1439
SOLUBILITY OF METALS IN LIQUIDSODIUM
A Study of the Solubility of Metals in Liquid Sodium. 11. The System Sodium-Lead by G. J. Lamprecht, L. Dicks, and P. Crowther National Nuclear Research Centre, Pelindaba, Pretoria, South Africa Accepted and Transmitted by The Faraday Society
( M a y 11, 1967)
The solubility of lead in liquid sodium has been determined in the temperature range 100-280". X-Ray data are given on a newly isolated intermeqallic compound nNa6Pb. From the solubility data the heat, Ar?r(aol),entropy, A&lol), and free energy, AFi, of solution have been calculated.
Introduction I n an earlier article,' methods for the determination of the solubility of dilute tin alloys in sodium were described. The present paper is the second in a series of investigations into the solubilities of metals in liquid sodium. Experimental Section Reagents. Helium used as cover gas in all experiments was purified as previously described2 and contained no detectable amounts of 0 2 , Nz, HzO, COZ,and CH,. The lead was spectroscopically pure as supplied by Johnson Matthey and Co. The sodium used was commercial, dry packed as supplied by E. Merck and Co. It was purified with respect to oxygen by successive filtration through 5-p sintered glass filters at 110". This method' yielded sodium with an oxygen content of 11 f 2 ppm. Procedure. Solubilitg Determination. Because no suitable radioactive lead isotope is available, it was not possible to carry out the solubility determinations by the radioactive technique used previously. The lead solubility determinations were therefore carried out with inactive lead in the apparatus shown in Figure 1. An amount of lead chips, in excess of its solubility under the particular experimental conditions selected, was placed on the filter of flask C and approximately 100 g of sodium was introduced into flask A. The whole apparatus was then heated to 120' and evacuated to a pressure of lou5 mm for 2 hr. The sodium was then successively filtered from A to B to C at 120". After the filtration the temperature of flask C was raised to the required value. The lead and the sodium were allowed to remain in contact with each other for 6 hr. Homogenization was assisted by bubbling small amounts of helium through the mixture from time to time. The sodium solution of lead was then filtered off into trap D and analyzed by direct acid-base titration for sodium and gravimetrically as chromate for
lead.* The above procedure was repeated at a number of different temperatures. Intermetallic Compounds. The solubility apparatus was also used for the preparation of the intermetallic compounds. The initial intermetallic compound which formed between sodium and lead was prepared in vessel C by the reaction of 30 g of sodium and 10 g of lead at 120". Owing to the exothermic nature of the reaction, the temperature rose (after 5 min) to 140". This temperature was maintained for 10 min. The excess sodium was then drawn off into vessel D and the compound remaining on the filter C was analyzed. A stoichiometric ratio of Na4Pb was obtained. (See Table I.) Table I
Melting point Mole ratio' X-Ray data
a
NarPb
NaoPb
374O 1:4.02 =k 0.06 Cubic a = 13.02
390" 1:5.01 =I=0.09 Hexagonal a = 10.6 c = 11.5
Average of six determinations.
The above procedure was repeated, with the exception that the sodium-lead mixture in vessel C was heated up to 160°, where it was kept constant for 70 hr, and the excess sodium then was filtered off. Analysis of the compound retained on filter C gave a stoichiometric ratio of Na5Pb (see Table I), indicating the slow conversion of NadPb to Na5Pb in the presence of excess sodium. (1) G. J. Lamprecht, P. Crowther, and D. M. Kemp, J . Phys. Chem., 71, 4209 (1967).
(2) J. Malgiolio, E. A. Limoncelli, and R. E. Cleary, "The Purifioation and Gas Chromatic Analysis of Helium," PWAC-362. (3) A. I. Vogel, "Quantitative Inorganic Analysis," 2nd ed, Longmans, Green and Co., New York, N. Y., p 420.
Volume 78, Number 6 May 1968
1440
G. J. LAMPRECHT, L. DICICS, AND P. CROWTHER
1qB
2.0
2.1
2.2
2.3
.+XI0
2.4
2.5
2.6
-3
Figure 2. Lead solubility in sodium; log (mole fraction of P b in N a ) us. reciprocal absolute temperature.
E:"'
VACUUM
Figure 1. Solubility and intermetallic compound preparation apparatus: 1, stopcocks; 2, glass ball joints; 3, B 29/32 gloss cones and sockets; TIthermocouple holes; Pa, 10-p filter; Pd, 5-p filters.
The compound K'asPb was also prepared as follows by crystallization. Sodium (30 g) at 110" was filtered from vessel A to vessel B where it reacted with 10 g of lead. The temperature was raised to 250" for 2 hr. The sodium plus dissolved lead was then filtered from vessel B to vessel C at 250" and the solution was cooled down to 110", followed by filtering off the excess sodium to vessel D. Long needlelike crystals crystallized out of solution. Analysis gave a stoichiometric ratio of NajPb. These crystals were used for crystallographic analysis.
Results and Discussion I n Figure 2, the plot of the mole fraction of lead dissolved in sodium us. reciprocal absolute temperature is given. The plot resulted in a good straight line with The Journal
of
Physical Chemistry
slope -2.64 X lo3, and the equation for the solubility based on the least-squares fit to Figure 2 is y = 3.515
-
2.639 X lo3 T
(1)
with a standard deviation in the value of y of =kOo.019, where y = log (mole fraction of Pb in Na) and T i s the absolute temperature. The heat ARf(so~) and entropy A s f ( s o l ) of solution can be calculated from the solubility data by means of the relation
where X,is the mole fraction of component i, and R is the molar gas constant. From the slope of the graph the heat of solution was calculated as 12.03 kcal mole-', and from the intercept on the log X axis at 1/T = 0 , the entropy of solution was calculated as 16.06 cal mole-' deg-'. The partial molal free energy of solution AF,, relative to the intermediate solid solute NasPb, is given by the following expression for solutions not saturated in the solute as
1441
SOLUBILITY OF METALSIN LIQUIDSODIUM
AFi
= 12,030 - 16.067'
+ 1.987T In X
(3)
The intermetallic compound Na4Pb has a melting point of 374". This compound has previously been reported in the literature4 where a melting point of 373" is given, whereas a compound NaI5Pb4 with a melting point of 386" is reported by H a n ~ e n . ~ The newly isolated compound Na5Pb which crystallized out of solution has a melting point of 390". From the results obtained in the experiments describing the preparation of the intermetallic compounds it may be concluded that in the temperature range studied, 120-245", the compound NarPb is initially formed but does not reach stable equilibrium, reacting further with the molten sodium to form Na5Pb. At equilibrium, therefore, the reference solute is nNa5Pb.
+ P b -%NarPb nNa4Pb + nKa nNa5Pb 4Na
(4) (5)
The stoichiometric integer n in eq 5 has been included since the crystallographic data given below preclude the simple formulation NasPb. Crystallographic Data Single crystals of the compound Na5Pb were prepared as described above. Inside a glove box filled with purified helium, a single crystal was sealed into a thin-
walled Lindemann glass capillary for rotation and Weissenberg photographs. Exposures were taken of the zero and first layer lines along the c axis. ~h~ cell emensions were: hexagonal, a = 10.6 A ; c = ll
,
l 1 . d A.
Considering the compound NadPb, Zintl and H arder6 have reported single-crystal work on a compound Na15Pbr and showed the compound to have a bod%-centered cubic structure with parameter a = 13.32 A. Shoemaker, Weston, and Rathlev' have confirmed the conclusion of Zintl and Hardera that this compound is body-centered cubic based on the composition Na15Pb4 and find no support for the conclusion of Stillwell and Robinson,s who claim on the basis of X-ray powder photographs that a phase to which they assigned a composition Sa31Pbg has a face-centered cubic lattice w i t h a = 13.30A. The relevant chemical and physical data for the two compounds NarPb and NasPb are summarized in Table I. (4) C.J. Smithells, "Metals Reference Book," Butterworth and Go. Ltd., London, 1955. (5) M. Hansen and K. Aderko, "Constitution of Binary Alloys," 2nd ed, McGraw-Hill Book Co. Ino., New York, N. Y.,1958. (6) E. Zintl and A. Harder, Z . Physik. Chem., B34, 238 (1936). (7) D.P. Shoemaker, N. E. Weston, and J. Rathlev, J . Am. Chem. SOC.,77, 4226 (1955). (8) C. W.Stillwell and W. R. Robinson, $bid., 55, 217 (1933).
Volume 72, Number 6 May 1968
