NUCLEATION-From
Gases On the basis of the results obtained in the continuous cloud chamber, it is apparent that crystals form a t -39” C. without recourse to the presence of ice crystal fragmentation (12) or splintering (1)phenomena, freezing nuclei (18), or sublimation nuclei (IS). In fact, the evidence seems to be clear that the air must be saturated with respect to water and contain either condensation nuclei or sufficient moisture to form spontaneous water drops. If these two conditions are not met, the multiplication phenonienon will not occur. \In this respect, the results are in agreement with ideas advanced recently by Mason and Ludlam (8). Observations recently reported by Dingle and Nexsen (6)are alsoexplained by the phenomena described. Additional experimental results concerning spontaneous nucleation phenomena with respect to the water-ice phase will be described in a subsequent paper. LITERATURE CITED
Brewer, A. W., and Palmer, H. P., Nature, 164,312 (1949). Brewer, A. W., and Palmer, H. P., Proc. Phyls. Soc. B64, 765-73 (1951).
Cwilong, B. M., N a t u r e , 155,361 (1945). Cwilong, B. M., Proc. Rou. SOC.(London). A190, 137 (1947). Dingle, -4.iY., and iiexsen, W.E., J . Meteorol., 8, 365 (1951). Findeisen, IT., Meteorol. Z., 57, 201 (1940); 60, 145 (1943). Langsdorf, 4.,Rev. S e i . Instruments, 10, 91-103 (1939). Mason, B. J., and Ludlam, F. H., Progress in Physics, 14, 147
Figure 7. Pinch-Off Effect When Condensation Trail of Ice Crystals Passes into Region of Ice Crystals Photographed a t angle of 20’ above horizontal
taneously a t the inversion interface. The number of primary crystals that form will depend on the concentration of condensation nuclei and ice nuclei in the moist air mass. The number and size of the secondary crystals that form will probably be some multiple of the effective number of condensation nuclei. Since these conditions for ice crystal formation are of a marginal nature, the variability and often unique appearance of true and false cirrus clouds may be closely related t o these spontaneous crystal formation phenomena. It is likely that the concentration of supercooled water droplets a t the transition temperature of -39” C. is of primary importance in the formation of cirrus crystals. Ice nuclei in the overrunning moist air will operate t o prevent the multiplication effect described in this paper.
(1951).
Pound, G. &I.,aiid Madonna, L. A., Division of Industrial and Engineering Chemistry, A x . CHEM.Soc., Nucleation Symposium, Evanston, HI., December 1951. Rau, W., Schriften deut. A k a d . L u s t . , 8 , 65 (1944). Schaefer, V. J., Bull. Am. Meteorol. Soc., 29, 175-82 (1948). Schaefer, V. J., “Compendium of Meteorology,” p. 221, Boston, American Meteorological Society, 1951. Schaefer, 5’.J., J . A p p l i e d Math. and P h v s . (ZASIP), 1, 153-211 (1950).
Schaefer, V. J., J . Jfeteorol., 6 , 283 (1949). Schaefer, V. J., Science, 93, 239 (1941); Nature, 149,81 (1942). Schaefer, V. J., Science, 104, 457 (1946). Vonnegut, B., C h e m . Revs., 44, 227 (1949). Weickmann, H., “Die Eisphase in der Atmosphare,” Rept. an3 Trans. No. 716, Volkenrode, Ministry of Supply, Great Britain. RECEIVED for review January 4, 1952. A C C E P r E D .%gril 4 , 1952.
Liesegang Rings of Ammonium Chloride W.
H. J O H N S T O N
AND
PETER J. M A N N O
P U R D U E UNIVERSITY, I A F A Y E T T E , IND.
R e c e n t experiments are reported on t h e phenomenon of Liesegang rings of ammonium chloride, including isothermal studies, experiments with x-rays, surface studies, and an investigation of t h e effect of water. Discoveries of a cloud chamber effect and t h e necessity of water vapor as a catalyst are described.
A
LTHOLGH many workers have studied the formation of Liesegang rings in gels (I), the phenomenon of rhythmic precipitations in the gas phase has received little atfention. One such example, however, is the rhythmic precipitation of ammonium chloride produced by ammonia and hydrogen chloride into opposite ends of a tube, as reported by Koenig ( 3 ) and Hedges ( 3 ) . Recently, Spotz and Hirschfelder measured the time of formation of a ring, and by diffusion theory that a 1000-fold supersaturation enists €or both gases prior t o precipitation (4). on the The presentpaper sulllmarizes further phenomenon of Liesegang rings of ammonium chloride including isothermal studies, experiments with x-rays, surface studies, and 1304
an investigation of the effect of water. The discoveries of a cloud chamber effect and of the necessity of water vapor as a catalyst are described. Experimentally, the vapors from sohtions of 1.5 and 2 F‘ ammonium hydroxide and 10 F hydrochloric acid were allo.l\-ed to diffuse into the opposite ends of 50- or 100-cm. lengths of 3 mm. borosilicate glass tubing. Under these conditions the rings formed near the center of the reaction tube. The isothermal runSWere done a t 25.0” 0.1’and 35.0” =I=0.1” C. The cloud chamber experiments were done with 10 mg. of radium and with a hfachlett AEG 50 x-ray tube operated at 40 kv. and 20 ma. with a tungsten target and beryllium window. The surfaces tested were clean borosilicate glass, Desicote, and powdered ammonium chloride. The dry runs were done with phosphorus as drying agentsfor the pentoxide and magnesium hydrogen chloride and ammonia, respectively.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 6
NUCLEATI ON-From The discovery of a cloud chamber effect gave direct evidence of the presence of supersaturation of ammonium chloride. In the presence of 10,000 to 30,000 roentgens per minute of x-radiation, the rings ceased to form and a uniform precipitate of ammonium chloride was deposited in the bottom of the reaction tube. In a series of isothermal experiments a t 25.0’ and 35.0’ C. the position of the first ring was reproducible. The ratio of the distances from either end of the tube was found to be approximately equal to the ratio of the products of the corresponding diffusion coefficients and partial vapor pressures. In a study of the possibility of surface catalysis, reactions were carried out in borosilicate glass, glass coated with Desicote, and glass covered with powdered ammonium chloride. No difference in the nature of the Liesegang rings was observed. Finally the effect of water was studied. In the absence of water vapor, no precipitation of any kind was observed in 30 days as compared with the usual time of 15 to 20 minutes for the formation of Liesegang rings in the presence of water. Subsequent diffusion of water into the gas mixture produced a rhythmic precipitation.
Gases
These results are taken as support for the suggestion of Hirschfelder that a supersaturation occurs because it is difficult for a molecular cluster of ammonium chloride to convert to an ionic lattice (4). Furthermore, water appears to be a catalyst for this conversion and is necessary for precipitation. ACKNOWLEDGMENT
The authors wish to thank Hubert J. Yearian of Purdue University for his helpful assistance with the x-ray bombardments and J. 0. Hirschfelder of the University of Wisconsin for interesting discussions. LITERATURE C I T E D
(1) Alexander, Jerome, “Colloid Chemistry,” 5th ed., pp. 513-26,
New York, Reinhold Publishing Corp., 194.4. (2) Hedges, E., J. Chem. SOC., 1929,1848-9. (3) Koenig, A.E., J. Phys. Chem., 24,473 (1920). ( 4 ) Spotz, E. L., and Hirschfelder, J. O., J. Chem. Phw., 19, 1215
(1951). RECEIVED for review March 7, 1952. ACCEPTED April 3, 1952. Abstracted from the master of science thesis of Peter J. Msnno, Purdue University, January 1952.
NUCLEATION FROM LIQUIDS
Nucleation in Sucrose Solutions ANDREW V A N H O O K A N D FLORO FRULLA COLLE5E OF THE HOLY C R O S S . WORCESTER. M A S S .
Established facts and procedures of sugar boiling are outlined, after review of the significance of nucleation and crystal growth in this instance. Previous work suggesting the applicability of the essential features of the Volmer-Becker theory i s reviewed, and followed by recent observations on variations due to size of sample and on t h e evaluation of the activation energy for nucleation. The ability of sonic irradiation a t high intensity t o induce cryst a l formation in difficultly crystallizable substances is described, and some of the faoIt is possible t o improve t h e size distribution of t h e final tors involved are indicated. granulated product by t h i s means.
N
OT only is crystallization a major hnit operation in the
sugar industry, but the very magnitude of this industry compels us to regard the process as one of the chief unit operations. The annual production of sucrose far exceeds that of any other crystallized product and of most heavy chemicals; and much of this tremendous production is actually recrystallized a second time from the original raw grade to give a refined product. Crystallization is the cheapest way of effecting this purification (9).
As in all crystallizations, there are the two consecutive steps of nucleation and growth to consider. Most investigations of the crystallization of sucrose have been concerned primarily with the second of these steps, since this is the more elaborate factor ( 6 )in the “boiling of a strike.” It is also common practice to eliminate or at least minimize the first factor by employing seed stock, or “footings,” upon which the final crop of crystals is built. However, under some circumstances and especially with older techniques-which are still used to some extent-matters of grain establishment become increasingly important throughout the boiling period. CRYSTALLIZATION OF SUCROSE
Webre (26)has outlined the supersaturation curves for sucrose in terms of Ostwald’s metastability concept:
June 1952
Existing crystals grow in the metastable region, but no new ones form. This zone persists to a supersaturation of about 1.2, this coefficient being the ratio of the actual concentration to the solubility in terms of units of sugar er unit of water. Beyond the metastable range is t f e intermediate, or period of false grain, in which not only existing crystals grow but new ones also form. This zone extends from a supersaturation of about 1.2 t p 1.3. Above this latter value is the labile zone, in which crystals form spontaneously without the presence of others. Although these figures are approximate only, in the sense that they merely represent conditions under which crystallization becomes visibly appreciable under ordinary operating end experimental conditions, they form a very practical working guide for the sugar boiler (8, 14). The division points are elevated considerably if precautions are taken to exclude foreign or heterogeneous nucleation (13, 23),and if the purity of the sirup is decreased (4, 23). The chain-reaction-like nature of the intermediate zone is also probably the result of attrition and fragmentation of the growing crystals (19). This supersolubility pattern suggests that growth will proceed, uncomplicated by nucleation, only below certain critical concentrations, and this conclusion is amply verified by both practical and experimental experience. Below this metastable limit the growth rate of sucrose may be represented adequately by the simple kinetic form
INDUSTRIAL AND ENGINEERING CHEMISTRY
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