In the Laboratory The Microscale Laboratory
The Development of a Microscale Continuous Hot Solvent Extractor Steve S. Wesolowski, Thomas Mulcahy, Christina M. Zafoni, and Wayne E. Wesolowski Department of Chemistry, Benedictine University, Lisle, IL 60532
The extraction of soluble materials from organic samples plays an important role in many industries. For example, extraction of oils and grease from soil samples using traditional Soxhlet extractors is an essential procedure in waste management laboratories. These extractors use from 25 to 1000 mL of solvent per sample and the waste solvent is typically distilled or simply disposed of upon completion of the extraction. To minimize the waste solvent produced, we have developed a microscale continuous hot solvent extractor that effectively extracts 100–500-mg samples using about 4 mL of solvent. Design Many microscale devices are miniature replicas of standard glassware while others are new designs (1). The fill-and-siphon system of the traditional Soxhlet was attempted in miniature, but problems with construction and solvent surface tension prohibited a direct mimic of the Soxhlet. The final continuous extraction design (Fig. 1) is quite simple. Approximately 4 mL of solvent is placed in a 10-mL round-bottom flask. From the round-bottom boiler, solvent vapors rise through a 5-mm glass pipe centered in a standard 25-mL bulb trap with four glass indentations using glass points (similar to a Vigreux column). From the glass pipe the vapors continue up the condenser. If the apparatus is tipped slightly, the condensed solvent drips back down into the round-bottom flask through the openings between the pipe and the neck of the bulb trap, where the sample rests in a filter paper cone. Because solvent drips through the sample rather than soaking it, fresh solvent is continuously rinsing the sample. A variable-temperature heating mantle was used to vaporize the solvent, with care taken to avoid boiling away the liquid completely. When heating is arranged to give about 20 drops (or about 1 mL) per minute the solvent completely recycles in approximately four minutes.
traditional Soxhlets (Fig. 2). A parallel set of tests was performed extracting tar from samples of roofing shingles with an unknown ratio of organic tars to inorganic pebbles and paper. Again the success of the microscale extractor matched that of the traditional Soxhlets. Furthermore, the weight percent recovered from the extractions (about 80%) is a very reasonable approximation of the weight of sand, paper, pebbles, etc. found in roofing shingles. Temperature profiles were conducted on both types of extractors to determine the actual extracting temperature as a function of time (Fig. 3). Since the microscale apparatus is so small, the temperature at the sample remains constant and very near the boiling point of the solvent, whereas without extensive insulation, the sample temperature of extraction of the Soxhlet fluctuates with the rising and falling of the solvent level. The 1000-mL Soxhlet (with a layer of aluminum foil as external insulation) extracted at temperatures substantially below the boiling point of the solvent—as much as 15° cooler for hexane (bp 68.7 °C). This represents an advantage for the microscale apparatus, since the extraction temperature can be accurately set very close to the boiling point of the solvent chosen. Conclusions The microscale continuous hot solvent extractor compares extremely well with traditional Soxhlet extractors while reducing the solvent waste produced in a 300-mL extraction by 99%. The extractor can be made by an amateur glassblower and has promise for use in the microscale lab as a teaching tool for extractions as well as introducing representative sampling.
Performance
Figure 1. The microscale continuous hot solvent extractor requires only 4 mL of solvent or less.
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The microscale extractor was tested against 25-, 500-, and 1000-mL standard Soxhlet extractors using a known sand–petroleum jelly mixture. After 20 trials with hexane as the solvent, the microscale extractor recovered an average of 99 ± 2% of the sand in the sample. The standard Soxhlets recovered between 97 and 100% of the sand, placing the microscale extractor at an efficiency level similar to that of
Figure 2. Mean weight percents of the extracted material remaining after extraction. The new microscale extractor compares very favorably to the larger Soxhlets for distinct organic–inorganic mixtures.
Journal of Chemical Education • Vol. 76 No. 8 August 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Acknowledgment Special thanks to the Geneva, IL, Laboratory of Waste Management, Inc. for the use of a variety of Soxhlet extractors for comparison. Literature Cited 1. Williamson, K. W. Macroscale and Microscale Organic Experiments, 2nd ed.; D. C. Heath: Lexington, MA, 1994.
Figure 3. The microscale extractor remains at a constant temperature near the boiling point of the solvent throughout the extraction. The macroscale Soxhlet (with moderate insulation) extracts at a significantly cooler temperature and exhibits a periodic fluctuation in temperature coinciding with the rise and fall of the solvent level.
JChemEd.chem.wisc.edu • Vol. 76 No. 8 August 1999 • Journal of Chemical Education
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