T H E FREEZING POINT CURVE OF THE SYSTEM WATERAMMONIA BY LOUIS D. ELLIOTT
The existence of two crystalline hydrates of ammonia is indicated by thr: work of Pickering,l Smits and Postma,2 R ~ p e r t and , ~ P ~ s t m a upon ,~ the freezing point curve of the system water-ammonia. Postma, agreeing closely with t,he work of Rupert, found the hydrate 2NH3. H 2 0to crystallize in very srr,all needles melting at - 78.go while the other hydrate, NH3.HzO,was found t o form much larger needles melting a t - 7 9 . 2 ' . From the form of their curves the hydrates are apparently largely dissociated in solution. Friedrichs5 has recently questioned the existence of these hydrates suggesting that it may be n case of mixed crystals in continuous series. In connection with the writer's investigation of the molecular lowering of the freezing point of liquid ammonia6 it was thought worth while to continue the study of the solute water over as large a portion of the freezing point curve of the syst,em water-ammonia as the adaptability of the apparatus would permit. Although the apparabus was not designed for this type of investigation the procedure was such as to render possible a careful study of the formation and appearance of crystals along the curve. Accordingly water was added to the freezing cell in increasing quantities and observations taken in the same manner as in the work upon the molecular lowering. After taking the freezing point temperature a lamp was held behind the cell so that the crystals could be critically examined. The freezing points obtained are tabulated below. Mol.%
0bs.F.P.
Mol.% 0bs.F.P. s"
"8
-77.73O 99.4 - 7 8 . 1 1 98.5 -78.58 97.3 -79.34 94.5 -81.01 91.0 -83.45 8 9 . 9 -84.13 86.6 - 8 7 . 0 5 84.6 -89.37 83.5 -90.07 81.8 -91.86 8 0 . 9 -92.83
100.0
80.6
-92.10~
-94.33 8 0 . 0 -90.56 79.7 -go.20 79.1 -888.80 77.7 -86.98 76.4 -85.54 75.0 -83.56 74.2 -82.05 70.4 -80.03 68.6 -79.10 66.2 -78.88 80.1
Mol.% NHs
0bs.F.P.
64.3 -79.39O 61.0 -83.12 60.0 -85.03 5 8 . 8 -85.47 57.9 -87.25 57.3 -88.39 55.8 -84.54 53.3 -80.93 50.8 -79.12 47.8 -80.15 37.0 -97.0
Eleven of the first thirteen readings are those determined in connection with the previous investigation upon the freezing point constant. For the determination of the freezing points of the solutions containing more than twenty five percent water the filling pipette previously used and described was replaced by one containing 26.0 grams of ammonia. The results are shown graphically in the accompanying chart in which our results are plotted together with those of Postma' for comparison. J. Chem. Soc., 63, 141 (1893). * Z.anorg. Chem., 71, 250 (1911). J. Am. Chem. Soc., 31, 866 (1909); 32, 748 (1910). * Rec. trav. chim., 39, 515 (1920). Z.anorg. Chem., 127, 228 (1923). 6 Elliott: J. Phys. Chem. 28.
888
LOUIS D. ELLIOTT
Ammonia crystals which separated out until the first eutectic was reached were flock-like in character without any distinguishable crystalline form. The crystals of HzO .NH3 separating between the first and second eutectic were very distinct, comparatively large needles. There was no indication of the formation of mixed crystals at any time. The increasing viscosity noticed by previous i'nvestigators began to evince itself in the neighborhood of about 60% ammonia. Hand stirring was resorted to a t this point. Supercooling became much more marked, soon amounting to about ten degrees. Crystallization under those conditions being quite sudden caused the formation of such a large mass of crystals that their form could not be determined with surety.
FIQ.I
Freezing Point C u r w of the System Water-Ammonia
In the last measurement recorded the viscosity was so great as to render efficient stirring impossible, the liquid resembling a semi-liquid glass. However, in each case a definite crystallization took place with the usual rise in temperature to a maximum in the course of a, few minutes. Friedrichsl notes that in his work with these viscous solutions equilibrium failed to become established even after several hours. It was found in our investigation that a differential of about twenty-five degrees between solution and cooling bath was necessary to bring about crystallization and subsequent rise to the maximum with such viscous solutions at the low temperatures encountered in this region. Though not definitely determined, earlier investigators calculated the freezing point of the eutectic between NH3.HzO and HzO to be in the neighborhood of - 120' while Postma's measurements place it at about - 100.3'. Owing t o the great viscosity encountered at this concentration no further measurements were attempted in the present investigation. The position on the curve of our last value of -97.0' seems to indicate that Postma's eutectic of - 100.3' is much more nearly correct than the lower value. The eutectic between NH3and3"2 .Hz0 was determined experimentally. After taking the reading of the freezing point of the needle crystals formed with 80.6 molecular percent ammonia the temperature was observed t o fall until typical flocks of ammonia cryst,als also began to appear. After a slight supercooling and subsequent rise the temperature remained constant a t - 92.59'. This figure is in close agreement with Postma's corresponding figure of -92.5'. As a whole the results corroborate those of Postma and of Rupert in showing the existence of two crystalline hydrates of ammonia. 'Loc. cit.
