A Simple Modification of a Domestic Microwave Oven for Improved Temperature Control Marco ~agnona,' Anna Nolan, and Leona Kim Barnard College, Columbia University, New York, NY 10027 Domestic microwave ovens have been finding ever widening uses in research laboratories. The high heat develo ~ e dr a .~ i d-l vin microwave ovens has been used for a variety of applications, including degradation reactions, drvine ofrrlassware, dryinc of silica and alumina fir use in c&o~ato&aphy,&d regeneration of molecular sieves and desiccants (13).Using domestic microwave ovens to promote chemical reactions by heating sealed vessels to high temperatures and pressures, on the other hand, has been less common (4,5). Relatively few reports address effects on reactivity caused by changes induced on the molecular level by absorption of &crowive energy. Jahngen noted an increase in the rate of hydrolysis of phosphoanhydride bonds in triphosphates for samples heated with microwaves as compared to samples heated conventionally. The obsenred acceleration tentatively was attributed to spectroscopic heating caused by preferential absorption of microwaves bv the substrate (6). However, questions concerning temperature stability as well as uniformity of heating havc cast some douht on the interpretation of these results (7). We have been investigating similar microwave induced effects by examining changes in selectivity for a series of competition reactions, performed a t identical temperatures and pressures, but heated either conventionally or with microwaves in a domestic microwave oven. Crucial to these studies is the ability to maintain a solution at constant temperature while in the microwave oven. We would like to report some of our simple modificationsto a domestic microwave oven that allow us to maintain reasonable temperature control and stability. Description of Modifications to the Microwave Oven To use a domestic microwave oven for our purposes, we needed to remove the excess heat generated by the microwaves and a t the same time to monitor continuously the temperature of the reaction. A domestic microwave oven2 equipped with a temperature probe and turntable was modified to accept inlet and outlet ports for circulating water and an independent temperature probe. Two 114-in. holes, one 10 cm above the other, are carefully drilled a t the approximate midpoint of the left wall of the oven. A 6-in. length of rigid polypopylene tubing is snugly fitted into each hole and fixed to both the inner and outer walls using molten beeswax. Flexible 114-in. Tygon tubing is used to connect the inlet and outlet ports to a 50-mL jacketed reaction vessel or to a small reflux condenser. Chilled water is circulated through the reaction vessel a t an approximate rate of S 1 0 Wmin. The exterior inlet and outlet ports are enclosed in a small (25 x 20 x 5 cm) cardboard box lined with aluminum foil. The system is checked for microwave leakage.3 With a beaker containing 250 mL of room temperature water placed i n the microwave oven and heated a t full power 'Autnor to whom correspondence shou d oe adaressed. 'AGE 600W mlcrowave oven, model numberJE1034T. was used 'We s e a a S mpson Mode Numoer 380 M mlcrowave lean derec-
tor, available from Thomas Scientific
(600 W), less than 2.5 mw/cm2 of leakage is detected a t a perimeter measured 5 cm from the end of the foil lined box. This is below the recommended safety limits of 5 mWlcm2 a t a distance of 5 an (8).Enclosing the protective box with a second aluminum foil lined box reduces the leakage around the 5-cm perimeter to less than 1 mWlcm2. With any microwave oven used in a laboratory, it is exceedingly important that the system be checked for leaks in and around the modified area as well as in the rear and front before any experiments are performed. The modified microwave oven should never be used without circulating water, which acts like a radiation sink for the small amount of microwave radiation that leaks from the inlet and outlet ports. Temperature Measurement Amodest degree of temperature stability (f5-7 'C) is realized using only water-cooled jacketed reaction vessels and the "temperature cook" option available on most microwave ovens. The thermistor temperature probe supplied with the microwaveoven can be used. However, this method is of limited use because the irradiation power is either drastically reduced or even shut offoncethe set temperature is attained. This method also does not allow for measurement of temperatures above the setpoint. What is referable is continuous measurement and control of temperature during microwave irradiation without activating the temperature cook option of the oven. Temperatures during microwave irradiation have been measured using shielded thermocouples (9), or, more recently, fiber optic thermometry (6);the latter method avoids local heating and electrical interference problems associated with using thermocouples in a microwave field. Thermistors, although they have a narrower temperature range than thermocouples, can also be used. We used the thermistor probe supplied with the microwave oven connected to an independent temperature monitor. ARer carefully removing the outer cover of the microwave oven, the leads connect& the temperature probe to the microwave Dower board are easilv detached and the female iack connected by wires lead& through the vents in the back of the ovento a NIST therbistor sensing electronic thermometer. This setuD bwasses the "tem~eraturecook" feature, that is usually activated by insertion of the probe, and allows probes to be changed quite easily. The probe is calibrated over the desired temperature range, in our case fromODtolOOoC. Using a 50-mL jacketed reaction flask and chilled cooling water a t a rate of 8-10 Wmin, we are able to maintain 10 mL of organic solvent, for example, ethyl acetate, at 50 f 2O C for greater than 2 h in a microwave oven operating a t full power. Temperature fluctuations are moderated by changing the flow rate and temperature of the circulating liquid. We also have successfully used this setup to heat solutions to reflux. &
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Conclusions Inexpensive, safe modifications can be made to domestic micmwave ovens that allow for independent temperature measurement a s well as reasonable temperature control. Such a setup has allowed us to investigate micmwave activation of organic reactions by giving us the ability to compare two reactions performed at the same temperatures and pressures. Acknowledgment Acknowledgment is made to the Donors of the Petroleum Research Fund. administered bv the American Chemical support of the research. Society, for
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Journal of Chemical Education
Literature Cited 1. Ki"gst0". H. M:Jassie, L. B., Ed. Inmduetion to Mkmwov. Sompk Pnpomtion; ACS Preaa, 1988. 2. Viiemin. D.;Thibault-Star~yk,F. J. C k n r Ed=. 1991.68.346. 3. Green.. H.A,:. Go&. R. W J Chem. Educ. 1991,68,429. 4. Gedye, R.: SmiUI,F.;Westststy, K.;Ali, H.;Baldisera,L.; Leberge, L;8080ll.J. mmh d m n h f f . 1Se427.219-282. 5. Gipere, R, J.; Bray, T L. ; Dunean, S. M.; Majetich, G.7MvhedmnLltt. 1986,27, 4945-4948. .E Ow 6. Sun,W . 4 . ; Cuy, P M. ;Jahngen. J.H.;Rossamon&, E.F;Jahngen. E.G.' ChPm. lssS,53,4414--4416. 7.Jahngen,E.G.; Len*, R.R.; Perahsk, E P: Sacks*, P H. J Org. C h m . 1890,56, 34063409. 8.rpltle 21 of the Code ofFederal Regulations,Part l(U0.10. 9. Bafd, M.; Brni, M.: Ch-mi, A: rgneati, A; Millanta. L.; Rubino, N.:vanni, R. J Chem. Soc. Fomdny I h n s . 1 1981,77,1503-1509.
