NOTES
124
moIes/l.). The number of moles released into the solution at the trailing meniscus, for each square centimeter of siliconed glass surface, was therefore (1/2) (1.5/100) (0.6/2) (1/41) (1/1000) = 5.5 X times the concentration (in moles/l.) of the solution entering the capillary. From this value, and the data in Fig. la, it can be calculated that only about 3% of the increment r obtained in the experiments described previously is due to testosterone from the water-siliconed glass interface. In order to compare the results of the direct measurement of concentration at the water-air interface with those obtained by an independent method, the surface tension, y, of the solutions was determined for several concentrations of testosterone. A du Nouy tensiometer (Central Scientific Co., Model 70535) with a platinum ring of 6 cm. circumference, was used for these experiments. The results are given in Fig. lb, expressed in terms of the surface pressure ?r = yo y, where yo is the surface tension of pure water. Within experimental error ( f 0 . 2 dyne/cm.) these points lie on a straight line. From the slope of this line the concentration at the interface, I’, can be calculated by means of the approximate form of Gibbs’ equation
-
r = - c-
dr
RT dc where c is the concentration of the subsolution, R is the gas constant, and T i s the absolute temperature. This calculation gives the straight line in Fig. la. It can be seen that the interfacial concentrations measured directly are slightly higher than those found by means of Gibbs’ equation. This result is to be expected, since no precautions were taken to maintain the solution in equilibrium with its own vapor; failure to take such precautions has been shown4 to lead to discrepancies of the kind observed. Errors also may have arisen from the fact that, at concentrations above 8 or 9 X mole/l. , the testosterone solutions are supersaturated.s The interfacial concentration measured directly and that calculated from equation 1 are strictly speaking not comparable,6 but the factor relating the two is in these experiments so close to unity as to be negligible. The present method seems to have some advantages over other m e t h ~ d s ~ for J - ~directly measuring surface concentration, particularly in regard to simplicity of equipment and technique. Although the accuracy in the experiments reported here is not high, there is no reason to suppose it could not be improved by introducing certain refinements. Among these, the most important would be a more accurate method for measuring small differences in concentration, such as the interferometric method used by McBain and H ~ m p h r e y sand , ~ the use of a vessel o&optimum size, with perfectly ffat bottom and vert?&alsides. Some arrangement for maintaining vapor pressure equilibrium above the (6) F. Bischoff and H. L. Pilhorn, J . B i d . Chem.. 174,663 (1948). (7) J. W. McBain, G. F. Mills and J. F. Ford, Trans. Faraday Xoc., 86,930 (1940). (8) E. Hutchinson, J . Colloid Sei., 4,699 (1949). (9) J. K. Dixon, C. M. Judson and D.J. Salley, in “Monomoleaular Layers,’’ed. by H. Sobotka, American Association for the Advancement of Science, 1954, p. 83.
Vol. 62
solution, and for controlling the temperature within narrow limits, should also improve the results. The author is indebted to Dr. Jesse F. Scott for advice and encouragement during the course of these investigations. HIGH TEMPERATURE HEAT CONTENTS OF STANNOUS AND STANNIC SULFIDES BYRAYMOND L. ORRAND A. U. CHRISTENSEN Contribution from the Minerals Thermodynamics Experiment Statipn, Region II, Bureau of Mines, United States Department of the Interior, Berkeley, Calif. Received Julv I O , I067
The literature contains no high temperature heat-content data for stannous and stannic sulfides except those of Krestovnikov and Feiginal for crystalline stannous sulfide between 288 and 973°K. The present paper gives data for stannous sulfide to 1225”K., including the heat of fusion and a 70”-portion of the liquid range. The data for stannic sulfide extend t o 1005°K. where the decomposition pressure is appreciable. Low temperature heat capacities and entropies at 298°K. of these substances have been reported by King and Todd.2 Materials.-The stannous sulfide was a portion of the sample used by King and Todd,Z who reported the method of preparation and the results of chemical and spectrographic analyses. The stannic sulfide also was a portion of the sample used by King and Todd,Z but it was treated further to increase the crystallne particle size. This was accomplished by heating the substance in an evacuated silica-glass container for 3 months at 500”. At the end of this period i t was reanalyzed and found to contain 64.93% tin, as compared with the theoretical 64.92% and with the 64.95% reported by King and Todd. Measurements and Results.-The measurements employed apparatus previously described.* The samples were enclosed in platinum-rhodium alloy capsules, the heat contents of which were determined in separate experiments. After being filled with sample the capsules were evacuated of air, helium was admitted, and the capsules were sealed by pinching shut and soldering the capsule necks. Pure platinum was used as the solderin material for the stannous sulfide capsule and pure golf for the stannic sulfide capsule. The furnace thermocou le was calibrated frequently a t the melting point of gold3during the course of the measurements. The experimentally determined heat contents, expressed in defined calories (I cal. = 4.1840 abs. joules) per mole, me listed in Table I and plotted against temperature in Fig. 1. Molal weights accord with the 1954-55 Report on Atomic Weights.‘ Stannous sulfide undergoes a minor, substantially isothermal transformation at 875’K. (indicated by the arrpw in Fig. 1); 160 cal./mole of heat is absorbed. The melting point of stannous sulfide was taken as 1153’K., in accordance with NBS Circular 500.6 The heat of fusion was measured as 7550 cal./mole and the corresponding entropy of fusion is 6.55 cal./deg. mole. The heat capacity of the liquid is substantially constant a t 17.90cal./deg. mole over the investigated 72O-portion of the liquid range. The results of Krestovnikov and Feiginal (1)A. N. Krestovnikovand E. I. Feigina, J . Phya. Chem. (U.S.S.8.1. 8, 74 (1936). (2) E. G. King and 6. 8. Todd, J . Am. Chem. Soc.. 76,3023 (1963).
(3) K. K. Kelley, B. J. Naylor and C. H. Shomate, U. S. Bur. Mines Tech. Paper 686. 1946. (4) E. Wichers, J . Am. Chem. SOC.,78, 3235 (1956). (5) F. D. Rossini, D. D. Wagman. W. H. Evans, 9. Levine and I. Jaffe, Natl. Bur. Standards Circ. 500, 1952.
v
?
NOTES
Jan., 1958 TABLE I MEASUREDHEATCONTENTS ABOVE298.15'K. T. "K.
388.4 389.9 482.4 483.6 580.5 804.5 675.7 726.6
HT Hm.n
'd.
1,070 1,090 2,220 2,240 3,500 3,780 4,760 5,450
SnS (mol. 764.9 793.7 843.6 863.6 868.1 872.8 876.3 892.1
T
HT
-
HZ98.16
(CAL./MOLE)
-
T
HT Hza.is
OK.
wt. 150.77) 5,960 980.3 6,430 1071.0 7,170 1114.1 7,510 1122.5 7,620 1143.8 7,880 1161.4 7,830 1185.1 8,030 1225.1
9,200 10,440 11,070 11,750" 14,890' 19,280 19,700 20,430
SnSz (mol. wt., 182.83) 400 0 1,710 592.0 5,100 834.4 489.6 3,260 594.9 5,200 872.3 491.2 3,300 603.1 5,310 902.5 498.7 3,450 621.0 5,670 969.3 524.0 3,860 692.6 6,960 1005.3 561.2 4,550 762.1 8,240 4 Shows premelting effects.
9,580 10,320 10,860 12,210 13,000
-
18 -
I
I I I
I I
15
TABLE I1 HEAT CONTENTS(CAL./MOLE) AND ENTROPY VALUES (CAL./DEG,MOLE)ABOVE 298.15"K. T,OK. 400 500 600 700
800 875 875 900 1000 1100 1153 1153 1200 1250
Y S n S HT - HWWI ST
1,210 2,450 3,750 5,090 6,520 7,650( a) 7,810(0) 8,140 9,470 10,840 11,580((3) 19,130(1) 19,970 20,8GO
- Sar.16.
3.49 6.25 8.62 10.69 12.60 13.94 14.13 14.50 15.90 17.21 17.86 24.41 25.13 25.85
-SnS.z----HT Ham6 ST
-
1,740 3,470 5,250 7,070 8,930
...
- SaDs.16
5.02 8.88 12.12 14.93 17.41
...
... ...
10,840 12,810
19.66 21.73
...
... ... ...
...
... ...
...
... ...
SnS(a): H T - Hzga.16 = 8.532' 3.74 X 10-a2'2 - 0.90 X 1062'-' - 2,574; (0.2%, 298-875'K.) SnS(B): ., , H T - Hzgs.la = 9.78T 1.87 X 10-*Ta - 2,180; (0.1 %, 875-1 153OK.) SnS(1): (O.l%, 1153-1250'K.) HT - H B S .= ~ ~17.902' - 1,510 SnS2(c ): H T - Hzss.,a = 15.51T 2.10 X 10-aT2 - 4,811 (0.2%, 298-1000°K.)
+
for the crystalline substance average about 2.5% higher than those reported here. The heat content of stannic sulfide is regular throughout the measured temperature range. Measurements beyond 1005.3'K. were not attempted, as the capsule already had started to swell owing to decomposition pressure of the stannic sulfide.
21
125
I
9
+
+
SOLUBILITY OF NITROGEN I N METAL-ACETATE SOLUTIONS BY ROBERT C. BRASTED AND CHIKARA HIRAYAMA Contribution from the University of Minnesota, Minneapolis, Minn., and Westinghouse Research Laboratories, Pittsburgh, P a . Received August 7, 1967
In connection with some recent studies relating nitrogen fixation in aqueous solutions to transition metal complex formation, the solubility of nitrogen gas in buffered solutions of some transition metal acetates was determined a t 25". In addition to the metal acetate, each solution was one molar in acetic acid and 0.5 ill in sodium acetate. The chemicals were of reagent grade and used without further purification. The vapor pressures of the solutions were determined by comparison against that of pure water in a static manometric system. The total volume of solution was approximately 97.6 ml. 3 Agitation of the system during the nitrogen solution process was achieved by a magnetic stirrer. The solubilities recorded in Table I are in terms of the Ostwald solubility coefficient a,which is calcu300 500 700 900 1100 1300 lated according to Lannung' T , 'IC. a=- WPO WLWP Fig. 1.-Heat contents above 298.15'K.: curve A, SnS; curve B, SnSz. where Smooth valucs of the heat-content and entropy increLY = vol. of gas, a t S.T.P., dissolved in 1 ml. of solvent
ments above 298.15"K. are listed in Table 11. The entropy increment.s were derived by the method of T
