In the Laboratory
Spectacular Breeding of Crystals on Silica Gel Ryszard Piekos* and Jacek Teodorczyk Department of Inorganic Chemistry, Faculty of Pharmacy, Medical University of Gdansk, PL – 80-416 Gdansk, al. Gen. J. Hallera 107, Poland; *
[email protected] General considerations underlying the sol–gel process have been reported by Buckley and Greenblatt (1) and more recently by Laughlin and co-workers (2) and by Celzard and Marêché in this Journal (3). The latter authors described in detail chemical reactions and procedures based on hydrolysis of labile M⫺OR bonds in alkoxides and condensation of the products to afford gels. The tetraethylorthosilicate (TEOS), Si(OC2H5)4, was a precursor that, after hydrolysis, produced unstable monosilicic acid, H4SiO4. The acid subsequently underwent polycondensation reactions resulting in silica gel. Alternatively, silica gels can be obtained by another, safer and more cost-effective procedure consisting in preparation of the silicic acid precursor from sodium metasilicate by ion exchange (4). When doing experiments with blends of silica aquasols obtained by this method with solutions of sodium thiosulfate pentahydrate, we noted an interesting phenomenon. Namely, after gelation of the system and evaporation of excessive water at ambient temperature, the gels cracked and on the surface of the solidified material white offsprings appeared that continued to grow like grass (Figure 1). A closer inspection of the resulting species showed them to be pure sodium thiosulfate slightly less hydrated (DSC examination) than the original pentahydrate. A literature search for explanation of this phenomenon led us to a class of minerals known as antholytes (5) whose representative is a rare mineral chalcanthite, CuSO4⭈5H2O, formed upon oxidation of copper(II) sulfide-bearing minerals, for instance chalcopyrite, CuFeS2, in humid air (5): CuFeS2 + 4O2 + 12H2O → CuSO4⭈5H2O + FeSO4⭈7H2O
a series of experiments using other salts: CuSO4⭈5H2O, Cr 2 (SO 4 ) 3 ⭈18H 2 O, Al 2 (SO 4 ) 3 ⭈18H 2 O, NiSO 4 ⭈7H 2 O, Na2SO4·10H2O, and (NH4)2SO4. All these experiments were successful. With colorful salts, the crystals of different colors appeared thus indicating variable hydration numbers of the salts. An equilibrium is likely to be established between hydrated silica and hydrated salts, for example, SiO2⭈nH2O + CuSO4⭈5H2O
SiO2⭈(n+2)H2O + CuSO4⭈3H2O
Another noteworthy observation is that crystals of some salts grown on the gel had different shapes suggesting that they belong to different crystallographic systems (Figure 2). When sodium thiosulfate was dissolved in the silicic acid sol, an opaque, yellowish gel was obtained. A closer inspection of the gel revealed the presence of highly dispersed sulfur in it. The presence of sulfur can be explained in terms of interaction of the thiosulfate with a weak monosilicic acid (Ka = 2 × 10᎑10 ) H4SiO4
H+ + H3SiO4−
S2O32− + 2H+ → H2S2O3
to produce the unstable thiosulfuric acid, which decomposes spontaneously to afford sulfur, H2S2O3 → H2O + SO2 + S↓
The procedure for preparation of highly dispersed sulfur by this procedure has been patented (7).
Depending on meteorological conditions, the water soluble chalcanthite either remains in the pores of the parent mineral in solution or, when the mineral becomes exposed to low humidity, it oozes out of the capillaries onto the surface where the water evaporates leaving behind needlelike crystals (antholytes) (5).1 Encouraged by this finding, we conducted
Experiment
Figure 1. Microphotograph of sodium thiosulfate appearing on the surface of silica gel (magnification ca. 15-fold). This image is featured on the cover of this issue.
Figure 2. Microphotograph of copper(II) sulfate crystals on silica gel (magnification ca. 15-fold). See the cover of this issue for a color version of this image.
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Preparation of Silicic Acids The silicic acid sol is prepared by ion exchange. A sodium metasilicate solution, 200 mL, (R-145, Na2O⭈xSiO2, manufactured by Enterprise WAMA, Lebork, Poland), also called a
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In the Laboratory
water glass solution, is diluted with 1 L of distilled water and passed through a glass column (420 mm × 60 mm) packed to a height of 300 mm with a swollen cation-exchange resin, Amberlite IR 120 (Fluka AG). The resin had been previously conditioned by passing 1 M hydrochloric acid through the column at a rate of 30 mL min᎑1 until a faintly acidic solution eluted and then was washed with distilled water to remove excess acid. The dilute water glass solution was then passed through the column to afford a solution of silicic acids (sol). The 140 mL of solution was dispensed into each of seven plastic containers (90 mm in diameter) and 12-g samples of Na 2 S 2 O 3 ⭈5H 2 O, Na 2 SO 4 ⭈10H 2 O, (NH 4 ) 2 SO 4 , CuSO 4 ⭈5H 2 O, NiSO 4⭈7H 2 O, Cr 2 (SO 4 ) 3 ⭈18H 2 O, and Al2(SO4)3⭈18H2O were dissolved in the aliquots of the sol in the containers. After dissolution of the salts, the containers were left to stand open for gelation at ambient temperature, ∼25 ⬚C. The gelation times ranged from about 3–5 hours with (NH4)2SO4 and Na2SO4 up to 48 hours with Cr2(SO4)3. Crystals on the solidified gels appeared after a week and continued to grow during the following days. The experiments are adaptable to a range of undergraduate students and may be accomplished either individually or in small groups. The results, as can be seen in Figures 2 and 3, are impressive. Magnified pictures of colorful specimens may even be exhibited on the walls of laboratories. The crystals of CuSO 4 ⭈nH 2 O are both navy blue (CuSO4⭈5H2O) and almost white, characteristic of the anhydrous salt (Figure 2). Also the crystals of Cr2(SO4)3⭈nH2O exhibit different hydration degrees of the Cr3+ ion as visualized by violet species characteristic of the hexahydrate, [Cr(H2O)6]3+, the greenish-blue pentahydrate, [Cr(H2O)5]3+, and the green tetrahydrate, [Cr(H2O)4]3+. The colors of the NiSO4⭈nH2O crystals were also different as were their shapes (Figure 3).
sue, especially lungs. Ammonium sulfate is moderately toxic by several routes. Nickel(II) and chromium(III) sulfates are hazardous, the former being suspect for carcinogenicity by inhalation. Precautions for the safe use of the hazardous reagents are on the labels.
Hazards
1. A similar phenomenon, referred to as drying shrinkage, is familiar to those tackling the problems of physical corrosion of cement (6).
Water glass is highly alkaline and corrosive. Avoid contact with skin and eyes. Safety goggles should be worn when handling this liquid. Dilute silicic acid sols are nontoxic, as are sodium sulfate and sodium thiosulfate. Copper(II) sulfate is moderately toxic to humans by ingestion. Aluminum sulfate hydrolyzes to form sulfuric acid, which irritates tis-
Conclusions From an educational point of view, the experiments can be used to demonstration of the following reactions and phenomena: • Preparation of silicic acids from sodium metasilicate by ion exchange (of these, monosilicic acid, H4SiO4 is soluble, while its first condensation product, disilicic acid, (HO)3Si⫺O⫺Si(OH)3 is only sparingly soluble) • Sol–gel transition whereby interconnected polymeric network is formed through condensation of silicic acids present in sol • Aging of silica gels to produce a porous glasslike solid termed a xerogel • Crystallization of salts on the surface of xerogels • Hydration equilibria between silica gel and hydrated salts • Breeding crystals belonging to different crystallographic systems • Labile nature of thiosulfuric acid
Acknowledgment We thank Boguslaw Markowski for technical assistance and valuable suggestions. Note
Literature Cited 1. Buckley, A. M.; Greenblatt, M. J. Chem. Educ. 1994, 71, 599. 2. Laughlin, J. B.; Sarquis, J. L.; Jones, V. M.; Cox, J. A. J. Chem. Educ. 2000, 77, 77. 3. Celzard, A.; Marêché, J. F. J. Chem. Educ. 2002, 79, 854. 4. Tanaka, T.; Tanigawa, T.; Nose, T.; Imai, S.; Hayashi Y. J. Trace Elements Exp. Med. 1995, 7, 101. 5. Kantor, B. Z. Khimiya i Zhizn 2002, 11, 54. 6. MacLaren, D. C.; White, M. A. J. Chem. Educ. 2003, 80, 623. 7. Piekos, R.; Teodorczyk, J. Silicic Acids-Stabilized ThiosulfateContaining Preparation with Highly Dispersed Sulfur and a Method forIitsPpreparation. P 326015. Patent application 1998.
Further Reading
Figure 3. Microphotograph of nickel(II) sulfate crystals on silica gel (magnification ca. 15-fold); note different morphology and hydration degrees of the crystals. This image is shown in color on p 338.
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1. The Chemistry of Silica; Iler, R. K., Ed.; Wiley, New York, 1979. 2. Chan, S. H. Geothermics 1989, 18 (1/2), 49. 3. The Colloid Chemistry of Silica; Bergna, Horacio E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994. 4. Mackenzie, J. D. J. Sol-Gel Sci. Technol. 2003, 26, 23.
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