In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Low-Cost “Vacuum Desiccator” Frederick Sweet Department of Obstetrics and Gynecology, Washington University School of Medicine, Saint Louis, MO 63110;
[email protected] For over a century, solids and liquids that had to be kept dry were stored in glass desiccators containing a drying agent such as anhydrous calcium sulfate. Today, the need for storing a variety of air- and water-sensitive chemicals under dry conditions is still a common practice in undergraduate, graduate, and research chemistry laboratories. Yet the commercial glass and plastic desiccators have become more expensive.1 Moreover, they chip, crack, and break with regularity in undergraduate chemistry laboratories. Vacuum desiccators are even more expensive than the simple two-piece variety. During the past eight decades, a number of inexpensive desiccators (1–3) have been reported, some made from a variety of novel materials such as: plastic sandwich bags (4), coffee cans (5), and cookie jars (6). The present article describes how a new kitchen appliance costing less than half the price of a single, large desiccator1 can be used in the laboratory for quickly producing low-cost, disposable “vacuum desiccators”. The appliance, which has a small foot print (6 in. × 15 in.) is marketed for quickly, conveniently, and inexpensively shrink-wrapping foods for storage in a refrigerator or freezer. But the same device can provide an entire undergraduate chemistry laboratory with easy-to-use and disposable desiccator bags for a small fraction of the cost of conventional glass or plastic desiccators. In the present example, the Seal-a-Meal device (Figure 1A) distributed by the Rival Company of the Holmes Group, Inc. had been purchased by the author for less than $100 at a neighborhood K-Mart store. This appliance came with two rolls of a double-walled, heavy-plastic material measuring 11 in. × 120 in. (Figure 1E); plus an assortment of specialized
plastic bags of various sizes. Two 11 in. × 120 in. rolls of the thick shrink-wrapping material (also distributed by Rival) costs less than $10 per box and the two rolls can be used in a laboratory for making from 30 to 60 desiccators. Conventional glass desiccators require careful greasing at the ground-glass interfaces to achieve a perfect seal. Plastic desiccators are known to eventually leak because over time the plastic interface tends to warp. But once heat-sealed, the disposable desiccator bags (Figure 1B–D) remain completely intact until they are cut open. Each 11 in. × 120 in. roll of this material can be used to make approximately 30 small (4 in. × 11 in.) disposable desiccator bags for about $0.17 each or 15 large desiccator bags (8 in. × 11 in.) for $0.34 each. After the contents are heat sealed together with a desiccant they remain dry indefinitely. Moreover, the shrink-wrapped item and desiccant do not take up more space than the original items as opposed to the space occupied by a conventional, glass desiccator. An oversized heat-sealed desiccator bag can be used several times by narrowly cutting it open in a straight line along the seal after its first use. The open side must be trimmed in a straight line so that the bag can be quickly resealed in the Seal-a-Meal device. In addition to serving as an unbreakable and inexpensive desiccator, the system is useful for sealing bottles and vials of corrosive and irritating substances and also large bottles of solvent that do not fit into a large desiccator (Figure 1D). Unlike Saran Wrap or similar cling film, once the double-walled, heavy-plastic material is vacuum-sealed it is indefinitely impermeable to vapors. Thus six months after
Figure 1. Adapting an automated kitchen gadget for making “vacuum desiccators”: (A) The Seal-a-Meal device can be used for heat sealing with or without a vacuum. A 120-inch roll of double-walled, heavy-plastic material is located under the shrink-wrapping section of the appliance that contains a horizontal cutting device. (B) A reagent bottle with Drierite in position for shrink wrapping. The entire automated shrink-wrapping process takes about 15–30 seconds, depending on the size of the bag. (C) A bottle of isobutyl chloroformate (100 g). (D) A 1-L bottle of dioxane that had been sealed with the kitchen gadget. (E) Extra roll of 120-inch double-walled, heavy-plastic material.
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In the Laboratory
having been shrink wrapped, several grams of moisture indicating Drierite (behind D in Figure 1) remained completely unchanged. Similarly, no HCl was detected inside the desiccator after a shrink-wrapped bottle of isobutyl chloroformate (Figure 1C), which forms HCl on contact with moisture, had been standing in a laboratory for five months. The author has not yet sealed materials under an inert atmosphere such as nitrogen or argon. However, this can easily be done by flushing the air out of a bag with a stream of inert gas immediately before vacuum sealing it. Note 1. The costs for desiccators are: $65 for 100 mm and $181 for 250 mm glass desiccators; $67 for 230 mm and $96 for
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280 mm Nalgene desiccators; $234 for 140 mm and $494 for 190 mm vacuum desiccators (Fisher Scientific Company Catalog 2003).
Literature Cited 1. Day, J. E.; Walke, E. W. J. Chem. Educ. 1928, 5, 597. 2. Birdwhistell, R. K. J. Chem. Educ. 1967, 44, 667. 3. Hendrixson, R. R.; Whitcomb, D. R.; Palmer, R. A. J. Chem Educ. 1976, 53, 593. 4. Thompson, H. B. J. Chem Educ. 1966, 43, 473. 5. Minnier, C.; Johnson, S.; Matusz, I. J. Chem Educ. 1976, 53, 520. 6. Sarma, B. D. J. Chem Educ. 1983, 60, 906–907.
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