Microwave Drying of Precipitates for Gravimetric Analysis Robert 0. Thompson and Mufaddal Ghadiali Oberlin College, Oberlin, OH 44074 Microwaves are electromagnetic waves with wavelengths in the range 0.1-10 cm. They are widely known for their ability to heat and cook food. Microwaves have also been used in the chemical laboratory for more than a decade now. The major uses have been in the processing of ceramics (I),crosslinking of polymers and resins (21, catalysis (31, and acid dissolution of biological, geological, and other solid samples (4-6). The microwave oven should also be useful in drying solids and precipitates. However, few applications have been reported, andthere are only two concerning the use of microwaves for gravimetric analysis (7, 81. The first of these was written in 1981 and described the determination of P205in fertilizers. The authors reported that less energy was expended to obtain the same results a s with conventional oven drying. The more recent (1990) paper dealt with the analysis of sulfate in triglycine sulfate for infrared detectors. Again the reported advantage of the microwave oven was a savings in time and energy. Comparing Microwave and Conventional Ovens Conventionally, precipitates are dried in a convection oven set at about 110 "C for a period of 1-2 h. In this pmcess the precipitate is warmed, and water is released from it. The filter crucible is also warmed. In fact. the crucible usually becomes hot to the touch and must be cooled for 30-60 min in a desiccator before weighing. If the cooling period is shortened and the crucible is weighed while still warm, the rising warm air will cause inaccurate and fluctuating balance readings. On the other hand, in the microwave oven the water molecules rotate rapidly in response to the radiation and are heated directly. Moisture can thus be removed without heating the crucible. The crucibles only get slightly warm and, consequently, return to room temperature in the desiccator in a few minutes. Thus, the microwave oven has features that make it superior to the conventional, mnvection oven for the drying of solids. Here we report the use of a microwave oven in the analvtical chemistrv laboratorv to drv oreciuitates. Three of the most common gravimecric determinations in analytical chemistrv-for silver (as silver chloride). for sulfate (as barium sulfate), and for' calcium (as calcium oxalatejwere carried out usinn the microwave oven for d r v i n ~the empty sintered-glass &cibles and for drying the coll&ted precipitates. The precision and accuracy of each determi. . nation are reported. "
A
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Experimental Equipment Experimental conditions matched those found in our teaching laboratories as much as possible. Pivets and vdmetric-flasks were of Class manufacture &d used without further calibration. The medium-porosity (15-km pores) glass-fritted crucibles (Pyrex) used to collect the precipitates were weighed on a n analytical balance (Mettler, AE200) with a precision of M.1 mg. The microwave oven (Kenmore, model 565) had a 900-W capacity and is usually sold for home use. 170
Journal of Chemical Education
Reagents ACS-grade NaCl, K2S04,and CaC03 were dried in a convection oven for 1-2 h a t 110 OC and cooled in a desiccator before being used to prepare standard solutions. (It would be interesting to know whether a microwave oven could also be used for this sort of drying.) ACS-grade AgN08, BaC12, and (NHJ2C204,were used to prepare the precipitating solutions, and ACS-grade stock acids or bases were used to control the pH. The desiccant was calcium sulfate (Drierite). Procedures The gravimetric analyses for chloride (91, sulfate (lo), and calcium (11)were carried out a s described in wellknown monographs on analytical chemistry. Drying to Constant Weight In this work, as in most gravimetric work, the empty or filled crucibles were dried until successive weiehinw were within f0.5 mg. The crucibles were heated to eEect;drying, cooled in a desiccator to prevent reabsorption of atmospheric moisture, and then weighed. Several cycles of heating, cooling, and weighing were necessary to achieve constant weight. These cycles will be designated in the following way: (dryingtime lldrying time 21drying time 31. . . ) (cwling time) For example, (41212)(15) indicates that a cooling time of 15 mins was used throughout with heating times of 4 min for the first heating, 2 min for the second, and 2 min for the third. We tried to minimize both the heating and cooling times to increase efficiency. When heating times are too short more cycles are needed, and when cooling times are too short the warm crucibles are dimcult to weigh. Optimizing the Heating-Cooling Program Empty Crucibles
At fir& one crucible at a time was heated at 10% power. A program of (2/"/212)~15)achieved constant weight. At 205 power the crucible was dry afler (4,2121115). ~ c ~ a s i o n aal crucible l~ was~stillwarm aRer the 4-min heating with this protocol as indicated by a slowly increasing reading from the balance. When the cooling time was decreased to 10 min this became a consistent problem, and when the cooling time was set at 5 min, constant weight was difficult to achieve. Consequently, the cooling time should be a t least 15 min. In a large laboratory class several crucibles must be dried at one time. ~ e & u s eour microwave oven and our analytical laboratory sections (about 15 persons) are fairly small, we chose 6 crucibles as the maximum number to dry a t any one time. This would typically allow 2 or 3 students to complete the drying step si~ultaneously. At 100% power the empty crucibles reached constant weight in two cycles with (4/2)(15)and three cycles with (2/2/2)(151A program of (2/2/2/)(10) gave fluctuating balance readings because the cooling period was too short.
Table 3. Drying of Calcium Oxalate Precipitate
Table 1. Drylng of Silver Chloride Precipitate
Cwcible NO. 1 2 3 4 5 6 Weighing # I 1.3793 1.3277 1.1836 1.3239 0.9978 1.0001 Weighing #2 1.3793 1.3270 1.1836 1.3238 0.9978 1.0001 Weighing #3 1.3270 Expected wt. 1.3794 1.3275 1.I879 1.3221 0.9969 1.0006 %Found 99.99 99.96 99.64 100.13 100.09 99.95 All weights are in grams.
1
1
2
3
4
5
6
Weighing#? 0.6104 0.8201 0.6110 0.6114 0.6043 0.6123 Weighing# 0.6114 0.6123 0.6121 0.6114 0.6047 0.6125 Weighing #3 0.6114 0.6117 0.6118 Expectedwt. 0.6064 0.6064 0.6064 0.6064 0.6064 0.6064 %Found 100.82 100.87 100.89 108.82 99.72 101.01 All weights are in grams.
Table 2. Drying of Barium Sulfate Precipitate
Cwcible NO.
Crucible No.
2
3
4
5
6
Weighing#l 0.2362 0.2360 0.2496 0.2364 0.2358 0.2367 Weighing= 0.2361 0.2359 0.2345 0.2363 0.2358 0.2365 Weighing #3 0.2345 Expectedwt. 0.2365 0.2365 0.2365 0.2365 0.2365 0.2365 %Found 99.83 99.75 99.15 99.92 99.70 100.00 All weights are in grams,
silver chloride and barium sulfate was obtained aRer only two heating-cooling cycles in all but two cases. The precision and accuracy of the calcium data (5 ppt rsd, 100.7% recovery) were very good, and conventional oven-drying gave similar results. It did not appear that the water of hydration was being lost from the calcium oxalate (CaC204.HzO) because repeated heating-cooling cycles gave constant rather than decreasing weight and since the recovery was over 100%. This makes sense because water is removed only at temperatures exceeding 200 OC (12).To achieve constant weight with calcium oxalate, three heating-cooling cycles were necessary in half of the trials. Conclusion
The (4/2)(15)pmgram was most efficient; six empty crucibles could be dried to constant weight in about 40 min. Filled Crucibles Crucibles containing precipitates showed d+g characteristics similar to those of the empty crucibles. The effect of decreased cooling times was even more evident, so at least 15min, ifnot more, should be allowed for cooling. The optimal heating-cooling program was (4/2/2)(15),and this program was used in all subsequent experiments. Results and Discussion
The data obtained for the determinations of chloride, sulfate, and calcium are given in Tables 1,2, and 3, respectively. The net weights of the precipitates as measured after each heating-ding cycle are shown for each of six trials. The chloride and sulfate data had excellent precision (3ppt or less) and accuracy (>99.7%recovery), which are as good or better than data obtained using a conventional oven for drying. The expected silver chloride values varied because solid sodium chloride was weighed out as a standard for each trial, whereas for sulfate Lnd calcium every expected value was identical because the same volume of the same standard solution was used for each trial. Constant weight for
These results show that the microwave oven is an important addition to the general or analytical laboratory where gravimetric analysis is carried out. The conventional drying process normally takes three heating-cooliig cycles of at least 1h each, for a total of 3 h or more. The microwave method requires only 40 min in most cases and less than 1h in the most difficult case. Accuracv and precision are not compromised at all by the use of themicrowave oven, and the & t h d is applicable to the full ranze of mavimetnc determinations that use dryLow cost, department-store microwave ing of ovens suffice for this application. Literature Cited 1. Sutton. W H . h . Cemm.Sm. Bull.1888.68,376.
2. Mijwic, J.; W~jaya,J.Rlym. Compos.1990.11, 184. 3. Deoev M. C.: Wan. J. K S. RFJ. C k m Infermod. 1980.13.221. . . 4. ~ & b & , M. k . ~ r & I ~ &l991,19,47. 6. Knapp, G.Mikmehim.Aeto 1991.2.445 6. De la Guardia, M.; Salvador, b: Burguera, J. L.: Burpera. M. J. Flm Injetlon
Anal 1988.5.121.
7. Melton, J.R.;Hmver, W. L. JAmm. MTAnol. Chem.l W l , 64,1319. 6. Tsdms, S. H.:Frazier, D. O . h l y s f 163%115,229. 9. Kennedy, J. H.Amlytieo1Chembfn:PmetloP; h d d . ; Saunders: New York. 1990,
12. ~olthoff,I. M.; Sandell,E. 8.; Meeh8n.E. J.; B~uckentein,S.Quantilofiue C h i d Analysis,4th 4.MacMillan: : London, 1969;p 622.
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