SOLUBILITY OF TININ LIQUID SODIUM
the fluorescence lifetime must be measured directly with a flashlamp, wavelengths of exciting light being selected in a similar way. In compounds such as pyrene, where both the rates of the radiative and radiationless transitions are affected by the vibrational state of the molecule, the quantum yield of fluorescence must also be determined. Only then can both rates be independently evaluated. In order to know more precisely which vibrational modes are being excited, it is necessary to use smaller compounds having line-like
4209
absorption spectra in the vapor, and to excite only single vibronic states. Acknowledgments. The author is indebted to Professor S. A. Rice for suggesting work in the area of radiationless transitions and for his generous support during its completion. This work was supported by the Directorate of Chemical Sciences, AFOSR, and the USPHS. The author also acknowledges a USPHS Predoctoral Fellowship between Oct 1962 and April 1964.
A Study of the Solubility of Metals in Liquid Sodium. I.
The System Sodium-Tin
by G. J. Lamprecht, P. Crowther, and D. M. Kemp National Nuclear Research Centre, Pelindaba, Pretoria, South Africa Accepted and Transmitted by The Faraday Society
(January 3, 1967)
The solubility of tin in liquid sodium has been determined in the temperature range 100260”. An experimental technique has been developed which overcomes the two major operational problems common to liquid metal solubility measurements, namely, sampling at temperature and ensuring the attainment of equilibrium. From the solubility data the heat, A R i c s o l , , entropy, A S i ( s o l ) , and free energy, A F i , of solution relative to the solid solute Na4Sn have been calculated.
1. Introduction Current interest in high-temperature compact nuclear reactors has emphasized the need for basic information on the nature of liquid-metal solutions. This paper presents the first results of current work on sodium systems in the authors’ laboratory. 2. Experimental Section Reagents. Purified helium was used as cover gas in all experiments. Purification was achieved by passing the gas over molecular sieves (previously activated by heating to 350” for 10 hr under a pressure of lo-’ mm), followed by an activated charcoal trap, cooled to liquid nitrogen temperature (Figure 1). This method of
purification has been shown‘ by gas chromatographic and mass spectrometric analysis to remove all traces of 0 2 , N2, H20, Con, and CH,. Commercial, dry-packed sodium (supplied by E. Merck and Co.), with the following maximum impurity limits, was used: chlorine (Cl), 0.002%; sulfate (Sod) 0.002%; phosphate (PO,), 0.001%; nitrogen (N2),0.005%; heavy metals, 0.002%; iron, 0.001%; calcium, 0.00570;potassium, 0.01%. The sodium, as described above, was purified with respect to oxygen by successive filtration into the (1) J. Malgiolio, E. A. Limoncelli, and R. E. Cleary, “The Purification of Gas Chromatographic Analysis of Helium,” PWAC-352, 1981.
Volume 71, Number 13 December 1967
G. J. LAMPRECHT, P. CROWTHER, AND D. M. KEMP
4210
Figure 1.
Solubility apparatus.
solubility apparatus. A three-stage filtering unit (Figure 2), with a final filtration at 120” through a 5-/1 porosity filter, yielded sodium with an oxide content of 11 f 2 ppm as determined by the vacuum distillation methods2 Spectroscopically pure tin (as supplied by Johnson Matthey and Co.) was used. The 118-day tin-113 tracer contained a small percentage of 60-day antimony125 impurity which was removed as follows. Tetravalent Sn was selectively extracted from 1 N HCl-1 N H2S04 and 1 vol. % H202 solution into 0.5 M 2-thenoyltrifluoroacetone in methylisobutyl k e t ~ n e . ~ A 1 M tartaric acid solution was used to strip the tin from the organic phase. Two successive extractions completely removed the Sb activity. The Sn tracer was electroplated onto a stable tin rod from a solution 0.25 and 2 M with respect to sodium tartrate and HCI, VACUUM respectively, and containing ca. 50 mg of Sn metal ion and 1 g/100 ml of hydrazine hydrochloride. The Figure 2. Intermetallic compound preparation apparatus: potential of the stable tin cathode was controlled (1) stopcocks; (2) glass ball joints; (3) B 29/32 glass manually at -0.7 v with respect to a saturated calomel cones and sockets; (T) thermocouple holes; (Pa) 10-p reference electrode, while the current density was apfilters; (Pd) 5-p filters. proximately 5 ma cm-2. After electroplating, the tin rod was washed and dried and then melted under an system (l), perchlorate and molecular sieve drying inert atmosphere of helium to obtain a uniform specific towers 2, 3, and activated charcoal traps operated a t activity of approximately 5 X lo4 counts/min g. liquid nitrogen temperatures 4; (b) assembly for sodium The isotope 118-day l13Sn decays through a 0.26purification by successive filtration (5a and b) ; (c) a Mev y ray to its metastable 1.7-hr l18In daughter radioactive detector system, consisting of a sodium which, in turn, decays through a 0.393-Mev y ray to iodide (Tl) detector (ll), lead shielding (12), electronic stable ll3I11. A period of 18 hr must, therefore, be counting equipment composed of a preamplifier (13), an allowed to elapse between readings to allow equilibrium to be established. (2) K. 9. Bergstresser, G. R. Waterbury, and C. F. Mets, “DeterSolubility Apparatus. The experimental apparatus, mination of Trace Amounts of Oxygen Added to Metallic Sodium” as shown in Figure 1, consisted of: (a) a helium purifiReport LA-3343, 1965. cation unit which was made up of a manometric inlet (3) J. R. Stokely and F. L. Moore, Anal. C h m . , 36, 1203 (1964).
Lcmi
The Journal of Phz/mkal Chemistry
SOLUBILITY OF TININ LIQUID SODIUM
amplifier discriminator (14), and a scaler (15); (d) reaction cells (7, 8) interconnected by means of capillary tubing; sodium could be transferred back and forth between the cells by means of vacuum or helium pressure; (e) furnace (6) with temperature control to & l o a t 600". The above apparatus was constructed in Pyrex glass for solubility determinations up to 250". Experimental Procedure. The radioactive metal for which the solubility was to be determined was placed in cell 8. After degassing the whole apparatus a t a temperature of 150" and a pressure of mm for 12 hr, approximately 30 g of purified sodium was introduced into cell 7. The sodium was transferred to cell 8 and allowed to remain in contact with the radioactive metal until, when transferred to cell 7, there was no increase in the measured activity. By determining the increase or decrease in specific activity of the liquid sodium, the variation of solubility with temperature was followed. Apparatus for the Preparation of Intermetallic Compounds. The apparatus, as shown in Figure 2, consisted of three filtering vessels, A, B, C, and a trap D, arranged in a cascade. Each vessel had an inlet through which either vacuum could be drawn or helium added. The metal which was to be reacted with the sodium was placed in vessel C as fine chips. Purified sodium was inserted in vessel A. After degassing the apparatus at a temperature of 150" and mm pressure for 1 hr, the sodium was filtered from h through B into C. At the completion of the reaction, the excess sodium was filtered off into trap D. The iIitermetallic compound formed was then removed from C and analyzed by direct titration for sodium and assayed radiometrically for tin. If the intermetallic compound was to be prepared by recrystallization, vessel C was heated to a temperature a t which all of the tin was taken up to solution. This was then slowly cooled to approximately 110", and the excess sodium was filtered off, leaving behind long needlelike crystals of the intermetallic compound. This apparatus has also been used as a check on the solubility measurements obtained with the apparatus described above. Excess Sn metal in cell C was allowed to remain in contact with the sodium at a particular temperature for approximately 6 hr. The sodium was filtered off into the trap D, the contents of which were then analyzed.
3. Results and Discussion The results of the experiments described above showed that tin, when in contact with liquid sodium, was completely converted into an intermetallic compound NarSn. The compound which crystallized out of solution also had the empirical formula NaBn with a melting
4211
i5
Figure 3. Tin solubility in sodium. Log [mole fraction of NQSn in Na] us. reciprocal absolute temperature: 0, heating sequence; x, cooling sequence; m, solubility from intermetallic compound preparation apparatus.
point of 411". Results are therefore given with N a S n as the intermediate reference solute. Figure 3 shows that the plot of log [mole fraction of Na4Sn] vs. reciprocal temperature is a good straight line of slope -2.303 X lo3. The equation for the solubility based on the least-squares fit to Figure 3 is y = 2.35
-
2.31
x
103
T
*
with a standard deviation in the value of y of 0.02, where y is log [mole fraction of Na4Sn in N a J and T is absolute temperature. The thermodynamic treatment of metallic solutions has been discussed by several The varia(4) 0. J. Kleppa, J. Am. Chem. SOC.,72, 3346 (1950). (5) 0. J. Kleppa, ibid., 73, 385 (1951).
(6) D. L. Johnson, "Phase Equilibria-Free
Energy Relationships
in Liquid Sodium Systems," NU-SR-8381, April 1964.
(7) 0. Kubaschewski and E. L. L. Evans, "Metallurgical Thermochemistry," Pergamon Press Inc., New York, N. Y., 1958.
Volume 71,Number I S
December 1967
G. J. LAMPRECHT, P. CROWTHER, AND D. M. KEMP
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tion of the solubility with absolute temperature, T,as a function of the heat and entropy ASi(*ol) of solutions is best given by the relation
the value reported by Hume-Rotherys at 290" and that calculated from eq 1, namely, 9.97 and 9.30 wt %, respectively . 4.
where Xi is the mole fraction of component i and R is the molar gas constant. From the slope of the plot in Figure 3 the heat of solution, AZ?i(sol), was calculated as 10.76 kea1 mole-' and the intercept of the log [X axis] at (1/T = 0) gave the entropy of solution, ASi(sol), as 10.78 cal mole-' deg-I. The partial molal free energy of solution AFi relative to the intermediate solid solution NQSn is given by the following expression for solutions not saturated in the solute as
AFi = 10.67 - 10.787' + 1.9877' In X
(3)
I n previous work reported on this system8a solubility increase from 1 wt % at 120' to 12.35 wt % a t 257" is given, compared with 0.153 to 5.24 wt % reported in this paper. However, there is fair agreement between
The Journal of Physieal Chemietry
Conclusions
The experimental technique described above overcomes the difficulty of ensuring that equilibrium has been attained, since the approach to equilibrium may be followed. Furthermore, sampling at temperature does not present any difficulties as measurements are made in situ. The apparat'us is applicable to solubility measurements ranging from a fraction of 1 ppm to several per cent, provided the metal for which the solubility is to be determined has a radioisotope with suitable nuclear decay characteristics. Thus, these solubility measurements should provide a means of determining heats and entropies of solution for systems whose range of compositions are so low that ordinary thermodynamic methods for their measurement are impracticable. (8) C. B. Jackson, Ed., "Liquid-Metals Handbook: Sodium (NaK) Supplement," USAEC Report TID-5277,July 1965. (9) W.Hume-Rothery, J . Chem. Soc., 131, 947 (1928).
