Modification of a Microwave Oven for Laboratory Use Judith Andrewsl and George F. Atkinson University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 The use of domestic or commercial models of microwave ovens for drying analytical samples has been recommended by previous authors (1-7). The savings of time are attractive, and many (but not all) samples, including some electrically conductive ones, are dried without damage ( I ). A computer search of the literature using the Chemical Abstracts database yielded a relatively small body of literature and indicated one principal problem. When complete dryness of a sample is approached-a condition never reached in microwave cooking-the oven loses its load. No material capable of absorbing microwaves efficiently remains. Energy is still being supplied to the enclosure. Under these conditions, there is a tendency for the magnetron to arc and destroy itself (7) \-,.
We were also concerned about ~ossihleinteraction of the material being dried with the oxygkn of the air, and wished to examine emitted eases from the drvina These con. -vrocess. . cerns led us to m;dify a Litton commercial microwave oven (Model 70108 Menumaster) as follows.
Figure 1. Water ballast far oven on a polypropylene stand.
Water Ballast The oven nameplate states its minimum permissible load is 50 ml of water. Accordingly, a small bottle was made with two sidearms and a threaded neck accepting a standard polyethylene screw cap. A stand made from two pieces of polypropylene sheet held the bottle above the oven floor and away from the wall. The sidearms were joined to two lengths of PTFE bellows tuhine bv usine PTFEiFEP shrink tubine. both from Pope ~ c i e n t l k > n co7Menominee . Falls, WI. T; canacitv of the bottle to the umer sidearm was a~~roximatelv 100 ml:~ottle and stand are'shown in Figure 1: Inert Atmosphere Enclosure The polypropylene lid of a Nalge polymethylpentene jar (1000 ml) was put on a lathe, and the upward flange provided for secure stacking was removed. The lid was then mounted on the sidewall of the oven, positioned so the jar itself could be screwed into it. Two 51s-kdiameter holes were made in the lid and in the oven chamber wall, and two similar holes on the same spacing in a polypropylene shelf. These parts were assembled as shown in Figure 2 using glass-filled polypropylene tee-fittings. The through arm of the inlet tee was provided with a glass tube with several lateral holes to sweep purge gas into various parts of the jar beneath the shelf, while the upward-turned stem of the tee was plugged with polypropylene rod. The gas exit tee had its arm extending below the shelf plugged, and the upward-pointing arm open. When desired, this arm may he fitted with an inverted U of tubing to collect gas from a sample container immediately above the sample. The shelf is braced with a half-slotted support and may he given a platform of ceramic foam to protect it from the heat of drying samples. Outside the oven chamber, elbows direct the bores to the rear, and polyvinyl chloride tubing is used to lead out through the ventilating louvers of the oven to an appropriate gas supply and vent or analyzer. Water Flow System The connections to the water ballast are made to glass-filled polypropylene elhows mounted in the chamber wall, and thence through polyvinyl chloride tubing out through the
Figure 2. Attachment of enclosure to oven chamber wall. See text tails.
for
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louvers. A water flow monitor (Instruments for Industry and Research, Cheltenham, PA, Model WFM-1800) was connected in the drain line to turn off oven Dower if the water flow failed. A constant water feed, either through a flow regulator or a constant head device, is helpful in setting a minimum adequate flow. Materials and Equipment Comments All construction materials were tested by placing them in the oven for at least 2 min of irradiation. If no temDerature rise was observed rapart from the slight general rise of the unloaded oven's temnerature). the material was iudeed suirahle for use. ~ o l ~ ~ r o ~ ; l ePTFE, n e , polymethylpehtene, polyethylene, and nylon were thus tested. The holes for fittings and the rim of the jar lid were generously coated with silicone bathtub sealant at the time of assembly. Although polyvinyl chloride tubing did not heat when
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This work was performed by Judith Andrews as her final-yearresearch project for her BS degree.
Volume 61 Number 2
February 1984
177
placed dry in the oven, and although it was successfully used for connecting the water ballast iar in early experiments, its use inside theihamher w n s a l ~ a n d knftr;themen l wasonrp or~era~ed with this tubing ill11 oI'u,ater hut withnut watrr flow. The tubing swelled and charred to resemble a burned sausage, and melted (hut did not otherwise degrade) nylon clamps securing it to the ballast jar. One disconcerting discovery made when the shell of the oven was -removed to fit the modifications was that the air flow . .~~ channeling fitted by-the manufacturer consisted of strips of cardboard secured with carpet tape, and that this cardboard was resting against the glass of the incandescent lamps provided to light the oven chamber. We recommend that these strips be fireproofed or replaced with fireproof material even if the oven is not being modified in the manner described. ~~~
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Performance
Copper sulfate pentahydrate is reported to lose four moles of water a t temperatures below 150°C, and the fifth mole only between 250 and 300°C (8). Our experiments agreed with literature reports that microwave drying can produce only the monohydra;e. Drying is nearly n)m&re in 10 min. and \,;A ations in lime to remwe the last tract's or the f w ~ moles r uf water probably reflect exposure during weighing to the varying humidity of days on which experiments were conducted. Drying oxalic acid dihydrate a t high heat settings of the oven resulted in sublimation of the a form of the anhydrous acid 1m.n. 188.5-189.F1~C).The sublimate which collected in the upp& part of the sample container was shown by infrared soectroscoov .. to he nnchaneed chemicallv. Such use of the ovens for sublimation from hydrated starting materials deserves exoloration. At medium vower settines, oxalic acid dihydrate does not sublime, but appears to ;each a stahle condition after 15 min and then abruptly loses the rest of its water. About one-third of the total weipht loss occurs in this second stev. Fr~shlvprrcipitnted calcium
