938
silver nitrate at its melting point was found to be 2960 h 60 cal./mole.
Discussion The values for the heat of fusion of silver nitrate reported from 1898 onward are summarized in Table 11, together with the value from the present investigation. In view of the markedly higher value gained from the present calorimetric measurements, an evaluation of the factors possibly contributing to the differences in the previously determined yalues is of interest. T ~ B LI E ENTHALPY O F SILVER KITRATE AT V.4RIOTJS TEMPER 4TURE3 (Tt) REL.4TIT-E TO STAXDARD TEhIPER.4TURE (3Oo'K. ) -Sample--Initial temp.,
T t , OK.
Vol. 64
NOTES
rlnal temp.,
OK.
Cal. Calorimevolved eter temp. by 1 mole change
AgNOz
Corm. t o 298.15OK.
HT HZO~.IK,
cal./niole
4.83 10945.61 41.22 11036.83 58i.2 302.3 301.3 3.43 9316.53 68.98 9385.51 547.7 301.3 :?.Xi 9245.78 68.98 9314.76 517.3 3.90 7916.64 106.79 8023.43 503.3 303.0 305.8 2.89 i913.53 168.99 8082.55 500.2 3.82 7733.45 131.25 '7864.70 491.5 304.1 300.2 1.99 5221.68 44.52 5266.40 483.2 l.i4 4479.91 60.09 4540.00 470.4 300.9 1.59 4110.81 53.41 4164.22 460.2 300.6 1.52 3939.52 80.10 4029.62 450.2 301.8 1.44 3752.74 42.29 3795.03 438.4 300.1 Wt. of AgNO; sample, g. 30.0256 16.0315 Wt. of Pt crucible, g. Heat equiv. of calorimeter 1st expt. 441.07 cal./deg.; remaining expt. 539.10cal./deg.
TABLEI1 T a r ~ r ~FOR s s HEATOF FUSION FOR AgSOs Inyestigator
H. Juptner (1898) H. Goodn-in and H. Kalmiis (1909) h. llaynus and F. Oppenheimer (1928j K. Kellry (1936) Present investigation ( 1959)
SIethod
Estimated
AHP,
where K is the modulus of elasticity and p is the density of the salt. No information relative to the temperature a t which the modulus or density were measured is given. The agreement of this estimated value with other results must be regarded as fortuitous. Further tests of this expression to evaluate its applicability would seem of interest. The value by Kelleys is based on seven sets of cryoscopic data varying from 2,300 to 2,800 cal. mole-'. Assumptions relative to the thermodynamic ideality of the solutions and the dissociation processes for the solutes are implicit in the calculations of the heats of fusion from the above cryoscopic results. For theoretical discussions of cryoscopic behavior of solutes in silver nitrate a value of the heat of fusion (and thus the cryoscopic constant) independently derived by a different experimental technique is almost essential. The values for the heat of fusion (2960 cal./mole) entropy of fusion (6.14 e.u./mole) and cryoscopic constant (26.47 deg./mole/1000 g.) derived from the present calorimetric measurements are recommended for practical and theoretical calculations. Acknowledgment.-This work was made possible in part by support received from U. S. Air Force, Air Research and Development Command, Office of Scientific Research. Active participation in the earlier phases of this study and continued interest by Dr. Cyril Soloinons is gratefulIy acknowledged.
THE PREPARATION OF SINGLE CRYSTALS OF CERTAIN TRANSITIOY METAL FLUORIDES BY H. GUGGENHEIM
eal./mole
Ref.
2755
6
Bell Telephone Laboratories,Incorporated, Murray Hall, .Vcw Jersev Reeemed January BS, 1960
Single crystal WiF2 was grown from the melt using a modification of Stockbarger's' method. NiF2 is reported2 to have 5t vapor pressure of one atmosCalorimetry 2757 4 phere a t about 1000" and a melting point over 1300". ........ 2550 5 For these reasons it is impractical to grow it from Calorimetry 2960 the melt using conventional methods, but a sealed It has been realized only recently how difficult it platinum container has been found to overcome the is to remove all traces of water from a crystalline difficulties. Dry NiFz was prepared by passing salt; in the present investigation a rigid vacuum- H F over "low cobalt" NiCl at 850" for 16 hours. elevated temperature technique wa,s employed as The resulting material was a light greenish-yellow. the most efficient drying method. Magnus and The Pt-10% R h alloy tube was used. To facilitate Oppenheimer4 used an isothermal calorimetric pinching off of the tube, a smaller Pt tube was method in which the sample was open to the air a t welded to the larger tube after charging with NiF,. all times. The presence of water thus possible in To prevent moisture and oxygen contamination, their samples would contribute to a lower value for the tube was heated to 250" while connected to a , ~ vacuum line. The vacuum was broken with argon AHf. In the work of Goodwin and K a l m u ~ in addition t,o this factor, a heat loss owing to chimney and then the small tube was pinched off near the convection in the design of drop calorimeter used is top with barrel jam pliers. The pinched end was seen possible and would contribute to the lorn re- welded with a gas flame and the tube was placed sult'. The melting point for AgSOs is reported as inside an alumina tube attached to a clock motor 219' by Goodwin and Kalmus3 (cf. 209.6') and this by a sprocket chain. The tube was lowered may indicate tha,t all temperatures in this work were through a hot zone (1420") a t 0.075" per hour. incorrect,. After 100 hours the tube was removed a t room The yalue attributed to Juptner6 was obtained temperature and opened by cutting along the not by direct measurement but is a value calculated length with a circular diamond saw. The KiF2 \I: w e of the empirical relation boule was emerald green, weighing 56 g., and was 2 Calorimetry
2580
3
(1) D.C. Stockbarger, Rsv. Sci. Inatr., 10, 205 (19391. (2) C. Poulenc, Ann. Chem. Phys., 2, 41 (1894).
July, 1960
939
SOTES
inches long and 3,/4 inch in diameter. Although there were many cracks, there were clear areas as large as a inch cube which were shown to be single crystal. The Pt tube was easily repaired for reuse. The cracking might best be minimized or eliminated by slowly cooling the boule in a uniform temperature zone after the last part has crystallized in the temperature gradient. The growth of mixed fluorides in a hydrothermal bomb has been illustrated by the preparation in a sealed vessel of Cu compounds which normally dissociate or form oxides in air. K. Knox3 has grown crystals of KCuF3 and K2CuF4 from the melt. These crystals were very small and the KCuF3 invariably twinned. By analogy with the KFKiFZ4and KF-MgFt systems, it is expected that KCuF3 and K2CuF4will exist in the KF-CuF2 system, the former congruently and the latter incongruently melting. An additional factor in the Cu Fystem is the fact that copper(I1) fluoride16and presumably potassium-copper-fluorides, have an appreciable dissociation pressure of fluorine at the melting point, so that in an open system loss of fluorine and reduction of the copper occur. The copper system should be several hundred degrees lower than the KF-NiF2 and KF-MgF2 systems. Because of oxide formation and the dissociation problem, the system was investigated in a sealed vessel. KHF, \vas used instead of K F because it is less hygroscopic, and has a melting point much below the dissociation temperature of CuF2. The melting point of KHF2 is 195", a t which temperature it begins to liberate HF. Because of the build up of pressure when heating over 200" a steel bomb7 with a platinum liner was used. This autoclave was designed for hydrothermal experimentation and was constructed to withstand pressures up to about 7000 p.s.i. A corrosion resistant steel was used which can be heated to 800" without vreep. Reagent grade CuF2,2H,0was dried in dry HF at 400". ;Ibore this temperature decomposition takes place. ;I, typical run was made as follows: The bomb, xt7hic3h has a capacity of 30 ml., was charged with 30 g. of liquid KHF, prepared by heating the powder in a platinum crucible. After solidification, 10 g. of dry CuF2mas added. A 0.005 inch thick platinum disc was placed over the top of the liner and the steel plunger was forced d o m on the disc by tightening the head in a vise. The bomb was heated in a muffle furnace to 500" for 16 hours. The temperature then was lowered at a rate of 3" per hour to 200", at which temperature the bomb mas removed from the furnace. This mixture of 20 mole % CuF2and SO mole ?& KHF? yielded single crystal plates of KCuF3, about 0.5 cm.2. They could be separated by dissolving the K F matrix in warm HzO whirh had no effect on the KCuF3. Single crystal plate. of K2CuF4were grown using the 3ame procedure but changing the mole ratio to 139, CuF, and S7% KHF,. These crystals were the same size and habit as the KCuF3 crystals. In this case, the flux could not be removed with HsO be(3) K. Knox. J Chem P h y s , SO, 991 (1959). ( 4 ) G. Wagner and D. Balz. Z Elektrochem., 66, 576 (1952). ( 5 ) R . C. DeVries and R Roy, J A m . Chem. SOL., 1 6 , 2481 (1953). ( 6 ) H. v. Wartenberg, 2. anorg. Chem., 241, 381 (193'JI4 ( 7 ) G . W. Morey, Am. M h t 91, 1121 (1957).
cause the K2CuF4hydrolyzed as evidenced by a change in color from clear colorless to opaque blue; however, the crystals could be separated mechanically without difficulty. The crystals were show1 to be KCuF3 and KZCuF4, respectively, by X-ray powder and single crystal pictures. Finally, single crystals of MnF,, ZnFz and KRInFa have been grown by zone melting in an inert atmosphere. -4significant reduction in the impurity content of the crystals, as well as quantitative impurity doping of the crystal, were accomplished by this method. The author is indebted to K. Knox for helpful discussions on KCuF3 and K2CuF4and J. W. Nielsen and E. Dearborn for useful discussions on thcx Stockbarger technique.
THE ASSOCIATION EQUILIBRIUM I K T H E METHYL BROMIDE-ALUMINUM BROMIDE SYSTEM. ESTIMATED BOKDING STRENGTHS OF ALURIINUIJI BROMIDEADDITIOK JIOLECULES WITH METHYL BROMIDE, P E N T E S E AKD B E S Z E S E BY D. G. WALKER Research and Detelopment Dzizaion, Humble OaZ & Refining Compnnii Baytozcn, Texas Receaied January 28. 1960
Brown and Wallace1 have reported an excellent and thoroughgoing study of the addition compounds of aluminum halides with alkyl halides. They obtained experimental vapor pressure-composition data for the system methyl bromide-aluminum bromide at -80, -64.4, -45.8. -31.3 and 0' and concluded that the aluminum bromide in solution was present in the form of an additional molecule CH3Br:A1Br3. A redetermination of the vapor pressure-composition curve for methyl bromide-aluminum bromide a t 0' was made in the course of some other work. In the homogeneous liquid phase, excellent agreement was found with the results of Brown and Wallace. Also, in agreement with their results, the appearance of a solid phase was found to occur a t a CH3Br/AlBr3 liquid-phase ratio of 1.19 and at a system pressure of 235 mm. However, a welldefined pressure plateau was obtained between CH Br/A1Br3 ratios of 1.19 to zero whereas Brown and Wallace found no such pressure plateau in this region corresponding to compound formation in the solid phase. They concluded that solid solution phenomena must be present. Possibly they did not allow adequate time for the system to equilibrat? between withdrawals of methyl bromide. It seemed that if aluminum bromide was the solid precipitate, the system might be simple enough that considerable information of interest could br gained by a further study. Results and Discussion I n Fig. 1 is plotted the system vapor pressure versus composition obtained experimentally at 0'. If one awumes that only the addition mole{-
(1) H. C. Brown and W. J. Wallace, THISJOURNAL, 76,16279
(iwa).
