DAVID T. PETERSON AND J. A. HINKEBEIN
1360
Vol. 63
TABLE VI QUATERNARY A N D QUINARY MELTING POINTDATA Composition, mole %
a
M.P., 'C.
Biphenyl 20 20 20 40
o-terphenyl 20 20 40 20
ni-terphenyl 20 40 20 20
naphthalene 40 20 20 20
33.5" 50.5 32.9 28.4
Biphenyl 20 20 20 40
o-terphenyl 20 20
m-terphenyl 20 40 20 20
phenanthrene 40 20 20 20
49.5 51.6 32.9 30.9
Biphenyl 20 20 20 40
o-terphenyl 20 20 40 20
naphthalene 20 40 20 20
phenanthrene 40 20 20 20
o-Terphenyl 20 20 20 40
m-terphenyl 20 20 40 20
naphthalene 20 40 20 20
phenanthrene 40 20 20 20
48.5 34.5 53.6 32.9
Biphenyl 20 20 20 40
m-terphenyl 20 20 40 20
naphthalene 20 40 20 20
phenanthrene 40 20 20 20
48.5 35.6 52.9 32.9
40
20
Biphenyl o-terphenyl m-terpheny 1 naphthalene 20 20 20 20 Did not solidify as a result of supercooling or a very low melting point.
However, it is possible to consider these systems qualitatively. Although the actual eutectic melting points are probably higher than the calculated values, it is quite probable that they are about 25" or less. It is very doubtful, however, that any of these systems have optimum compositions which melt a t 25" or less. All the quaternary and quinary systems contain either biphenyl or naphthalene, or both. These compounds have low boiling points and, consequently, the vapor phase is markedly
50.5 35.6 (1
32.9
phenanthrene 20
30.9
enriched in them which raises the vapor freezing point considerably above the melting point of the original mixture. Acknowledgments.-The author wishes to acknowledge the valuable discussion and assistance given by H. C. Bowen and C. Groot. The author would also like to thank Mr. Frances De Mers for providing the gas chromatographic analyses, and Mrs. Pauline Stewart for her assistance in weighing out the sample mixtures.
EQUILIBRIA I N THE REACTION OF BARIUM WITH CALCIUM CHLORIDE1 BY DAVIDT. PETEHSON AND J. A. HINKEBEIN Contribution N o . 686 from the Institute .for Atomic Research and Department of Chemistry, Iowa Stale College, Ames, Iowa. (1) W o r k was performed in the Ames Physical Chemislry Laboratory of the I;. S. Atomic Energy Conimission Received December 16. 1958
The reaction of barium with calcium chloride in the liquid state was investigated by quenching and analyzing equilibrated samples and by thermal nnalwis. Thc reaction appeared to obey the law of mass action over a large part of the salt composition range. The eyui1ib;iiiin constant at 900' was 116 and a t 950" was 87. The solubility of metal in the salt phase varied with composition in B manner which indicated that this solubility was controlled by the composition of the liquid metal phase. The solubility of metal chloride in the liquid metal decreased from the solubility in calcium metal to a minimum at 10 mole yo barium and then increased to the value for barium.
Introduction metal-metal halide equilibria. The phase systems The use of metallothermic processes for the prep- formed by a number of metals with their halide salts aration of reactive metals has stimulated interest in have been investigated but only a few reactions of a
Sept., 1959
EQUILIBRIA IN REACTION OF BARIUM WITH CALCIUM CHLORIDE
metal with another metal halide have been studied. A system of this type can be specified by an equilibrium constant if the salt and metal phases are insoluble in each other, and the activity of each component in its respective phase is known as a function of concentration. However, if the phases are soluble in each other to a significant extent, the solubility must also be known as a function of concentration. The reaction of barium with calcium chloride was investigated a t 950 and 900" by determining the composition of the two liquid phases. The Ca-CaClz and Ba-BaClz binary systems were known to exhibit appreciable mutual solubility of the liquid phases. The Ba-BaClz-CaClz ternary system permitted the determination of the solubility of metal in the liquid salt phase and of salt in the liquid metal a t various compositions. The law of mass action was applied to this reaction, using mole fractions of total barium and calcium in each phase to determine whether it would accurately express the completeness of the reaction. Experimental Methods Materials.-Anhydrous calcium chloride was prepared by dehydrating J. T. Baker Analyzed hydrated calcium chloride (99.9570)by heating it slowly to 140" for two days and fusing in a platinum boat by heating t o 850" for 8-10 hours under pure dry HC1. The maximum calcium oxide content as determined by titration with acid was 0.006~0. The m.p. of this material determined by thermal analysis was 770". Anhydrous barium chloride was prepared by drying J. T. Baker Analyzed barium chloride (99.85%)a t 140"for twodays and fusing in a platinum boat at 1000" under pure dry HC1. The product was neutral t o phenolphthalein. The m.p. was 962" and the transformation temperature was 926". Calcium metal, prepared by alumothermic reduction of calcium oxide, was purified by vacuum distillation. The purified product contained 0.00270 AI, 0.005% Fe, 0.005% Nz, 0.00370 manganese and 0.03% magnesium as the principal impurities. No analyses for carbo;, oxygen or hydrogen were available. The m.p. was 836 Barium in the form of cast sticks was purified by double distillation under reduced pressure. The product contained 0.0073% Nz, 0.0035% C, 0.0023% Fe and 0.008% Mn as the principal impurities. The m.p. of this Ba was 729", which is significantly higher than most reported values. The higher m.p. probably was due to lower oxygen contamination resulting from handling the barium under a n argon atmosphere. Methods .-The tie-lines of the isothermal sections were determined by equilibrating mixtures of either calciuni chloride and barium or barium chloride and calcium at 900 or 950" and quenching into cold water. The mixtures were 304 stainless steel capsules which were 6" heated i:,type long, 7/16 inside diameter, and had a 0.035" wall. Stainless steel was quite resistant to attack by these mixtures and no evidence of corrosion was observed. The mixtures after heating were analyzed for iron, chromium and nickel. No increase in the content of these elements was observed. The capsules were charged in a glove box under purified argon and sealed by arc welding in the same glove box. The capsules were held at temperature for 2 hours with intermittent shaking and were held an additional hour to allow separation of the phases before quenching. The attainment of equilibrium under these conditions was verified by reaching the same equilibrium point starting with either barium and calcium chloride or calcium and barium chloride. Holding the samples a t temperature for up to 24 hours, or heating t o a higher temperature and cooling to the desired temperature, did not change the observed compositions. The capsule was opened in a glove box after slitting the walls with a silicon carbide cutoff wheel. Each phase was sampled and the samples placed in weighing bottles in the glove box. The salt phase was analyzed by determining the chloride content by precipitating and weighing as AgCl and hy titrating the alka1init.y to measure the amount of free motal.
.
1361
The barium and calcium contents were calculated from the chIoride analysis and free metal content. The high BCcuracy of the chloride analysis permitted a more accurate determination of the calcium and barium content than was achieved by flame photometry or by separation and gravimetric determination of the calcium and barium. The amount of chloride in the metal phase was determined by precipitating and weighing as silver chloride. The calcium and barium were precipitated together by adding sulfuric acid to an aqueous solution and diluting with absolute methanol t o a final concentration of 90% methanol by volume. The calcium and barium contents were calculated from the weight of the total sulfate precipitate and the chloride analysis. Since the reactants were weighed accurately and no material was lost from the sealed capsule, the analytical results were checked by a material balance. Thermal analyses were made on samples enclosed in type 304 stainless steel capsules 2.5" long and 7 / 8 1 1 in diameter with s / ~ 6 " diameter thermocouple well. These capsules were filled under argon in a glove box and sealed by arc welding. A differential thermal analysis method was used in which the sample temperature and the difference between the sample and the furnace temperature was recorded with a recording potentiometer. The thermocouples were standardized with a U. s. Bureau of Standards aluminum sample and a coulometer-grade silver sample. At both temperatures the thermocouple agreed within 0.5"with the table for chromel-alumel thermocouples in the N.B.S. Circular 561.
Results and Discussion The Ba-BaClz and Ca-CaClz binary systems were checked by thermal analysis and analysis of quenched equilibrium mixtures. The data on the Ba-BaClz system agreed very well with the phase system reported by Schiifer and Niklas.2 The monotectic was a t 15 mole yoBa and 890", the consolute temperature was 1017" and the eutectic temperature 710". The monotectic and eutectic temperatures of the Ca-CaClz system agreed with those reported by Cubicciotti and T h ~ r m o n d . The ~ monotectic was a t 09.5 mole % Ca and 820", and the 0 Ca and 768". The solubilities eutectic a t 2 mole 7 of each phase in the other a t 900 and 950", determined by analysis of quenched samples, ape reported in Tables I and 11. The observed solubility of calcium in calcium chloride was much lower than that reported by Cubicciotti and Thurmond. Water, calcium oxide or other impurities could result in high values for this solubility. Several samples, which were prepared from calcium chloride containing traces of water, gave solubilities as high as 20 mole yo. Thermal analysis results agreed with the lower solubility values as the monotectic arrest was observed in a 5 mole % calcium sample. The Ca-CaClz system differs from the Ba-BaClz system in the lower solubilities, a t the monotectic and eutectic temperatures, and in the much smaller increase of the solubility of each phase in the other with increasing temperature. The phase equilibrium data a t 950" are shown in Fig. 1 as an isothermal section of the ternary phase diagram. The diagram was plotted as a square figure to avoid the unequal ordinate dimensions of a truncated triangle. The solubility of metal in the salt phase decreased rapidly as the mole fraction of total barium in the salt decreased to 0.90. This solubility was nearly independent of the saIt phase composition over the remainder of the composition (2) H. Schafer and A. Niklas, Angew. Chem., 64, 610 (1952). (3) D. D. Cubicciotti and C. D. Thiirmond, J . A m . Chem. Soc., 71, 2149 (19.29).
DAVID T. PETERSON AND J. A. HINKEBEIN
1362
TABLE I EQUILIBRIUM COMPOSITIONS Te,mp..,
C.
950
900
Salt phase
NEB
NMat.1
1.000 0.198* 0.990 ,2780 .983 .I45 .964 .IO5 ,895 ,041 .885 .042 .841 .039 .613 .045 .454 ,030 .201 .010 0 .042 1.000 O.16Oa 0.920 .0326 .905 .0329 .896 .0314 .887 .0393 .884 ,0369 .849 .0276 .789 .0615 .763 .0358 0 .0380
Metal phase Concn. NE. N M C ~ quotient'"
1.000 0.856 .588 .335 .096 .070 .056 ,019 .010
... 0
1.000 0.081 .OS1
,063 .O63 .063 .044 .038 .027 0
0.120* .082 .027
.016 .0071 .0067 .0068
.021 .009 .012 .0156 0.052 .0050 .0036 .0038 ,0042 .0038 ,0044 .0048
.0050 .0102
... 17 40.5 53 80 102 89 82 82
... 130 108 128 117 113 122 94.7 116
a Concentration quotient = 1 - N,Ma )/ (1 - NF)(N,Mtal).* Determined by thermal analysis.
Incomplcte separation of metal phase.
Vol. 63
phase which was present as calcium chloride and calcium which was present as dissolved calcium metal. At the present time the nature of these solutions is not well understood and the value of making such a distinction has not been established. The concentration quotients calculated using the mole fraction of total barium and calcium are given in the last column of this table. The values are reasonably constant up to barium contents above 0.9 in the salt phase. At 900" the composition range was restricted by the boundary of a three phase (liquid metal, liquid salt and solid BaC12)field a t high barium contents. This was a result of the monotectic temperature being raised to a maximum of 914' in the ternary system. The concentration quotients did not change with composition over the limited composition range which was studied. The activity coefficient quotient was assumed to be unity because of the similarity of the component metals and of the component salts. Calcium and barium are reported by Sheldon4to form a complete series of solid solutioiis and should form nearly ideal liquid solutions. The CaC12-BaC12 system is reported by Bukhalova5 to have a eutectic a t 595" and 37 mole % barium chloride and a compound, CaC12-BaC12, melting a t 629'. The free energy for the reaction Ba(L)
+ C~CL(L)
BaClz(L)
+ Ca(L)
was calculated for each temperature, assuming that the concentration quotients were equilibrium constants. At 900°, the equilibrium constant was 116 and the free energy change -11.1 kcal. Using only the data up to 0.90 mole fraction of barium in the salt phase, the averaged equilibrium constant a t 950" was 87.0 and the free energy change was - 10.8 kcal. The enthalpy of reaction calculated from these two equilibrium constants was - 16.5 kcal. These results are compared with values calculated from compiled thermodynamic data in Table 11. TABLE I1 THERRIODYNAMIC
D A T A FOR THE
WITH
Source
This investigation B re wer6 Villa7 Ostertap
Fig. 1.-Isothermal
section of the Ba-CaClz-BaClz system a t 950".
range. The solubility of salt in the metal phase decreased as the metal phase composition varied from pure calcium to 10 mole yo barium and increased smoothly with further increase in barium concentration. In Table I are given the compositions of the equilibrium phases a t 950". Since the metal phase contained dissolved salt and the salt phase contained dissolved metal, the compositions were expressed by the mole fractions of total calcium and total barium and the mole fraction of the metal or metal chloride in solution. Chemical analysis of quenched samples cannot differentiate between calcium in the salt
REACTION OF BARIUM
CALCIUM CHLORIDE 9000
AF
9.500
- 11.1 - 8.4
- 10.8 - 8.2
-13.0
-13.1 -12.5
AH
- 16.5 -16.7 (At 1000")
The uncertainty of the compiled thermodynamic data makes an evaluation of the assumptions used in calculating the free energy from the experimental data impossible. This reaction has been investigated at 1000° by Ostertags using a similar method of equilibrating the mixtures. The average equilibrium constant was (4) E . Sheldon, "Exploration of the Phase Diagram of the CalciumBarium Alloy System," unpublished P1i.D. Thesis, Syracuae University Library, Syracuse. N. Y.,1949. (5) G. Bukhalova and A. Bergman, J. Den. Chem. (I.S.S.R., 21, 1723 (1951). (6) L. Brewer, L. Bromley, P. Gilles and N. Lofgren, "The Thermodynamic Properties of the Halides," L. L. Quill Ed.: "The Chemistry and Metallurgy of Miscellaneous Materials," McGraw-Hill Book Co., Inc., New York, N. Y., 1950, p. 76-192. (7) H. Villa, Soe. Chem. Ind., 69, Supp. 1, S 9 (1080). (8) €1. Ostertag, Compt. rend., 246, 1082 (1058).
Sept., 1959
HEATOF SOLUTIOX OF
A
BINARY MIXTURE OF FLUOROCARBONS
larger than that obtained by extrapolating our data to 1000". No explanation of this difference is obvious from the description of the procedure. Since the accuracy of the equilibrium constant depends on the determination of small amounts of barium and calcium in the presence of large amounts of the other, a small systematic error in the analytical method used in either investigation could be responsible. The solubility of metal in the salt phase shows an interesting dependence on composition. The amount of dissolved metal does not vary in proportion to the amount of each component of the salt, as would be expected if the solubility were governed by the solvent properties of the salt. A solubility curve which agrees quite well with the observed solubilities can be calculated by assuming that the solvent properties of the salt do not change with composition and each metal dissolves independently in the salt. Then N(ca,0s) = k l ' y t c a , ~ ) where N(Ca,Os) is the mole fraction of calcium metal dis-
1363
solved in the salt phase, N ( c ~ , M )is the mole fraction of total calcium in the metal phase and ICl is the mole fraction of calcium dissolved in pure calcium chloride. A similar expression was used to calculate the amount of dissolved barium metal. The calculated solubilities a t 950" were somewhat higher than the observed values. Assuming that each metal interacts only with its own ion, a solubility expression N(ca,Us) = h * N ( C a , M ) ' N ( C a , S ) where N(ca,s) is the mole fraction of total calcium in the salt, can be formulated. This leads to calculated solubilities which are low and do not agree as well as the first expression. A very close fit can be obtained by postulating that calcium interacts less with barium ions than with calcium ions. However, this introduces an empirical parameter and the significance of the better fit is questionable considering the approximation made by assuming the activity of each metal was directly proportional to the total mole fraction in the metal phase.
THE HEAT OF VAPORIZATION AND SOLUTION OF A BINARY MIXTURE OF FLUOROCARBONS1 BY E. I?. NEILSONAND DAVID WHITE Contribution of the Cryogenic Laboratory, Department of Chemistry, The Ohio State University, Columbus, Ohio Received December PS0IO68
A new method is discussed for the determination of integral heats of solutions for a completely miscible binary system. The method involves the use of a conventional low temperature calorimeter from which a mixture of known composition is completely vaporized. The results for the binary mixture chlorodifluoromethane-dichlorodifluoromethane over a wide composition range are given. The method appears quite satisfactory for systems exhibiting relatively small integral heats of solution.
The heat of vaporization of a pure substance is a well defined quantity. For binary mixtures, this quantity depends on the conditions under which the vaporization takes place, and in fact five such quantities can be defined.2 Four of these can be related to one another, provided additional thermodynamic information is available. From the experimental standpoint, the determination of the heat of vaporization of a binary system has required special types of calorimeter^,^ and possibly for this reason very little work has been reported on this subject. A method employing a conventional low temperature calorimeter is reported here. The quantity measured is related to the integral heat of Vaporization at constant temperature and composition, and can be used to determine the heat of solution of the binary liquid mixture provided the thermodynamic properties of the pure components are known. The heat of vaporization and solution over a wide composition range, of a liquid mixture consisting of chlorodifiuoromethane (Freon-22) and dichlorodifluoromethane (Freon-12), is reported here. These liquids are completely miscible a t the temperature at which the experiments were performed. (1) This work was supported b y the General Electrio Company, Sclienectady, Ncw York. (2) V. Fischer, Ann. Phusik., [ 5 ] 17, 209 (1933). (3) L . I. Dana, Pror. Amer. Acad., 60, 241 (1925).
Theory.-Consider the reversible vaporization of a binary mixture at constant temperature T from the calorimeter shown schematically in Fig. 1. Assume initially the calorimeter is completely filled with 1 mole solution containing nl moles of component 1 and nzmoles of component 2, and then completely vaporized. During the vaporization, the valve at the top of the calorimeter is adjusted so as to maintain constant temperature during the run. The material escaping the calorimeter (outside the adiabatic shield) is continuously removed to the gas storage system. Let Pi be the initial pressure in the calorimeter, and Pfthe final pressure, when the last drop of liquid is vaporized. For an infinitesimal amount of material vaporized from the calorimeter we can write clE
+ PdV
=
DQ
where DQ is the amount of heat added to the calorimeter or d H = DQ
+ V dp
where V is the volume of the calorimeter, equal to the initial nio1:ir volume of the liquid solution Vis. Integrating the above expression between the initial and final state (Pi+ Pf)we have H g s (T, P k , ' n k ( l ) , nk(2)) - HIS(2') Pi, 'nt, nz) =
C? + VI.
(Pf - Pi)
where Q is the amount of heat added to the calorim-
