NOTES
3425
Acknowledgment. The financial support of the National Science Foundation through grant SSFCP-1985 is gratefully acknowledged.
(14) D. H. Geske, J. L. Ragle, M. A. Bambenek, and A. L. Balch, J Am. Chem. SOC.,86, 987 (1964)"
Discussion Transpiration runs, not shown In Fig. 1, have also been carried out using helium a t flow rates of 160-240 cc./min. as the carrier. I n eight out of ten of these experinlents the boats gained weight. X-Ray crystallographic examination of a white residue on the outside
Heat of Formation of MgA1204 by Robert L. Altmaiii' Rice University, Houston, Texas
(Received J u n e 4, 1964)
t
Grjotheim, Herstad, and Toguri2 report a AHo,, of 1.1kcal./mole and a A.So298 of 10.1 e.u. for the formation of RilgA1204from MgO and AlzOs. These values were obtained by a second-law treatment of experimental Mg vapor pressure data according to the reaction
+ 2~1(1)--+ n/[gAlzo4(s)+ 3 ~ 4 g ( g )
4nilgo(~)
0
0
0
(1) 0
A third-law treatment of the same data by this author3 yielded a value of -8 rt 2 kcal./mole. The third-law entropy of formation of MgA1204 froin the oxides a t 298°K. is 0.53 e.u.4 This note reports weight loss results in transpiration experiments similar to those of Grjotheim, et c ~ l . , ~ and also the results of experiments in which argon was transpired over heated mixtures of A1 and MgA1204.
Experimental RSullite boats were loaded with 0.25-g. A1-MgO or Al-nlgA120s samples. The source and preparation of the MgO and MgA1204 powders are described elsewhere.3 The boat was pushed into a preheated PtRh wire-wound tube furnaces into which argon gas flowed in the opposite direction. The mullite furnace tube was closed a t one end by a Pyrex optical flat through which the temperature of the boat was read with a Leeds and Northrup optical pyrometer. The pyrometer temperatures, corrected for the light absorbed by the optical flat, were in fair agreement with readings obtained from a chromel-alumel thermocouple attached to the mullite boat. After heating in flowing argon for 3 to 7 hr. a t pressures of -1.5-2 atm. and flow rates of -100 cc./min., the boat was moved to the colder part of the furnace and cooled in the flowing gas before being removed froin the furnace a,nd reweighed. The weight loss results for both series of runs are given in Fig. 1.
0
0
0
1
I
I 1306
I
1
TEMPERATURE
I-
I
lbSO
1400
I
I
(0 H)
Figure 1. Trmspiration weight loss ofAl-MgO ( 0 ) and A1-MgA12O4 (0) in flowing argon.
of the boat showed it to be MgO and similar examination of the contents of the boat showed AlZO3. The weight gain can be attributed to oxidation of the gaseous magnesium product and aluminum starting material by diffusion of air through the wall of the mullite furnace tube. (1) California State College at Hayward, Calif (2) K. Grjotheim, 0. Herstad, and J. M.Toguri, Can. J. Chem., 39, 443 (1961). (3) R. L. Altman, J . P h y s . Chem., 67, 366 (1963). (4) E. G. King, ibid., 59, 218 (1955). ( 5 ) R. T. Grimley, Ph.D. Thesis, University of Wisconsin, 1958, p. 16.
Volume 68, Number 11
November, 1964
3426
KOTES
Of thirty-one runs with the argon carrier, twentyeight lost weight. However, a t argon flow rates as low as 50 cc./min. the boats gained weight and only those runs with argon flow rates between 100 and 200 cc./min. are shown in Fig. 1. The magnesium vapor pressures calculated from the data in Fig. 1 are about one order of magnitude lower than those of Grjotheim, et aL2 X-Ray examination of several successful Al-1lgO runs showed A1203 among the contents of the boat. Again, the difference between all these runs and the results of Grjotheim, et al., can be attributed to partial oxidation of the aluminum starting material. Similar examination of successful A1-I\IgA1,04 runs showed the presence of AI, Al203, and the spinel phase. However, the bcak-reflection lines of this spinel phase were somewhat broader than for the unheated RlgA1,04. The incorporation of excess A1203in the spinel lattice reduces the lattice parameter ,6 while similar incorporation of AIgO increases it.* The L41-MgA1204 weight loss data suggest that some of the aluminum present in the A1-1IgO heatings was incorporated into the spinel lattice. As shown in Fig. 1, weight loss in the L41-h'IgA1204 system is negligible at temperatures below 1300'K. Since weight loss in the A1-XlgO heatings is still significant a t these temperatures, the composition of the spinel phase must be close to RIgA1204. The third-law treatment of the lowest temperature runs will give the most meaningful heat for reaction 1. A AH'298 of 131 kcal./mole was obtained from such a treatment of the Grjotheim results for temperatures as low as 115OoK.3 This value yields an upper limit for the heat of formation of MgA1204, -551 kcal./mole, and a A H 0 2 9 8 for the reaction
of - 7 kcal./mole. This last value is in agreement with previous Knudsen effusion data. Acknowledgments. The author wishes to thank Professor John L. RIargrave for his interest and for having made possible this experimental work at Rice University during the summer of 1963. Financial support was provided through a grant from the National Aeronautics and Space Administration. ( 6 ) G. L. Clark, E. E. Howe, and A. E. Badger, J . Am. Ceram. 17, 7 (1934).
A Solid Benzene-Tetra-n-butylammonium Yitrate Complex'
by Thomas J. Plati and Edward G. Taylor Thompson Chemical Laboratory, W i l l i a m s College, Williamstown, Massachusetts (Receited June 27, 1964)
Following up an earlier observation2 that tetra-nbutylammonium nitrate crystals contain benzene when recrystallized from this solvent, we have established the existence of a solid 1: 1 mole ratio complex between the two compounds. Vapor pressure-temperature rneasurements have been used to determine AH for the dissociation.
Experimental Naterials. Tetra-n-butylammonium iodide was prepared in the usual manner from fractionated tri-nbutylamine and n-butyl iodide and was converted into the nitrate by metathesis with silver nitrate in ethanol solution. The nitrate was purified by niultiple recrystallizations from benzene and nielted at 120° after vacuum drying a t 90'. Benzene was either of reagent or spectrophotometric quality and was used without further purification. Procedure. The vapor pressure-composition diagram was determined a t 25' in a simple type of apparatus consisting of a flask attached to an open end manometer leading to a vacuum pump. Starting with approximately 5 g. of the nitrate and 10 g. of benzene in the flask, the usual procedure of evacuation followed by equilibration and subsequent weighing of the flask plus contents was carried out. A step curve, shown in Fig. 1, was obtained, the sudden break establishing the existence of a 1: 1 mole ratio complex. The vapor pressures of the molecular complex were measured at a series of temperatures using a flask to which was attached a closed end manometer. I n this way it was possible to submerge most of the apparatus in the thermostat. After equilibration the difference in the mercury levels was observed using a cathetometer; it is estimated that the vapor pressure measurements are good to within +0.2 mm. Solubility nieasurenients were carried out by sealing the appropriate mixtures of benzene and the nitrate in heavy walled glass tubing. The tubes were attached to a wrist action shaking device and immersed in a
SOC.,
(1953).
(1) Abstracted from the Honors Thesis of T. J. Plati, Williams College, 1964.
(8) A. RI. Alper, R. N. RIcNally, P. H. Ribbe, and R . C. Doman, J . Am. Ceram. SOC, 45, 263 (1962).
(2) N.L. Cox, C. A. Kraus, and R. M. Fuoss, T r a n s . Faraday Soc., 31, 749 (1936).
(7) D. SI. Roy, R. Roy, and E. F. Osborn, Am. J . Sci., 2 5 1 , 337
T h e Journal of Physical Chemistry
