Julie, 1960
SUBLIMATIOS Pamsvrzm OF SOLII)SOLUTIONS
molecular weight of the compound as a whole must be of such magnitude that the sedimentation coefficient can be measured with reasonable accuracy. Acknowledgments.-The author wishes to express apprtxiation to Professor Louis C. W. Baker, in whose laboratory this work was carried out, for helpful discussions and for introduction to problems involving heteropoly electrolytes, which prompted
737
the author to undertake this derivation. He is indebted to Professor R. Haase for advice aiid critical reading of the manuscript. This research was supported in part by a grant from Monsanto Chemical Company and in part by the US.Atomic Energy Commission, through Contract hT(30-1)1833. The author also wishes to thaiik the F~ilbright Commissioii for a Travel Grant.
SUBLIhJATION PRESSURES OF SOLID SOLUTIOSS. I. THE S1'STEMS TIS(1V) BROMIDE-TIS(1V) IODIDE ,4SD TIS(1V) BR0114IDE-TIT,1SIUI11 (11') BROMIDE. THE SYSTEM TIS(1V) BROMIDE-TIS(1V) IODIDECARBOX TETRACHLORIDE1 BY JAMES J. KCAVXEY~ AND NORMAN 0. SMITH Contrzbutzon from the Department of Chemistry, Fordham Universzly, h e w York, N . Y. Rccezved November 6,1969
A manometer has been designed for measuring vapor pressures of materials the vapor of which slowly interacts with or dissolves in tne manometric liquid. It was used to measure the sublimation pressures of pure SnBr, and TiBr4, of solid solutions of SnBr4in Sn14over a small range of temperature, and of solid solutions of SnBr4 in TiBr4 a t 22". Marked negative deviations from Raoult's law were found in the system SnBr4-Sn14,interpreted as a tendency toward the formation of mixed halides in the solid state. A supplementary study of the solid-liquid equilibria in the system SnBrr-Sn14-CC14 a t 12" was undertaken to examine the distribution of SnBr4and SnIa between solid and saturated liquid solution. The distribution is of Type I1 in the Roozeboom classification, although it gives some evidence of compound formation. Measurement of the sublimation pressures in the system SnBr4-TiBr4 proved very difficult, and the results, although erratic, showed positive deviations from Raoult's law.
In conti:ast to liquid solutions comparatively few measurements of the thermodynamic properties of solid solutions, especially non-metallic ones, have ever been made. The e.m.f. approach, used by Wachter3 for NaC1-AgC1 and PbC12-PbBrz solid solutions, is not conveiiieiit for covalent substances, and what few studies have been made with the latter have been through sublimation pressure. S p e r a n ~ k i ,and ~ Perlman and Daviesj measured the vapor pressure of naphthalene in pnaphthol. Speraiiski6 and Kiister' determined the vapor pressures of p-dichlorobenzene-p-dibromobeiizeiie and of p-dibromobenzene-p-bromochlorobenzene solid solutions, and Tanstone* studied the system borneol-camphor. HollmanQ measured t'he dissociation pressure of hydrated salt pairs. In an att'empt to add to the limited available data of this kind the present investigation was undertaken. The Group I V t'etrahalides were chosen because, in addition to their \videspread isomorphism, the tetrahedral structure makes for- comparative simplicity in any theoretical treatment that might result. Hildebrand'O has dran n atteiition to their approximately (1) T h e niaterial of tliia paper is taken f r o m a thesis uresented by , I . J. Kcavney for t h e degree of Doctor of Philosophy a t Fordham University, Jiine. 1957. Portions of the work mere reported to t h e Division of Physical a n d Inorganic Chemistry of t h e American Chemical Society in N e w York, September, 1954. and t o the New York Section's Mee,:ing-ir-lIiniatrire. February, 1954. ( 2 ) Nations1 Science Foundation Fellow. 1953-1954. (3) A. Wac'Iter. J . A m . Chem. Soc., 64, 919, 2271 (1932). (4) A. Spermski. Z. phvsik. Chern.. 46, 70 (1903). ( 5 ) E. P. 1'1 rlniun :Ifill .J. 11. l)nvic.s, J . C h ~ n iSoc., . 91, I 1 1 L (1007). (6) A. Spermski, Z. phusik. Chem., 61, 46 (1905). (7) F. W. Kiixter, i b i d . . 6 0 , 65 (1905). (8) E. Vanstonr, J . Chem. Soc.. 97, 429 (1910). (9) R. IIollnim, Z. p h y s i k . Chem., 37, 103 ( I W I ) .
spherical symmetry and recommended them for study. The experimental difficulties cncouiitered were formidable, however; the data obtained permit only a qualitative interpretation and so fall short of the original goal of thermodynamic treatment. ,lmoiig the difficulties may be mentioned the hygroscopic nature of the components, the fact that the pressures to be measured lay in a difficult range (a few tenths of a mm.), and the slowness with which equilibrium is attained. Pure Components Experimental-SnBra was prepared by the method of Loren7P from reagent grade mossy tin and liquid bromine (N. F.), and distilled from the reaction mixture over a range of 1" and stored in a glass stoppered bottle in a desiccator; m.p. 29.2-29.4". It was analyzed for tin gravimetrically (ignition to SnOz) and volumetrically (reduction to stannous ion and titration Fvith KIOa-KI soln.), and for bromine (potentiometrically with standard AgN03). Calcd.: Sn, 27 08; Rr, 72.92. Found: Sn, 27.14 (grav.), 27.21 (vol.); Br, 72.78. It was distilled as needed into the apparatus described below. SnIa was prepared from the elements according to the method of ?rlcDermottlz; m p. 144-145". I t was analyzed for tin gravimetrically and for iodine volunietrirally as aboye. Calcd.: Sn, 18.95; I , 81.05 Foiind: Sn, 10.12; I , 79.82. The CC1, was obtained from Eastman sulfur-frce material by distillation in a 3-ft. column and collection at 75.0-76.1". TiBr4 was prepared from sugar charcoal and reagcnt grade Ti02 by the method of Raxter and Butler13 in a stream of nitrogen a t 450", and purified h,v distillation under reduced pressure; m.p. 38.7-39.2'. The vapor pressures of SnBr4 and TiRr4 were measured using a static method with a liquid manometer. (SI& vias assumed to hape negligible volatility a t all temperatuies (10) J. H. Hildebrand, J. Chem. Phys.. 15, 727 (1947); J. H. IIililebrnnd and R. I,. Scott. "Tlie Sollihility nf ~ o n ~ l i - c t r , , l y t r a , " Reinhold Publ. Corp., New York, N. Y., 1960, pp. 58. 313. (11) R. Lorenz, Z.anorg. Chem., 9, 365 (1895). (12) F. A. hlcDertnott. J . A m . Chem. Soc.. 33, 196:s (1011). (13) G. P. Bnxtar a i d A . Q. Butler. ibid., SO, 408 (19283.
JAMES J. KEAVNEY AND NORMAN 0. SMITH
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Vol. 64
temperatures were held constant within 0.05" and are accurate to 0.1", by comparison with a certified N.B.S. thermometer.
A n
Results and Discussion.-Table I gives the measured sublimation pressures for SnBre and TiBr4, and includes some values for liquid SnBr4. Figures 2 and 3 show the data plotted as log p (mm.) us. l/T("K.). Both sets of data were fitted to the equation log p = A - B / T by the method of least squares, weighting the points as the square of p so as not t o overweight the smaller values of TABLE I SUBLIMATION A N D VAPORPRESSURE OF SnBrl ~ S n B r r P, mm. t , oc.
Fig. 1.-Apparatus. studied.) The low pressures, and the reactivit,y of the halide vapors with mercury, eliminated its use and the Pyrex apparatus shown in Fig. 1 was designed which would require contact of the vapors with the manometer liquid for only the brief interval of time needed for a measurement. Concen trated sulfuric acid was used for this purpose for the SnBrd, and mineral oil, thoroughly dried by filtering through a&vated alumina (ignited for an hour), for the TiBr4. The density of the sulfuric acid was taken from the International Critical Tables. The density ( d ) of the mineral oil, measured with a pyocnometer, obeyed the relation d = 0.8774 6.6 X 10-4 ( t -20) g./m!. The manometer M , internal diameter 8 mm., led to an oil diffusion pump, forepump and McLeod gage. The volume of the bulb B was 1 liter. The entire apparatus shown was immersed in a water-bath. SnBr: or TiBrl was introduced into t'he sample holder S from a small flask (not shown), fitted with a ground glass stopper, sealed onto S, as fo!lows: The stopper was put on and the manometer liquid introduced into M through tube 4,which was then sealed off. With the T-bore stopcock in a position 90' to the right of that. shown, the system was evacuated for about an hour. During this t,ime the apparatus was flamed in order to remove moisture. After the hour, dry air was admitted sloady through an air leak until the pressure was atmospheric. The stopper of the flask was removed and the SnBr4 introduced into it. The apparatus was then evacuated and ice-water passed through the jacket of S. The sample was distilled from the flask into S and the tube from the flask to S sealed off under vacuum. The Tbore stopcock was turned to the position shown and the stopcock a t the top of tthe manometer closed. The whole apparat.us was now allowed to stand overnight in a thermostat a t about 40". During this time gases which had been trapped in the sample slowly escaped. The sample was then frozen by passing ice-water through the holder jacket. SnBr4 supercooled easily and often required considerable time a t 0' before solidifying. The bulb was then evacuat.ed until the McLeod gage showed a pressure of 1 9 for 15 min. or more. The thermostat was then set a t the desired temperature and t,he sample allowed to equilibrate. About 30 min. was allowed for this, although a constant reading was obtained after 10 min. The measurement was made as follows: the pump was started and the stopcock at the top of the manometer opened. The pressure was checked with the gage until it reached a constant value of no more than 1 E / . The stopcock was then turned 90" to the left and the difference in the manometer levels measured with a Gaertner cathetometer graduat,ed to 0.01 mm. The stopcock was then returned to its original position and the manometer pumped out. The degassing procedure was repeated and measurements made until the same reading was obtained after two degassings. Once degassing had been completed it was found that readings could be reproduced within 59; this indicated that the gases produced by the interaction of the vapor and the monometer liquid did not approach significant amounts. Pressures were reproducible when the equilibrium was approached from both lower and higher temperatures. Thc pressiires recorded below art: rciducsed t,o 0" and accurate t'o 1 p. Any error introduced by allowing the vapor to expand into the evacuated bulb cannot be greater than 0.4y0, since the volume of t,he space between the stopcock and manometer liquid was only 3-4 ml. Bat.11
a
7.4 9.8 11.4 13.4 15.5 17.2 18.6 21.0 22.2 24.6 25.3 28.0 20. 5b 20. 6b 24. 8b 30.7" 31.0" 34.9" 35.8" 37.4" 41.4" Liquid.
0.098 .116 .133 .149 199 ,223 ,254 ,321 ,359 .424 ,467 ,577 ,359 ,339 ,486 .746 .753 ,985 1.031 1.160 1.493 Supercooled liquid. I
AND
TiBra
-TiBrat, o c .
P, mm.
14.4 16.0 17.4 20.0 23.0 24.8 25.2 26.9 26.9 29.2 30.2 30.5 33.4 36.4
0.053 ,060 ,071 ,082 ,116 ,135 ,141 ,165 ,157 I202 ,235 ,225 ,297 ,376
p . The following parameters were thus obtained: SnBr4(s) A = 10.7569, B = 3311.19; SnBrd(1) A = 8.9702, B = 2766.55; TiBr4(s) A = 11.0081, B = 3538.18. From the slopes of the lines the heats of vaporization, sublimation and 'fusion of SnBr4were found to be 12.65 f 0.14, 15.13 f 0.17 and 2.48 f 0.21 kcal./mole, respectively. The heat of sublimation of TiBrl is 16.17 f 0.15 kca1.l' mole. Figure 2 also shows the data of Seki,l4 and of Kabesh and Nyholm15 (SnBr4(l) only), and Fig. 3 includes the values of Seki.l4 It can be seen that the data of Seki are uniformly higher than those reported here. Recently, Hall, Blocher and Campbe1116 measured the vapor pressure of TiBr4 a t temperatures above the melting point. An analysis of their data and Seki's led them t'o the conclusion that Seki's values were high because of residual moisture and consequent hydrolysis prodducts. Our results confirm this conclusion. Hall, Blocher and Campbell also calculated the heat of sublimation of TiBr4 at 2 5 O , using their vapor pressure equation and the heat of fusion determined by Kelley," and obtained (14) S. Seki, J . Ckem. SOC.J a p a n , 62, 789 (1941). (15) A . Kabesh and R. 9. Nyholm, J . Chem. Soc., 3215 (1951). (16) E. H. Hall, J. M . Blocher a n d I. E. Campbell, J . Electrochem. SOC.,106, 271 (1958). (17) K. K Kelley. Quarterly S t a t u s Report t o O N R iroiii t h e E. S. Bureau of Mines, Project N R 037-054. October-December, 195.5.
16.2 kcal.:/riiolc. This is in excelleiit agreement with our result,s. I n addit~ion,they have calculated t'he spectroscopic entropy of T i B s and, in combinat'ion with the entropy of sublimation, find that it agrees with t>het'hird law entropy determined by Kelley. l8 The System SnBr4-Sn14 'This system was chosen because t'he compoiients are thr. least misitire to moisture of the Group halides I ) E C ~ U S C it had been deniori~trated'~ that they forin a. complete series of solid solutions, and because one of the components could be considered involatile, thereby avoiding vapor analysis. It was ant icipated, however, that t'he tendency toward chemical interaction, claimed by R a e d e P t,o he present in the rnolteii state, would also he showri in the solid. Experimental.---?'he:tpparntus of Fig. 1 was usetl. Solid solutions v-eIe medc in situ by evacuating and flaming the vntire apparu,t,us, introducing solid Sn14into S and distilling i3nBri ont,o the SnT4 as described above. Occluded gases wrre removed by allowing the whole to stand overnight under v:tiuuin a t a, t,emperature above the m.p. of the sample. Yilration rnrised by the bath stirrer homogenized the melt. The sample lras now qiienched, or supercooled and allorved t o frceze sutltienly, thus favoring the formation of a honiogi:iieous solicl. The system held at, 0" was now pumped clown to 1 p . The st,opcock n'as t h w turned to the position stion-n in Fig. 1, t,he bath set nt the temperature of measure,tern allowed to equilibratr. I'ressiire m:& everj- two days with frequent 10-15 days the same pressure was obt:tiiied aftiti tivo siiccessive degassings. Measurements now m:ule :it a new t8eniperaturewithout further dcng, m d stcxatly values obt,ained after 2 days. Values giwn trinl)(Jribi\lr
