NOTES o n the MOVEMENT of CRYSTALLINE IODINE in SILICA GEL MIKE A. MILLER University of Michigan, Ann Arbor, Michigan*
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H E phenomenon of precipitate formation in gels has received the attention of numerous investigators, few of whom agree as to the exact mechanism of such deposition. Any observation that may tend to clarify the problem is therefore of value. In the course of certain studies on crystal formation in gel media it was observed that crystalline iodine offered an excellent means for studying the mechanics of crystal formation, movement, and re-solution. In general, any strong oxidizing agevt will deposit crystalline iodine in silicic acid gel, the deposition being periodic in the cases where no simultaneous gas formation takes place, and especially so when the rate of diffusion is slow, as when a concurrent or secondary reaction product is also deposited. Such reagents as potassium dichromate or pemanganate with sulfuric acid, and nitric acid alone, have been used to deposit excellent specimens of crystalline iodine in silica gel. It is interesting to note that in the case of nitric acid the deposition of iodine takes place on the outlines of the flattened gas bubbles which are simultaneously
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* Present address: Aluminum Research Laboratories, Aluminum Company of America. New Kensington. Pennsylvania.
formed, resulting in a non-periodic structure. This non-periodicity follows as a result of the rapidity of the diffusion process, the inhomogeneity of the gel structure due to the distortion by gas bubbles, and the natural tendency of depositing materials to arrange themselves a t any point of irregularity or tension (I). Chlorine displaces iodine from potassium iodide very readily, yielding periodic stratification. Slight movement, en messe, results, however. Movement is accentuated hy the presence of hydro6hloric acid. The ideal deposition for the study of iodine crystal transfer in gel media results from a replacement reaction by bromine. The gel was made by mixing equal quantities of 1.06 density water-glass with N acetic acid, the gel being made N/10 with respect to potassium iodide before solidification. Upon covering the above gel with bromine water, immediate crystal formation resulted. Further action produced rapidly growing rhombohedra1 masses of crystalline iodiie possessing the general form of the lead ferrocyanide crystal masses described by Hatschek (2, 3); in this case, there is end-to-end fusion of single large rhombohedra of iodine.
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As pointed out by Taber (4), the particular shape of the deposited crystals and their subsequent growth will result from the operation, individually or concurrently, of a t least three factors: the tendency to produce forms of a somewhat regular polyhedral character under the simultaneous action of the surface tension and the forces of molecular orientation, and the modification of growth in different directions by external forces. Longitudinal growth of crystal masses was observed to take place along pre-determined planes. These forceor stress-planes were, in general, continuous with the faces of the end crystal, i. e., a t the growth zone, and manifested themselves as minute cracks sufficiently different from the medium in refractive index to he readily observable. These stress-planes were observed in single rhombohedra as well, growth invariably following the pattern pre-determined by them, their pattern, in turn, having been pre-determined by the shape of the original ultramicroscopic crystal. After growth had resulted in crystals about a centimeter in length, the whole crystalline mass moved downward into the tube as a result of re-solution of the crystals in the excess of entering reagent. The diffusion front, as shown by the deep red color of the gel, always preceded the crystal front by a relatively large distance. Immediately below the crystal front, throughout the area containing the crystals, and for a short distance above the crystal band, the gel was stained a decided orange color. The remainder of the distance to the surface was light yellow. The color scheme was intekpreted on the basis of analysis of carefully sectioned areas as follows. Displacement of iodine from potassium iodide by bromine resulted in the formation of potassium bromide and free iodine. The latter formed the soluble reaction product KIs, which diffused forward and backward. Moving backward, it met the higher concentration of bromine, forming KBrIBr. This product also diffused in both directions. In its backward movement it encountered more bromine, thus freeing iodine which deposited a t the crystal front. At the rear of the crystal band, the bromine dissolved the precipitated iodine, forming KBrIBr, which under the influence of the greater concentration of bromine in the rear moved forward, keeping a short distance behind the dissolving iodGe crystals. The uppermost strata contained only potassium bromide and free bromine. When diffusion of bromine was stopped three distinct mechanisms were apparent. The masses grew slightly by the deposition of iodine on the crystal front. Resolution having been discontinued, the crystal band as a whole remained stationary and only single scattered crystals of iodine grew in the region below the band. After a time, the whole crystal band began to move backward, i. e., up. The phenomenon of backward diffusion of initial reaction product here noted is similar to that described by Lloyd and Moravek (5) for the periodic precipita-
tion from the system potassium iodide-lead nitrate, the visible reaction product in their case being KI.PbI2. A further example of diffusion was shown by the following experiment. A blank silica gel, i. e., one containing no potassium iodide, was covered after solidification to a depth of about one centimeter with a silica gel N/10 with respect to potassium iodide. After gelation, the composite gel was covered with bromine water. The resultant crystal band moved through the interface with no apparent distortion or discontinuity in the individual crystals, and on down into the blank gel. The color scheme was identical with that already described. The most interesting characteristic of the redissolving of the iodine crystals was the skeletal formation that invariably preceded the complete disappearance of the iodine. The outlines of the faces of the crystal, shown by a series of planes parallel to each face, were last to dissolve. Since these last traces of iodine followed so closely the crystal pattern, we must infer that when the crystal was formed, adsorption took place on the crystal faces as they were built up, perhaps on the stress-planes already noted, and that the rate of resolution was appreciably diminished in these planes due to the protective action of the sorbed material. After complete disappearance of the crystal, its outline remained indelibly stamped into the gel, as portrayed by the difference in index of refraction of the volume previously occupied by the crystals and of the surrounding gel media. The following conclusions derived from a consideration of these observations seem iustifiable.
1. Deposition of crystalline iodine in silica gel may be periodic or non-periodic depending upon the rate of diffusion of entering reagent and the chemical nature of the reaction. 2. Crystallization does not result a t the diiusion front but some distance in its rear. 3. An iodine crystal apparently exerts a pressure in a plane continuous with its faces,'this pressure resulting in a visible stressing of the gel structure. 4. Movement of iodine crystals in silica gel appears to be the result of re-solution a t the rear of the crystal mass and deposition a t the crystal front. The moving crystal mass leaves an outlined path ba a record of its passing. 5. It is the outline of the iodine crystal which dissolves last when re-solution takes place. LITERATURE CITED
(1) WULPP,J., "Versuche iiber das Wesen der Neiderschlagsbildune." Z. bhvsik. C h ~ m .B6.43-61 . (1929). (2) . . HaTscm Ckm. lna., lo), au, zor (IHIIJ. (3) H A T S C ~ E.,K"Reactionen , in Gelen und die F o m und Teilchengresse der unldslichen Reaktionsprodukte," KolloidZ.,8, 1 9 2 4" " O n '
'he origin of veins of asbestifom minerals,"Naf.
