminum bromide is reduced. First the liquid phase volume is measured, (1) This work was made possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract NO.AT (04-3)- 106. (2) J . W. Johnson and D. Cubicciotti, J . Phys. Chem., 68, 2235 (1964). (3) J . W. Johnson, D. Cubicciotti, and W. J. Silva, ibid., 69, 1989 (1965). (4) J. W. Johnson, W. J. Silva, and D. Cubicciotti, ibid., 70, 1169 (1966). (5) E. A . Guggenheim, J . Chem. Phys., 13, 253 (1945). (6) D . I. Zhuralev, Zh. Fiz. Khim., 10, 325 (1937).
CRITICALTEMPERATURE AND COEXISTENCE CURVEFOR ALUMINUM BROMIDE over a range of temperature, in a given tube. Then all the salt is distilled to the bottom of the tube and a portion of the top is sealed off, reducing the total volume of the tube while the mass of aluminum bromide remains the same. The measurement of the liquid phase volume as a function of temperature was repeated for the new tube volume. For such a system the masses and volumes of each phase are related by the two equations
ill
= p1v1‘
ill = plU1”
+ pzvz’ + pzvz“
(la) Ob)
where ill is the total mass of aluminum bromide, plvl and pzvz denote the density and volume of the coexisting liquid and vapor phases, respectively, and the single and double primes refer to different total volumes of the system. The vapor phase volumes can be expressed as VZf
:=
V’ - ul’;
v2“
=
V” -
yl”
where V’ and V” represent the total volume of the system in the two cases. Substituting these relations into eq l a and l b and solving simultaneously gives Pl(V1’ Pz=
(V’ - V”)
- VI”)
+
(11’1’
- Ul”)
The reduction in total volume, V’ - V”, is a constant at all temperatures; the other quantities in eq 2 are temperature dependent. It is apparent that the larger the total volume reduction of the system the greater the difference in the liquid phase volumes at a given temperature. Thus, the larger the volume reductions the greater the accuracy of determination of the vapor densities. To cover the lower range of temperatures and densities, ie., 540 to 650OK or 0.017 to 0.100 g/cc, a dilatometer having an initial volume of 1.6106 cc and a final volume of 0.1429 cc or a volume reduction of 1.4676 cc was used. This dilatometer consisted of a 5-cm section of nominal 2-mm bore quartz tubing sealed to a 2-cm section nominal 8-mm bore quartz tubing. The 8-mm bore section was sealed off in the volume-reduction step, leaving approximately 4.5 cm of 2-mm bore tubing as the final tube. To cover the range from 660 to 745”1
