Norman 0. Smith and D O U ~ I ~J.S Hennessy
Fordham
University New York
Demonstration of Triple Buoyancy Reversal
In the course of a search for a liquid which would have only slight solvent action on ammonium chloride it was discovered that the latter, when suspended in 1,2,4-trichlorobensene, settled readily when the suspension was cold but rose when it was hot. Ammonium chloride undergoes a reversible transition at 183'C1 from simple cubic (Form 11) to face-centered cubic (Form I), and the densities of these forms a t 1 8 3 T are 1.47 and 1.23 g/ml, respectively.' The density of 1,2,4-trichlorobenzene a t the same temperature lies between these two figures. Further study revealed that, with suitable adjustment of the density of the organic medium, two additional buoyancy reversals could be demonstrated by utilizing the fact that the temperature coefficient of expansion of solids is less than that of liquids. About 2 g of dried USP ammonium chloride was laced in a Carius tube along with 20.00 ml of 1,2,4trichlorobensene and 1.58 ml of 1,2,3-tribromopropane. The density of the latter solution is 1.515 g/ml a t 28°C. The tube was placed in an ice-water mixture and sealed off at atmospheric pressure so that the contents occupied about one-half of the total volume. By immersing the tube upright in a stirred bath of silicone oil, the temperature of which could he altered from 10' to 250°C,2 the following changes were evident: Below
' POYH~NEN, J., Ann. Acad. Sci. Fennicae, Ser. A
V I , No. 58,
.52 pp., (1960).
The usual precautions taken in heating sealed tubes should be observed.
15'C the suspended salt floated on the organic liquid; from 15-185'C it sank; from 185-240°C it floated; and above 240' it sank. The explanation for this is clear from the figure. The changes were reversible and were repeated several times, hut with continued use the suspended solid gradually darkened. The temperatures just given are only approximate for
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1BS
210
remmr0m ~'cl Schematic density-temperature relations in the ~yslemNHtl-liquid medium.
several reasons, among which may be mentioned: (1) it was the temperature of the bath and not that of the suspension that was measured; (2) there is, as expected, considerable delay in the polymorphic transformation; (3) the bath temperatures a t which buoyancy reversals were observed depended upon the rate of heating and cooling, and the extent of convection within the sealed tube. Cooling to 10°C is likely to cause freezing of the organic solvents. The changes at 15 and 185'C could he observed without sealing the tube, but boiling occurred a t about 210°C if the tube was left open.
Volume
41,Number I, knuary 1964
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43
