J. Warkentin
University Hamilton, Ontario
Versatile Magnetic Stirrer
McMaster
and Flexible Coupler
T h e determination of reaction rates by monitoring gas volume as a function of time poses a t least two problems to the experimenter. First, he must assure adequate stirring in a gas-tight flask which is immersed in a thermostatingfluid. Stirring is necessary not only to assure good temperature communication but also to prevent an induction period or rate-limiting evolution of gas from the solvent.' The second problem is to provide for freeze-thaw degassing capability and subsequent immersion in the thermostating fluid while maintaining gas-tight connections, at least for reactions which are sensitive to oxygen. Either a flexible connection between flask and vacuum line or a movable thermostating bath is therefore required. We wish to describe a system which we have found very satisfactory for kinetic work of the type described.
1,200 rpm to 3,200 or more rpm. Within this range and with chlorobenzene as solvent, we have had no problems with the driven magnet dropping away. The latter jumps back to the drive magnet in any case, with a flask of suitable design. Our coupling between flask aud vacuum line is an Sshaped piece of glass tubing ending with ball-joints (Figs. 1 and 2). By means of this coupling the flask, complete with stirrer, can be kept vertical while being swung through the arc of a circle, from a degassing position beside the constant-t,emperature bath to a position inside that bath.
Figure 2. Figure 1. Flork and stirrer arrembly connected to the vacuum line through the S-shaped connecting arm; degorring porition. Conrtont temperature bathinthe background. Power supply for motor ot lower right.
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The stirrer (Fig. 1) is a one-piece tol,-driven magnetic type, which plugs into t,he rcactiori vcssel by means of a standard-taper ground glass joint. Thus there is no alignment problem in assembling the stirrer and flask, and the flask can he totally immersed in a heating or cooling medium. hIoreovcr, there are no O-rings with associated problems of contamination, lubrication, leaking, and wear. The stirrer imparts very little vibration to the rest. of the system from its stalling speed of about BUNNETT,J. F., in "Technique of Organic Chemistry," edited by A. Weissberger, Interseienee Publishers, (a division of Johu Wiley & Sons, Inc.) New Yark, 1961, Vul. VIII, Part 1 (2nd =I.), p. 183. 1
4. 5 6.
7. 8.
Portr list: Quick-connector cord. Motor, Delco 5069267, 27.5 VDC, Lectronic Rereorch Loborotorier, 7 1 5 Arch St, Philadelphia, with leads modified for microphone plug. Stoinles steel shaft. '/$ -in. diameter, fastened to motor rhoft with two Allen tcrewr The bottom of the shaft is mochined ond thremded to in. Eclipse button magnet IElectro Sonicr, pars through the hole of o 543 Yonge St., Toronto) and to fasten with o b r a s nut. The shaft must be precisely machined. with recessed groove to receive lip of part Coilor of laminated 5. Hoier match the threaded holes orovided in the motor end date. Pyrex 9tirrer envelope, T 34/35. ~ i otop t rim fits recessed groove of part 4. Hole permits occerr to Allen screws to odjust dirtance between magnet pole faces and bottom of envelope. Tenon-covered driven magnet; 1 in. with spin ring. Flask. S-shaped connecting link, with water jacket,
e have found it very desirable, for reactions involving gas evolut~on,to pre-equilibrate the bulk of the solvcnt to the bath temperature in the reaction vessel and then to inject the reactant, in the minimum volume Volume 43, Number 5, May 1966
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265
of solvent, through a rubber septum. As a result the time needed for warm-up and vapor-equilibration is negligible, and V oand &are accurately known. The stirrer can he used to advantage for other purposes. I n addition to the fast setrup time and the convenience of heating and cooling that it offers, it has some advantages over other magnetic stirrers. Stirrer and flask can be raised or lowered in an instant, without interrupting stirring. The stirrer performs well with solids on the bottom of the flask (e.g., Grignard reactions), and it is not readily stalled by precipitates or gums that may form during reaction. I n addition, the driven magnet is withdrawn from the flask as part of the stirring unit. Total cost of parts (Figs. 2,3), machining, and glassblowing is about $60.
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Journal of Chemical Education
Acknozuledgment. The author would like to thank Mr. C. Schonfeld for designing the power supply and the plastic-laminate collar.
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D
Figure 3. Power supply. T Transformer, 5 0 vst, ,275 omps [Hommond 167X) R Rheostat, 1 7 5 a, 2 % [Ohmite, Model H, Series Al D Silicone Diode, 5 0 0 mo, 1 0 0 v, PIV C Electrolytic Capacitor, 1 0 0 wF, 5 0 v S Toggle switch F 1 Amp Fuse (AGCI)
