CONCLUSIONS The versatility and sensitivity obtained by our apparatus are evident from the examination of the concentration curves and the titrations shown. Therefore, the apparatus is proposed as a valid instrumental complement in the resolution of inorganic and analytical problems such as the study of complexes in solution or precipitated, the determination of the dielectric constant, and all other applications where a resistive or dielectric constant variation may furnish helpful indications. In addition, the possibili-
ty of taking automatic measurements with great advantage regarding personnel and time without any loss in sensitivity and instrumental reproducibility is to be considered. ACKNOWLEDGMENT We thank C. D’Arrigo for his helpful collaboration. Received for review April 17, 1973. Accepted February 19, 1974. The present paper was supported by the Consiglio Nazionale delle Ricerche, Roma.
Use of a Microwave Oven in In-Process Control James A. Hesek and Robert C. Wilson Analytical Services, J. J. Baker Chemical Company, Phillipsburg, N.J. 08865
Microwave energy is receiving increasing use in the cooking of foods and in some industrial drying applications. Laboratory application for microwave energy has largely been in the research sector with little attention directed toward its use as an aid in routine analysis. The in-process control of inorganic chemicals manufactured by batch operations often requires partial analysis of the product during washing operations or before drying. Determinations required may include loss on drying, assay or metal content by titrimetric procedures, or the content of one or more impurities. Conventionally, a wet cake or paste is dried on a hot plate or in an oven (110 “C); for slurries, initial filtration is sometimes expedient. For some slurries, the elapsed time for drying may total 3-4 hours. Any measures that can be taken to reduce the laboratory time for such in-process assessment can often yield savings in plant costs and time, and improvement in utilization of equipment. EXPERIMENTAL Apparatus. A 550-watt Litton Minutemaster, Model 350 microwave oven was purchased in mid 1972 to assess its merit in the laboratory drying of in-process materials. This has recently been replaced with a Litton Minutemaster 70/20 with a 700-watt output. Our accumulated experience reveals that for most inorganic slurries and wet cakes, microwave drying can be accomplished successfully within 15 minutes. A portion of the slurry or cake (weighed if necessary) is placed in a porcelain or glass container and dried for a suitable period of time. Where experience has shown the material may spatter, a ribbed watch glass may be used as a cover. The establishment of an appropriate drying period is largely empirical. The water must be substantially removed (usually to less than 0.1%) and the compound must not decompose. Usually the appropriate drying time can be found with a few trials by drying a portion of the material to constant weight. The drying time may vary with sample size or the number of samples being dried. This time, however, is not always relatable to the number of samples. For example, it has been established that either one or six 20-gram samples of barium carbonate containing 68% water can be dried to constant weight in 15 minutes; 10 minutes is not adequate. (In a 105 “C drying oven, 3 hours was needed to reach a constant weight.) This indicates that small sample quantities do not approach a “full load” situation in the oven.
RESULTS AND DISCUSSION Materials dried successfully by microwave energy include carbonates of barium, cobalt, lead, magnesium, manganese, nickel, and zinc, and sulfates of barium, calcium, and lead. The savings in production time by the 1160
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
earlier availability of dry basis analytical results has been significant, Our studies suggest that materials that can be dried in a conventional oven maintained somewhat below 150 “C can be dried successfully in a microwave oven. Copper(I1) sulfate pentahydrate, for example, loses all of its water of hydration at 150 “C; however, persistent microwave drying yielded only the monohydrate. Spent silica gel (20% water) can be regenerated to a useful desiccant in the microwave oven. A 50-gram sample reaches constant weight within 30 minutes; on subsequent furnacing of this material, the weight loss is typically only about 1.5% after 30 minutes at 400 “C. The drying of organic materials has not been of general interest. Some limited experience, however, may be noted: saccharin can be dried successfully, but the microwave heating of oxalic acid monohydrate resulted in the prompt sublimation of oxalic acid. Potassium biphthalate in the microwave oven showed a weight loss after 5 minutes, identical to that after drying for the conventional 2 hours a t 120 “C; however, on longer microwave treatment, continual small weight loss occurred. PRECAUTIONS Use of the microwave oven to dry small samples involves a situation where there is extremely small “load” of material capable of absorbing microwave energy. This situation normally exists for only short periods. Over 20 months, no effects to the magnitron tube were observed. However, a coincidental failure of the timer and the thermal safety switch resulted in a 20-minute cycle being extended to about 1 hour. At the end of the period, one of six samples of nickel carbonate was converted to the black oxide, and the Pyrex beaker was fused to the bottom of the oven. The magnitron tube was damaged during this cycle, probably by arcing, and heat damage to other parts made it uneconomical to repair the oven. Highly acidic materials should not be dried in a microwave oven, since the acid fumes may attack the unit. Small amounts of free ammonia, however, can be tolerated. Following good occupational safety practices, a microwave unit should be checked at intervals of 6 months to assure the absence of spurious radiation. Received for review December 13, 1973. Accepted April 8, 1974. This note is paper X in the Series, “The Practical Analysis of High-Purity Chemicals.”
