J. Phys. Chem. 1981, 85, 275-280
the closeness of the static and spin susceptibility data3J1 as observed for sodium ammonia solutions. The same type of experiments should be performed in lithium-ammonia solutions to check the closeness between static and spin susceptibility.
Conclusion The magnetic susceptibility of lithium-ammonia solu-
275
tions has been measured for the metallic solutions and for solutions in the metal-nonmetal transition range for the classical technique. These measurements confiim previous data related to Na- and Cs-NH3 solutions, but the present measurements have been extended to low enough concentrations to display nonambiguously the strong diamagnetic pairing of the electronic species on the dilute side of the metal-nonmetal transition range.
Ytterblum-Ammonia Solutions R. Hagedorn and J. P. Lelleur’ Laboratoire des Surfaces et Interfaces, L.A.253 du CNRS, L i k , France (Received: JuW 17. 1979; In Final Form: August 6, 1980)
Differential thermal analysis and vapor-pressure and density measurements were performed on the ytterbium-ammonia system. Vapor-pressuremeasurements, as a function of ytterbium concentration, between -65 and 4 0 “C, show the existence of a liquid-liquid phase separation between -0.6 and 4.5 mol % of metal (MPM) and the existence of the compound Yb(NH3)=,where x is 6.5. The critical temperature of the liquid-liquid phase separation is above room temperature. Thermodynamic data are obtained from vapor-pressure measurements for concentrated solutions. The densities of ytterbium-ammonia solutions increase with ytterbium concentration, while in Li-, Na-, K-, and Ca-NH3 solutions, the density decreases with increase of metal concentration. Introduction It is often quoted that rare-earth metals ytterbium and europium are soluble in liquid ammonia. Warf and Korstl were the first to observe solutions of ytterbium and of europium in liquid ammonia and to suggest the existence of a compound between each of these metals and ammonia. However the solutions of ytterbium and of europium in liquid ammonia have not been as well studied as, for instance, solutions of alkali metals in liquid ammonia. Schroeder, Thompson, and Oerte12measured the electrical conductivity and noted a close similarity in behavior between ytterbium- and alkaline-earth-ammonia solutions and suggested that ytterbium be treated as divalent in these solutions. From electrical-conductivity measurements they established the composition (7.1 mol ?% of metal (MF’M) and temperature 183K) of the eutectic point of these solutions. Thompson, Stone, and Waugh3 investigated the composition dependence of the vapor pressure of ammonia over solutions of europium and of ytterbium in liquid ammonia at -75.9 OC. They also found that these two metals formed M(NH3), compounds where M is either europium or ytterbium, and n obtained by extrapolation of the ammonia vapor pressure to zero value was 6.3 and 6.4 for europium and ytterbium, respectively. More recently Frisbee and Senozan4measured the equilibrium pressure of ammonia over the europium- and ytterbium-ammonia compound. Therefore, it must be realized that basic physical data for ytterbium-ammonia solutions are rather sketchy. In the present paper, information about the phase diagram obtained by differential thermal analysis and vapor-pressure measurements are reported with the densities of dilute ytterbium-ammonia solutions. (1) J. C. Warf and W. Korst, J. Phys. Chem., 60,1590 (1956). (2) R. L.Schrder, J. C. Thompson,and P. L. Oertel, Phys.Reo., 178, 298 (1969). (3) D. S. Thompson, M. J. Stone, and J. S. Waugh, J. Phys. Chem., 70, 934 (1966). (4) R. H.Frisbee and N. M. Senozan, J. Chem. Phys.,57,1248 (1972).
Experimental Section Vapor-Pressure Measurements. The vapor pressure of ammonia over the ytterbium-ammonia solutions was determined in a vacuum line with calibrated volumes by using a procedure similar to that of Marshall and Hunt.s The vacuum line was connected with an ammonia reservoir, where ammonia was stored over sodium for drying, a mercury manometer, and a tube with known volume, which could be isolated from the vacuum line by means of a stopcock. The reaction cell where the solution of ytterbium in liquid ammonia is made was connected to the tube with a grinding. An ionization gauge indicated that a pressure less than 5 X lo4 torr could be attained in the system. The reaction cells, which had a cylindrical or spherical form, with a volume of 20 mL were cleaned by standard proceduree before use. The ytterbium (99.5% purity from Alpha Inorganics) was stored under argon, and before use its surface was cleaned mechanically; the ytterbium was cut into pieces, weighed with a microbalance, and introduced into the reaction cell. The reaction cell was then connected to the vacuum line and was pumped down to a pressure less than 5 X lo4 torr for more than 12 h. The vacuum line was then isolated from the vacuum pump, and ammonia from the storage reservoir was evaporated into it. The ammonia pressure in the line was determined with a cathetometer of f0.02-mm precision. The room temperature was read with a mercury thermometer, and the number of moles of ammonia in the line was calculated with the van der Waals equation, the constants of this equation for ammonia gas being taken from the “Handbook of Physics and Chemistry”. Ammonia was condensed on ytterbium at dry-ice or liquid-nitrogen temperature. An alcohol thermostated bath was then placed around the sample tube, the temperature of which was held constant within *0.05 “C at N
(5) P. R. Marschall and H. J. Hunt,J.Phys. Chem., 60, 732 (1966). (6) A. Demortier, P. Chieux, and G. Lepoutre, Bull. SOC.Chim. Fr., 10, 3421 (1971).
OO22-3654/81/2O8~-0275$01.OW0 0 1981 American Chemical Society
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the temperature of the experiment. After reaching the equilibrium, the amount of condensed ammonia, and thus the concentration, were calculated by reading the ammonia vapor pressure of the solution at the equilbrium and calculating the difference between the total amount of ammonia and the amount of ammonia in the gas phase. The concentration of the solution was then changed by isolating the sample from the line with the above-mentioned stopcock and recondensing a small part of the ammonia gas in the line into the ammonia reservoir. The stopcock was then reopened and the new equilibrium pressure at the new concentration waited for. This procedure was then repeated several times. After the end of each run, all of the ammonia in the line was condensed on the solution at liquid-nitrogen temperature, in order to detect any decomposition by means of the residual hydrogen pressure. The experimental results have not been perturbed by decomposition of the solutions since only negligible amounts of hydrogen were detected. Two different series of runs were performed, one starting at -2 MPM, the other starting at -7 MPM, because an error analysis shows that the relative error of the concentration of the solution depends on the amount of ammonia present in the condensed phase and of the number of removals of ammonia. For the two'different series of runs, since the amount of ammonia initially evaporated from the reservoir was approximately constant (-0.2 mol), only the amount of ytterbium was varied between 0.2 and 1.5 g depending on the starting concentration and the temperature of the experiment. Thus the amount of ammonia present in the condensed phase of concentrated solutions was greater when starting from a 7-MPM solution, and the relative error of the metal mole fraction of solutions of 14 MPM was always less than 2%. Differential Thermal Analysis (DTA) Experiments. The DTA experiments were performed with an apparatus previously described by Debaecker7 and Flipo.* The sample and reference cells were constructed from 4-mm Pyrex tubing. A weighed amount of ytterbium (20-50 mg) placed in the sample cell, and the sample cell was then connected to the above-described vacuum line. The amount of ammonia to be condensed was determined to obtain a chosen metal concentration. Ammonia (of the order of -0.005 mol) was condensed on ytterbium at liquid-nitrogen temperature. The sample was sealed off and stored at dry-ice or liquid-nitrogen temperature. The more concentrated solutions were submitted to an ultrasonic beam to assure complete dissolution of ytterbium in liquid ammonia. For the DTA experiment, the sample and the reference cells were located in holes of a cylindrical copper block. A good thermal contact between the cells and the copper block was assured by aluminum powder placed around the cells. The reference cell was filled with aluminium powder. Copper-constantan thermocouple junctions were imbedded with aluminum powder in a well at the bottom of each cell. A two-track Sefram recorder displayed the emf corresponding to the temperature of the sample cell, and the difference in emf between the sample and the reference cells. The temperature thermocouple was calibrated with some fixed points. Differences in emf of the order of 5 p V between the sample and the reference
-
(7) F. Debaecker, D.E.A., Orsay, France, 1973. (8)J. Flipo, ThBse, CNAM, Lille, France, 1972. (9) J. Jander, "Anorganische und allgemeine Chemie in flllssigem Ammoniak, Interscience, New York, 1966. (10)P. Damay, ThBse, University of Paris, Paris, France, 1972. (11) P. R.Marschall, J. Chem. Eng. Dota, 7, 399 (1962). (12) C. A. Kraus, E. S.Carney, and W. C. Johnson, J. Am. Chem. SOC., 49, 2206 (1927).
Hagedorn and Lelieur
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Flgure 1. Ammonia vapor pressure in mgHg over ytterbium-ammonia Solutions vs. ytterbium concentration in mol % metal (MPM) at different temperatures.
cells could be easily detected. The copper block was suspended in a Dewar flask containing liquid nitrogen and was cooled slowly down to liquid-nitrogen temperature. When the block was at the liquid-nitrogen temperature, the Dewar flask was lowered so that the block was not in contact with liquid nitrogen any more. Heating was then obtained by a resistance wire wound around the copper block, and the transition thermal effects were detected with a warming rate varying between ca. 1 and 5 OC/min. Density Measurements. Density measurements of ytterbium-ammonia solutions have been performed with a Pyrex pycnometer. The spherical part of the pycnometer had a volume of -4 cm3,to which a calibrated tube of 3 or 5 mm was sealed. The volume of the spherical part of the pycnometer and the diameter of the calibrated tube were calibrated with mercury. These density measurement cells were cleaned by the usual procedure previously mentioned. A weighed amount of ytterbium was introduced into the cell, and the cell was connected to the vacuum line and pumped down to a pressure less than 5 X lo4 torr for more than 12 h. The amount of ammonia to be condensed on ytterbium was adjusted to obtain a chosen metal concentration and the level of the liquid in the calibrated tube for the temperature range of the experiments. Ammonia was condensed on ytterbium at liquid-nitrogen temperature; the cell was then sealed off, warmed up to --30 "C to dissolve the metal, and submitted to an ultrasonic beam to assure a complete dissolution of the metal. To perform the measurements, we placed the sample cell into a thermostated alcohol bath, whose temperature could be held constant within fO.05 O C . The thermostated bath had windows through which it was possible to read the level of the liquid in the calibrated tube with the above-mentionedcathetometer whose accuracy is 10.02 mm. The temperature of the bath was changed gradually, and the position of the level of the liquid and the thickness of the meniscus were followed with the cathetometer vs. the temperature of the sample. Results Vapor-Pressure Experiments. The resullx of two series of vapor-pressure measurements are presented graphically in Figures 1 and 2. The experiments reported in Figure
The Journal of Physlcal Chemlstry, Vol. 85, No. 3, 198 1
Ytterbium-Ammonia Solutions
277
TABLE I: Vapor Pressure (VP) in mmHg and Concentration in MPM
-64.5"C concn
-60.2 " C
VP
2.1 2.3 2.4 2.6 2.8 3.0 3.2 3.5 3.9 4.3 4.7 5.4 6.0 6.3 7.2 7.4 8.0 8.4 8.6 10.2 11.1 11.6 1l.7
125.46 125.60 124.88 125.38 125.38 123.76 123.72 125.02 125.00 122.08 118.52 113.68 107.34 107.24 101.70 99.96 95.32 91.72 89.04 86.22 84.6 2 69.20 53.58
concn 2.1 2.2 2.4 2.7 2.9 3.3 3.7 4.2 4.9 5.6 6.5 7.2 8.2 9.6 11.4 11.5
VP 167.00 167.36 166.74 167.67 167.36 167.18 166.78 165.26 161.16 152.92 144.04 133.40 121.42 112.30 103.96 79.66
- 56.7 "C
concn 1.4 1.5 1.6 1.7 1.9 2.1 2.6 3.2 4.4 5.4 5.8 7.1 8.0 8.5 9.2 9.4 10.9 12.1 12.2
-49.1 "C
vp 216.00 215.26 213.66 213.94 213.26 214.06 214.06 213.70 210.67 198.16 193.04 175.90 164.46 157.62 148.20 ,144.16 140.12 129.34 110.82
concn 3.1 3.4 3.8 4.1 4.4 4.8 6.1 7.3 8.5 8.8 10.5 12.4 12.5 12.6 12.7
-45.4 "C
vp 317.88 318.88 320.18 316.64 307.88 301.08 277.70 256.44 233.48 224.60 199.36 182.20 167.80 128.26 98.39
concn 1.6 1.7 1.9 2.1 2.3 3.0 4.2 5.0 5.9 7.0 7.9 8.4 9.0 9.1 11.0 11.5
-40.2 "C
vp 393.02 393.88 394.12 392.98 393.16 392.86 389.68 372.14 350.10 324.18 299.02 282.16 262.04 256.06 243.54 228.16
cOncn 2.2 2.5 2.8 3.3 3.7 4.4 6.3 8.2 10.3 11.6 11.7 11.8
vp 546.28 547.02 547.06 546.90 543.46 529.26 471.92 397.47 328.42 3 25.84 298.96 272.84
TABLE 11: Vapor Pressure (VP) in mmHg and Concentration in MPM
-59.8"C
- 55.5 "C
-45.0 "C
-37.0 "C
- 29.9
"C
concn
VP
concn
VP
concn
VP
concn
VP
concn
VP
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137.26 131.52 127.80 121.90 119.50 114.58 110.00 103.42 102.80 103.80 103.16 100.38 81.36 69.84 54.62 37.02 22.52 11.60 5.24 2.80 2.21 1.96 1.80 1.88
7.2 7.5 8.1 8.7 8.8 9.0 9.6 10.3 11.1 11.4 12.3 12.5 12.5 12.7 12.8 12.8 12.9 12.9 13.0 13.1 13.3 13.4
175.80 169.60 161.94 153.24 151.83 148.14 137.48 137.56 136.96 137.02 128.92 106.52 86.24 54.48 39.94 24.16 19.76 16.04 8.26 6.08 3.14 3.22
7.8 8.3 8.4 8.5 8.8 9.0 9.3 9.5 9.8 10.4 11.0 11.7 12.3 12.6 12.7 12.8 12.8 12.9 12.9 13.0 13.0 13.1 13.4 14.0
306.74 298.10 292.60 287.54 281.20 273.52 264.12 255.08 245.90 244.59 244.63 244.20 219.92 158.40 127.56 99.50 73.78 52.68 40.26 25.46 18.08 11.94 8.98 9.26
8.0 8.2 8.7 9.3 9.7 10.3 12.3 12.4 12.6 12.9 13.0 13.1 13.2 13.2 13.3 13.5 13.9 14.6 15.4
490.28 475.84 461.22 431.10 405.76 387.00 381.88 358.38 317.26 219.08 161.90 117.64 75.68 51.56 36.46 20.68 17.60 17.72 17.76
8.3 8.5 8.9 9.2 9.4 9.8 10.0 10.1 10.4 11.4 12.2 12.5 12.8 13.0 13.2 13.2 13.4 13.5 13.6 14.5 15.6
683.94 663.28 640.46 620.42 610.09 576.50 561.30 546.50 532.30 524.32 524.42 509.02 390.10 279.54 194.82 134.74 62.90 49.74 29.82 31.84 29.30
1 have an initial concentration of -2 MPM before the removal of ammonia, while in Figure 2 the initial concentration is -7 MPM. Therefore the uncertainty of the concentration in concentrated solutions is smaller in the experiments of Figure 2 than in those of Figure 1. The experimental values corresponding to Figures 1 and 2 are reported in Tables I and 11. These experiments allow the determination of the phase boundaries of the ytterbiumammonia system. Following Gibbs's phase rule, the system in monovariant if two phases are present and nonvariant if three phases are present. The regions of Figure 1with constant vapor pressure indicate the regions of nonvariance of the system. The boundaries of the constant-vapor regions correspond to phase boundaries in the phase diagram. Considering Figures l and 2, three regions with constant vapor pressure vs. metal concentration are easily seen. Starting from the less concentrated solutions, the first plateau corresponds to a liquid-liquid phase separation, the second plateau to the equilibrium between a
saturated solution and the ytterbium-ammonia compound, and the third plateau to the equilibrium between the ytterbium-ammonia compound and ytterbium metal. The liquid-liquid phase separation was visually detectable, especially at relatively low temperatures, and shows blue- and bronze-colored liquids, the bronze liquid being the heavier one (as also confirmed by our density measurements), in contrast to the previously observed liquid-liquid phase separations in other metal-ammonia systems. To our knowledge, this is the first time that the liquid-liquid phase separation has been observed in ytterbium-ammonia solutions. However, it must be noted that the contrast in color between the two liquid phases is not as evident as, for instance, in lithium-ammonia solutions. With our vapor-pressure experiments it was not possible to detect the more dilute side of the liquid-liquid phase separation because the vapor pressure of pure ammonia is very close to the vapor pressure observed in the region
278
The JOUrn8l of Physical Chemistry, Vol. 85, No. 3, 1981
Hagedorn and Lelieur
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of the two liquid phases, but visually it was estimated to be -0.8 f 0.2 MPM in the temperature range of the vapor-pressure experiments. The critical temperature of this liquid-liquid phase separation could not be detected with our experimental arrangement because we were limited to vapor pressures less than 1 atm. After ammonia is removed from the saturated solution, the vapor pressure could be expected to drop abruptly to yield the pressure in the third region of nonvariance corresponding to the Yb/Yb-(NH3), system, the start of this region indicating the stoichiometry of the Yb(NH3), compound. In fact, the drop of the pressure is not abrupt but is apparently better described to be exponential. This phenomenon has been mentioned by Senozan et al.4J3in their work on metal-ammonia compounds. It is not quite clear, in our opinion, whether this behavior reveals only experimental effects or whether a real physical phenomenon is observed, such as an absorption of ammonia on the compound or a solid solution of ammonia with the compound. When considering Figure 2, one gets the impression that the composition of the compound is not constant, but varies with temperature, being Yb(NH3)6.6(i.e., 13.2 MPM) a t -60 "C and Yb(NH3)6.4(Le., 13.6 MPM) at -30 "C, but this difference in composition is of the same order as our estimated experimental error, which is larger at the end of the ammonia-removal procedure at a given temperature. The experimental error could be of the order of f0.2 MPM, so we can take this variation in composition of the compound found by our measurements only as a hint that the composition of the compound might be temperature dependent, not as convincing proof. Our experimental results and the analysis of the experimental errors prove that the composition is not Yb(NH3)6(Le., 14.3 MPM). DTA Experiments. Further information about the phase diagram has been obtained by means of DTA experiments, performed in a wide range of concentrations, from ca. 0.4 to 20 MPM, the temperature of each sample (13) S. Dickman, N. M. Senozan, and R. L. Hunt, J.Chen. Phys., 62,
2657 (1970).
being raised from liquid-nitrogen temperature to room temperature. Two main thermal events could then be observed. The first peak, from the low-temperature side, corresponds to the fusion of eutectic. It was only observed when the concentration of the sample was smaller than the concentration of the ytterbium-ammonia compound. If this compound, or this compound and metallic ytterbium, were present, this peak could not be observed. The temperature of the eutectic was found to be -88.5 f 0.5 "C, a result which has to be compared with the -90 "C value given by Thompson et a1.2 For a solution of ytterbium in ND3, the temperature of the eutectic was found to be -83.0 f 0.5 "C. If the eutectic concentration is approached, the relative intensity of the eutectic peak becomes larger and becomes the only event observed at the exact eutectic composition. In this way, a value of 7.7 f 0.2 MPM has been obtained for the eutectic composition, a value which can be compared with the 7.1-MPM value given by Thompson et al.2 Theoretically it is possible to determine in a similar way the composition of the compound, since, as the metal concentration increases from eutectic composition to the compound composition, the surface under the eutectic peak should linearly approach zero. The value found by using this procedure gives a stoichiometry of Yb(NH,),, at -83.6 "C (the temperature for the eutectic area vs. composition plot). It is not as precise as the value found by our vapor-pressure measurements but does confirm it. The other peak observed in the thermal diagram corresponds to the peak of the liquidus and allows the establishment of further information about the phase diagram. Its form depends on the form of the liquidus in the phase diagram, and more precisely on the slope of the liquidus. We have observed three different forms of this peak (Figure 3): (a) one on each side of the eutectic composition, from ca. 6 to 10 MPM, where the slope of the liquidus is large; (b) one between ca. 4.5 and 6 MPM where the slope of the liquidus is smaller; (c) the last one under the liquid-liquid phase separation, where the slope of the liquidus is zero. Thus the different forms of the liquidus peak allow the location of the liquid-liquid phase separation zone. However it must be noted that the signal, corresponding to the change from the liquid-liquid zone, is not clearly observed, probably because it is above room temperature, except in a narrow concentration range. Finally the experimental points of the phase diagram found with DTA technique are reported (Figure 4) together
The Journal of Physical Chemistry, Vol. 85, No. 3, 198 1
Ytterblum-Ammonia Solutions 30 .
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TABLE 111: Density of Dilute Ytterbium-Ammonia Solutions at - 50 "C concn, density, concn, density, MPM g cm-3 MPM g cm-3
0.0109 0.0125 0.0189 0.0254 0.0369 0.0594
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with the points found with the vapor-pressure-measurement experiments. Density Measurements. The density-measurement results are presented graphically in Figure 5 as a function of the decimal logarithm of the concentration and in Figure 6 as a function of the concentration (also see Table 111). The advantage of the plot of Figure 5 is to expand the range of the dilute solutions. In contrast to other metal-ammonia solutions, the density of ytterbium-ammonia solutions increases with increasing metal concentration, a t least for concentrations larger than 0.1 MPM. I t is clearly seen in both Figures 5 and 6 that the density does not decrease continuously toward the density of pure liquid ammonia when the metal concentration decreases. The experimental data suggest a maximum in the density curve between 0 and 0.05 MPM. Work is continuing to confirm these results. Such a maximum in the density corresponds to a minimum of excess volume. The existence of such a minimum in the excess volume has been controversial in dilute sodium and potassium-ammonia solutions. Orgell, Filbert, and EversI4 showed the existence of such a minimum in dilute solutions of sodium and potassium in liquid ammonia at -45 "C. However Gunn15 did not confirm these results, and it was never clear, in our opinion, whether this effect existed or not. In our experiments, the effect seems to be much larger and directly visible on the plot of the density vs. the metal concentration curve. Even if the experimental errors Ap are *1.5 X the trends in the experimental curve cannot be due to the experi(14)C.W.Orgell, A. M. Filbert, and E. C. Evers, Collog. Weyl, 1963, 67 (1964). (15)S.R.Gunn, Collog. Weyl, 1963, 67 (1964).
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mental errors. However, as a consequence of the controversy about the minimum of the excess volume in sodiumand potassium-ammonia solutions, more experiments are needed to confirm whether we are dealing with a true physical effect or with an experimental artefact. Accurate density measurements for concentrated ytterbium-ammonia solutions are difficult with the present technique, because the walls of the cell at the upper level of the liquid are not free of traces of solutions and as a consequence the meniscus is not well-defined. However preliminary experiments indicate that between 4.5 and 8 MPM the density increases from 0.87 to 0.97 g ~ m - ~ . Therefore, except for the possible anomaly of very dilute solutions, the densities of ytterbium-ammonia solutions increase with the ytterbium Concentration. The volume expansion AV of the metal cannot be reported accurately
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The Journal of Physical Chemistry, Vol. 85, No. 3, 1981
at the present time, but in the concentration range of 4.5-8 MPM preliminary experiments indicate that it is positive and of the order of 15 cm3mol-' of ytterbium in solution. Thermodynamical Calculations. The fundamental laws of thermodynamics can be used to calculate various thermodynamical values, if the vapor pressure is known as a function of concentration and temperature. If one considers ammonia as an ideal gas, the activity, a', of ammonia is expressed as a function of the vapor pressure, PI,of ammonia for the solutions and of the vapor pressure, Po, of pure ammonia by the ratio a1 = PdPO The activity coefficient y1 of ammonia is then expressed with the mole fraction x1 of ammonia by 7 1 = al/xl The excess free enthalpy of dilution, AG,,, is then expressed vs. the activity coefficient y1 and related to the corresponding excess values of enthalpy and entropy by the relation AG,, = RT In y1 = AH,, - TAS,, It is therefore clear that AHexis the slope of the plot of In y1 against 1 / T . In practice these calculations offered some difficulties since the temperatue of the experiments had to be known quite accurately in order to have correct values for the vapor pressure Po of pure ammonia which is strongly temperature dependent. Therefore it appeared necessary to find an internal calibration to bring the different experiments to a common temperature scale, in order to minimize the errors in temperature measurements resulting, for instance, from the use of different thermometers or thermometer position. In order to realize this internal temperature calibration, we did not use the experimentally measured temperature of the experiment for the calculation of Po, but we used the ammonia vapor pressure found in the region of liquid-liquid phase separation, and we recalculated the correspondingtemperature by using the vapor-pressure values of pure ammonia? The differences could be of the order of f0.5 OC between the measured temperature during the experiment and the temperature deduced from the ammonia vapor pressure in the region of liquid-liquid phase separation. In order to check the validity of our procedure, we performed control runs in measuring the vapor pressure of pure ammonia and of a solution in the region of liquid-liquid phase separation as a function of temperature with experimental arrangements as similar as possible. Within experimental errors, the same vapor pressure against temperature curves were found for both experiments, and the same heat of evaporation was calculated. In fact, our procedure means that we force the excess values to be zero in the region of liquid-liquid phase separation, an approximation which is quite reasonable. For the thermodynamical calculations, only the experimental runs starting from dilute ytterbium concentrations have been considered. To compare the different experiments, we fitted a polynomial with a least-squares procedure, to the experimental values of the vapor pressure at a given temperature, in the range of homogeneous liquid solutions from -4.5 to 9 MPM, and at a given concentration the values of the vapor pressure vs. the temperature could then be interpolated. The temperatures were corrected with the procedure described above, and the thermodynamical data were calculated. The results of these calculations are presented in Figure 7. Similar excess thermodynamical quantities had previously been determined by DamaylO for Na-NH3 and Li-NH3 solutions.
Hagedorn and Leiieur 300 cal/mol
250 200 150
1 A
100
1
/
i
58
1
-108
f
-150
1
-208
-250 -300
1 1
I
+
1
\\\ \ \
-T*AS.I:
\
\
Figure 7. Excess thermodynamicalquantities vs. the ytterbium concentration (MPM). Note that the concentration scale starts at 5 MPM.
During our experiments, vapor-pressure measurements over the ytterbium-ammonia solid compound have been made. Similar measurements had been made by Frisbee and Senozan4 which summarized their results, with a least-squares analysis, by the expression In p = -4488/T + 21.93 In the same manner, our results are summarized by the expression In p = -4432/T + 21.63 Our results are therefore in reasonable agreement with the results of Frisbee and Senozan. Conclusion More basic information has been obtained for the ytterbium-ammonia system. Some aspects of the phase diagram have been obtained. It has been found visually and with vapor-pressuremeasurements that a liquid-liquid phase separation exists, as for Li-, Na-, K-, and Ca-NH3 solutions, but, contrarly to the previously observed liquid-liquid phase separation in metal-ammonia solutions, in ytterbium-ammonia solutions the concentrated phase is more dense than the dilute phase. This is coherent with the observed increase of density with metal concentration. The existence of an ytterbium-ammonia solid compound with a stoichiometry greater than 6 has been verified by vapor-pressure and DTA experiments. The fusion temperature of this compound is above room temperature. The measurements of ammonia vapor pressure over this compound vs. temperature confirm the previous measurements of Frisbee and Senozan. Vapor-pressure measurements of the ytterbium-ammonia solutions have been used to determine the excess thermodynamical quantities. Acknowledgment. One of us (R.H.) expresses thanks for a Deutsche Forschungsgemeinschaft-Centre National de la Recherche Scientifique postdoctoral exchange grant. We thank P. Bailleul, A. Depriester, D. Dewally, P. Duquennoy, J. Fakeure, J. F. Lambert for technical assistance during the experiments, P. Damay for helpful discussions and suggestions, and G. Lepoutre for a critical reading of the manuscript.
