In the Laboratory
Magnetic Stirring with the Stirring Bar Length Exceeding the Vessel Diameter Kirk W. Payne Department of Chemistry, Oklahoma State University, Stillwater, OK 74078 James M. Lucas Department of Physics, Oklahoma State University, Stillwater, OK 74078 Edmund J. Eisenbraun* Department of Chemistry, Oklahoma State University, Stillwater, OK 74078;
[email protected] As more chemical laboratories begin using micro quantities for experimentation, the problem of effectively stirring these small quantities or viscous reaction mixtures increases. Without good stirring many types of reactions cannot be properly carried out. References to magnetic stirrers, stirring bars, and magnetic stirring in the chemical literature are meager; the major developments are found in the patent literature as follows: basic invention of the magnetic stirrer and stirring bar (1), plastic-encapsulated stirring bar (2, 3), multiplicity of the stirring bar (4), magnetically stirred hot-plate (5), improvement of the magnetic stirrer (6 ), and shapes of the stirring bar (3,7,8). Smaller magnetic stirring bars are frustratingly ineffective because of reduced size and reduced magnetic strength. Traditionally, magnetic stirring of solutions is carried out with the stirring bar freely aligned in the magnetic field of the stirrer magnet. If the moving bar is disturbed from this orientation, stalling or uncontrolled tumbling usually results. To improve magnetic stirring of small quantities of viscous liquids, trial studies were carried out with magnetic stirring bars of varied shapes and sizes (football, cylindrical rods tapered from the middle, wedges, cylindrical rods with two opposing flat sides, cylindrical rods with uniform diameter, and octagonal rods) placed in test tubes or graduated cylinders whose internal diameter was slightly larger than the diameter of the stirring bar but considerably less than the length of the stirring bar (Fig. 1A). Vigorous stirring was achieved despite the vertical constraint of the stirring bar. Performance, at maximum rpm (strobe-light measurements) in precisely measured volumes of water, glycerol, or glycerol–water mixtures, was evaluated. Use of different diameter test tubes showed that an inclination of 15–25° from vertical is the most effective for the stirring bar. This inclined position was shown to be superior to that of a completely vertical stirring bar by comparing the performance of a cylindrical, rod-shaped stirring bar constrained vertically in a Teflon sleeve (Fig. 1B) and the same bar held at 15° from vertical in a second identical Teflon sleeve (Fig. 1C). The stirring performance for the stirring bars listed above was found to be dependent on the shape and size of the bar, the shape and size of the stirring vessel, the magnetic strength of the individual stirring bar, the volume of liquid contained in the stirring vessel, and the angle that the stirring bar was permitted in the stirring vessel. The football-shaped stirring bars were most effective when used in test tubes with round bottoms. Their effectiveness is likely the result of their form allowing a favorable tilt in the test tube. Rod-shaped stirring bars were more effective when the stirring vessel had a flat bot-
A
B
C
2 cm Figure 1. Three types of stirring bars and setups.
tom. Cylindrical rods stirred smoothly, whereas octagonal rods, because of the uneven shape, showed a tendency to vibrate and chatter. The length of the stirring bar and its placement in the magnetic field also had an effect. The longer cylindrical stirring bars (1.5–2 in.) showed a tendency to periodically leap in the test tube. For operation, a football-shaped stirring bar in the conventional orientation required 1.5–3.4 times the liquid volume needed for an inclined orientation. The above experiments demonstrate that more powerful mixing of small samples may be achieved by constraining the stirring bar at an inclined position in the field of the stirring magnet resulting in a more favorable stirring bar volume to liquid volume ratio. Literature Cited 1. Rosinger, A. Magnetic Stirrer. U.S. Patent 2,350,534, 1944; Chem. Abstr. 1944, 38, 4841. 2. Clark, R. C. Anticorrosive Sealed Magnetized Stirring Bar. U.S. Patent 2,844,363, 1958. 3. Asp, H. L.; Roffe, J. W. Magnetic Stirring Bar. U.S. Patent 2,951,689, 1960. 4. Steel, J. Y. Magnetic Mixing Bar. U.S. Patent 3,245,665, 1966. 5. Hug, T. Hot Plate and Magnetic Stirrer. U.S. Patent 3,028,476, 1962. 6. Lane, K. Liquid Agitating Apparatus. U.S. Patent 3,116,913, 1964. 7. Cook, G. B. Magnetic Stirring Apparatus. U.S. Patent 2,518,758, 1950. 8. Vogtle, F.; Strasse, W.; Schmitt, W. Agitator Member for a Magnetic Agitator. U.S. Patent 3,514,214, 1970.
JChemEd.chem.wisc.edu • Vol. 79 No. 2 February 2002 • Journal of Chemical Education
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