Magnetic Stirrer Improvement and Design of Clustered, Teflon-Encapsulated Magnetic Stirbars R. L. Hertzler, M. Lucas, and E. J. Eisenbraun Oklahoma State University. Stillwater, OK 74078
The invention of the magnetic stirrer and the Teflonencapsulated magnetic stirbar has altered the stirring of liquids in organic chemistry laboratories such that overhead stirrers are rarely used, if magnetic stirring is satisfactory. The design of magnetic stirbars has evolved to a variety of shapes including~rods,spheres, disks, triangular prisms, dumbbells, crosses, and those resembling footballs (egg shape is a poor description since magnetic stirhars must be symmetrical). Stirhars also have been enclosed in a ring to increase stability and they have been joined as pairs. A complete listing is available.' For stirring of solutions and some light suspensions, the commercially available magnetic stirrers and stirhars are satisfactory. However, for viscous materials [polyphosphoric acid (PPA)] and sticky or heavy slurries (Grignard reactions, catalytic hydrogenation, etc.) they are largely inadequate. Our need to stir PPA a t 50+ "C has been met by the local construction of a heavy-duty magnetic stirrer using an earlier design2 and substituting a larger motor (Dayton Electric Mfg. Co, Chicago, IL, model 5K004,O-1590rpm, 115 Vac, 11 20 hp, 60 Hz; or Eastern Air Devices, Inc., Dover, NH, P N H34ADL-6,O-3600 rpm, 115Vac, 1/20 hp, 60 Hz) as wellas a larger and more powerful horseshoe-shaped magnet (Alcomax, BP326, Model D, 360 g, 56 mm wide, 44 mm thick, 35 mm high, pole gap 35 mm; Laboratory Supplies Co., Inc. 29 Jefry Lane, Hicksvitle, NY 11801). The resulting magnetic stirrer when used with a laree, foothall-shaped stirbar, 1in. diam. X 2.5 in. long1,provedadequate tostir PPAat 50+ "C. All similar but smaller stirhars were inadequate. Because of the size andmass of this stirbar and the mabetic strength of the stirrer magnet, it was used in a heavy-wall, pipe-neck flask (Ace Glass Co. No 6476 or 6477). Flasks with ordinary wall thickness are easily broken if a stirhar of this size and mass soins out of control. Before use. flasks should be carefully ckecked for stars and small cracks. With this oowerful maenetic stirrer available, further improvement in stirring through design change in the magnetic stirbar was soueht. The ohiective was to vigorously stir sticky, suspend& solids encountered in ~ r i g n & dandrelated reactions. Despite the above mentioned success in stirring PPA with a football-shaped stirhar in a round-bottomed flask, a flat-bottomed, stainless steel reaction vessel was selected because this shape provided better contact with the rotating stirhar which in turn permitted more effective scouring of the bottom of the vessel. The disk-shaped "Star head" and the triangular-prism-shaped "Spinwedge" were tried also could he exoected to scour the bot~ - because ~ these ~ ~ tom of the flask and thus keep solidssuspended. While these stirhars have some utilitv. thev failed to oerform as needed. However, they easily could de improved through use of a more powerful or larger encapsulated magnet. T o increase the magnetic strength of the stirbar, three Teflon-encapsulated cylindrical bars (2 in. X 5/16 in.) were clustered in a triangular arrangement and held in place with spacers as shown in the figure. This arrangement provided a ~~
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me stirbar arrangement. greater overall magnetic strength with a good balance of mass and hulk. For maximum magnetic strength, the bars are aligned withmagnetic fields inopposition. The use of the triangular spacers is advantageous. They provide a wear surface that protects the envelope of the stirbar3, are readily dismantled for cleaning, and they probably contribute to turbulence as compared to a solid object. I t would he of interest to compare this design with a solid stirhar having the same outside dimensions and magnetic strength. However, such is not available. Several experiments with known volumes of water, water with dissolved salts t o increase the density, water with insoluble salts as a simulated slurry, toluene, and toluene with added salts were conducted to test the capability of the magnetic stirrer and the clustered stirhar. In each of these tests. the clustered stirbar showed a better oerformance than individual rtirbars making up the cluster or individual stirbars of a different desien. The ~erformancestudies ioeluded measuring the maximum operating distance of the stirhar to the stirrer maenet. .. . maximum rom a t which the magnetic stirrer lost control of the stirbar either by having the stirbar s ~ i out n of rontrol or begin shimmvina. In this respect, the iocally constructed magnetic stirrer&asput into comoetition with fivedifferent commercialmagnetic stirrers c u r r k n t ~found ~ in our laboratories. In all cases, this stirrer was found to he substantially superior, which points to the need for improvement in both the magnetic stirrer and in magnetic stirhar. A large, heavy-duty, hut very expensive magnetic stirrer is available.' The combined apparatus (stirrer and clustered stirhars) was effectively used in converting methylmagnesium bromide to dimethyl cadmium and suhsequently using the latter in the synthesis of methyl ketones. e chanees and test exoeriments sueeest These s i m ~ l desien that more can he done to improve common-sized, commercial maenetic stirrers and maenetic stirhars. The followine simple iesign parameters deseke attention and each shoula be brought t o its maximum potential: magnetic strength of the stirrer magnet and the atirbar as well as the hulk, mass, and shape of the stirhar. ~~~~
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' Eel-Arts Products. Pequannock. NJ 07440.
Hall. H.; Eisenbraun, E. J. J. Chem. Educ. 1977, 54, 229. Cowan. K. 0.;Bymaster, D. L.; Hall. H.; Eisenbraun. E. J. C
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hd. 1986, 105.
Volume 68 Number 1 January 1991
85
