Wesley W. Wendlandt Texas
Technological College Lubbock
I I
A (ontrolled Atmosphere Thermobalance
The rapid growth of interest in the t,echnique of thcrmogravimetric analysis (TGA) during the past ten years has generated a need for automatic recording thermobalances which will determine the pyrolysis of compounds eit,her in a vacuum or in a controlled atmosphere such as argon, nitrogen, carbon dioxide, or hydrogen. A number of thermobalances, both manually operated and automatic recording inst,ruments, which will operate under the above condit.ions, have heen described in the literature. A large number of instruments have been described which are hased on the change in elongation of a quartz or metal spiral (1-5), similar to that first described by McBain and Bakr (6) in 1926. In many cases, an appropriate transducer element has been added such as a magnet and solenoid (7-9) a phot,ocell and light beam arrangement (10, 11), a linear voltage differential transformer ( I $ ) , or a mirror and light beam with a photographic recorder (15). Besides spiral type instrument,~,a number of torsion wire balances have been described (14-17) which will operate in a controlled gaseous atmosphere or in a vacuum. One of the inherent problems of the quartz spiral type of balances is t,he fragility and limited load capacity of the spiral. To overcome this difficulty. an automatic recording t,hermobalance was constructed having a strain gauge as the weight-sensing transducer. A thermobalance has previously been described (18) employing a conventional analytical balance to which was attached, as the weight-sensing device, a length of strain gauge wire. However, the instrument could not be operated under controlled atmosphere conditions. The strain gauge thermohalance is illustrated in Figure 1. The strain gauge used was obtained from Statham Instruments, Inc., Hato Rey, Puerto Rico, Model No. GIOB-0.15-350. The transducer has a range of +0.15 oz with a sensitivity of 20.93 mv per oz a t 9 v excitation voltage. Other models are available up to 1 5 oz if a larger capacity instrument is desired. The strain gauge is a rather robust device in t,hat i t can stand a 1 lb force overload without damage. The excitation voltage can be direct or alternating current and it has a non-linearity plus hysteresis effect of less than 1% of full scale. The gauge is small, 1.50 in. long, and weighs approximately 1.5 oz. The strain gauge was mount.ed on an aluminum block attached to the aluminum cylinder housing by two small setscrews. The aluminum cylinder was 2.5 in. in diameter by 6 in. long and was machined from a solid metal block. One end of the cylinder was enclosed with a 0.25-in. thick plastic disk so that the four
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lournal of Chemical Education
bridge leads could be connected to the outside power supply and recorder. The plastic disk was attached to t,he metal cylinder by six screws, along with an appropriate gasket and sealing compound. The glass system was attached to the aluminum cylinder by three setscrews and then sealed with Apiezon "W" cement.
7 FRONT
PUMP
4 P
Figure 1 . Sehemotic drawing of strain g a u g e thermobolance. A, oluminum cylinder housing; SG, strain g a u g e transducer; C, plastic dix; W , woter-cooled stondord t a p e r joint; F, furnoce; and P, somple pan.
To increase the sensitivity of the strain gauge, a length of aluminum metal, 0.25 in. X 1.25 in., was attached to the strain gauge probe. A platinum wire and sample pan was connected to the extension probe and allowed to hang freely into the glass system and furnace. The furnace chamber consisted of a 2.5-cm diameter Vycor glass tube which was connected to the Pyrex glass system through a water-cooled standard taper joint a t W. The bridge circuit for the strain gauge is given in Figure 2 and is a modification of the original circuit described by the manufacturer and also by Jackman (19). The power supply voltage must he quite stable, so two 6 v automobile-type lead storage batteries, connected in series, were employed. A 5KO Helipot was used to control the voltage into the bridge circuit. It should be mentioned that the bridge voltage must not exceed 9 v. An output filter circuit was necessary because of the pick-up of natural building vibrations
TO
RECORDER 0 - 1 MV
Figure 2.
Schematic circuit for strain gouge.
and disturbances by the strain gauge when operating under this high sensitivity. The output from the bridge circuit was recorded on an E. H. Sargent and Co., Model SR, potentiometric recorder, using a 1.0 mv range plug and a chart speed of 0.1 in. per min. Full scale on the recorder corresponded to a change in weight of about 80 mg. However, other full-scale weight ranges could be obtained by decreasing the bridge circuit voltage by means of the 5KQ Helipot. The furnace consisted of a ceramic tube, 3.2 cm in diameter and 24 cm long, wound with No. 26 gauge Nichrome wire, and had a total resistance of 36 ohms. The furnace temperature was controlled by gradually increasing the input voltage by means of a motordriven 0.135 v Powerstat (ZO). Maximum temperature of the furnace was about 850°C, although 500°C
was the usual temperature limit employed. A typical furnace temperature-time curve is shown in Figure 3; the heating rate was 7% per min. Also included in Figure 3 is a curve showing the change in weight of the pan as a function of temperature. I n practice, a correction is made on the weight-loss curve to allow for this change in empty pan weight. The operation of the thermohalance is illustrated by the thermogravimetric curves in Figure 4. The compounds studied were CuS04.5H10 and NasEDTA. 2H20 in nitrogen, and Na2EDTA.2H20 in air. The thermal decomposition of CuS04.5Ha0 should he independent of either air or nitrogen atmosphere; thermal decomposition of anhydrous NaiEDTA, since it involves oxidation of carbonaceous material, should he dependent on the type of atmosphere present in the furnace. The weight-losses involved for CuSO4.5H20 occurred at the following temperatures: CuS04.5H.0(s) CuS04.3HeO(s) + 2HnO(g). . . 45-100°C CuSO,.3HaO(s) CuS0,.H20(s)
--
+ H1O(g) . . . . . 20&262T
The only differences between the two curves occurred above 300°C. Initial decomposition of the compound appeared to independent of the atmosphere employed and only at higher temeratures does this effect appear
100 TIME,
CuSO,(s)
For the Na2EDTA.2H,0, the following weigbtlosses occurred, both in nitrogen and air: NarEDTA.ZHsO(s)-NenEDTA(s) + 2HnO(g) . . . 110-210'C N s 9 D T A ( s ) - N&Oa + carbonaceous products . . 230%
I MIN.
Figure 3. Furnace tempero
