Rudolph S. Botlei and Paul L. Gerace University of Notre Dame Notre Dame, Indiana
A Thermobalance for Student Experiments
Thermogravimetric analysis (TGA), an extremely valuable analytical technique, involves the weighing of substances while they are being heated. A complete treatment of the theory of thermogravimetry is available in the reference work by Duval.' This paper presents the details of the construction and operation of an inexpensive, manually-operated thermobalance which was used for the study of the thermal stabilities of some metal chelates and which could be used for student experiments in thermogravimetry in advanced laboratory courses in analytical and inorganic chemistry.
heated to about 500°C was constructed from two concentric transite cylinders approximately 4 in. and 6 in.
Thermobalanca ConrCuction
The thermobalance is shown diagrammatically in Figure 1. In constructing the thermobalance, the glass base of an ordinary analytical balance was replaced with a base of hakelite so that a hole approximately ' / a in. in diameter could be drilled under the center of the left balance pan. The balance was equipped with a magnetic damper. For heating the sample, a furnace which could be 1 DUVAL, C., "Inorganic Thermogravimetric h l y s i s , " Fdwevier Publishing Go., Inc., New York, 1963.
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Figure 1. Thermobolmce: A, gor inlet tube, B, temcomotor themecouple; C, iron-cowtantan thermocovplei D, sample viol; E, sample chamber, F, heating coils and insulation; G, temcomster; and H, potentiometer.
in diameter and 12 in. high. The heating element was a nichrome wire heating coil which was wrapped around the outside of the smaller cylinder and wired so that the total output was approximately 600 watts for the 110 volt input. The coils were covered with asbestos cement and the entire assembly placed inside the 6-in. cylinder. The two cylinders were attached to a 6-in. circular base cut from transite hoard. The space between the inner and outer cylinders was packed with mica insulation. The hot-junction of an iron-constantan thermocouple was sealed into the side of a piece of Pyrex tubing, 1in. in diameter and 15 in. high. This tube was then put into the center of the smaller transite cylinder and secured using supports fashioned from triangular crucible supports made of nichrome wire. A space of approximately one-quarter inch was left between the bottom of the furnace and the Pyrex tube to permit the flow of nitrogen gas through the sample chamber. An inlet for nitrogen gas was provided by inserting a 15-in. length of 7-mm Pyrex tubing into the inner transite cylinder. The top of the furnace was covered with an asbestos mat, and the entire top area, except for the Pyrex tubes, was sealed with a layer of asbestos cement, about '/a in. thick. As shown in Figure 1, the balance and furnace were assembled on a small desk. The furnace was supported on a platform which was attached to the underside of the desk. The balance was secured to the top of the desk by putting the supporting legs into holes drilled into the desk top. I n this way, any movement of the desk would not affect the alignment of the apparatus. Care was taken to protect the balance from the heat of the furnace. The top of the desk was covered with transite hoard and the underside covered with asbestos paper. Glass wool was packed into the drawer of the desk (not shown in Figure 1) to provide additional insulation. To reduce air currents, the sides and back of the platform bupporting the furnace and the sides and hack of the desk were enclosed with asbestos paper. The temperature of the furnace was controlled by a Temco input controller, Model CP-515T. The overall rate of heating was nearly linear, averaging 4-5" per minute. Temperature measurements of the sample chamber were made with a Leeds and Northrup doublerange potentiometer indicator, Model 8657-C, using an iron-constantan thermocouple. Small glass vials, 12 mm X 35 mm, with support loops made from 26-gauge nichrome wire, were used as sample containers. The vials were suspended in the furnace just opposite the iron-constantan thermocouple using a chain made from 20-gauge nichrome wire. Static heating tests showed that the vials could be heated to 450°C without showing any loss in weight; higher temperatures were not investigated. A study of the effect of the furnace temperature on the balance using two different empty sample vials of known weight indicated that the balance was unaffected up to about 220°C. Above 220°, there was an apparent continuous loss in weight of the sample vial amounting to 1.7 mg at 450°C. A calibration curve of weight loss versus temperature (Fig. 2) was prepared from two such determinations and all data were corrected using this curve. As a check on the accuracy of the final weight loss
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Figure 2.
Thermobolonce cdibrotioncurve.
readings obtained on the thermobalance, all sample eontainers were cooled in a desiccator after completion of a run and reweighed on a chainomatic balance. I n every instance, the weights obtained were within 0.2 mg of the corrected weight loss as determined by the thermobalance. Operation of Therrnobalance
Before beginning a run, the furnace was flushed with dry nitrogen for a short while. A samplea of the compound to be studied was accurately weighed into a dry, weighed glass vial which was then suspended in the furnace. The balance was adjusted by first placing the 5-mg rider on the right-hand side of the beam over the 5.0 mg mark and sufficient weight was added to the right balance pan to bring the pointer to the desired rest poinL3 Nitrogen. was passed through the sample chamber and the rate of flow, which could he approximated as the gas passed through a sulfuric acid bubbler, was regulated so it would not disturb the equilibrium of the balance. The sensitivity of the balance4 was then determined so that weight loss could he easily determined by noting the deflection of the pointer from its equilibrium position. After a weight loss of 2 mg, the rider was moved 2 mg to the left to restore the pointer to its equilibrium position. By successive reading of the pointer deflection and adjusting the rider to the left, when necessary, weight loss was easily determined as a function of temperature. Since both weight and temperature readings were taken together a t definite time intervals, as measured by a stopwatch, the weight losses obtained were a function of time as well as temperature. A typical thermogram obtained using the thermobalance described is shown in Figure 3 for the zincnaphthazarin complex, ClOH4O4Zn.2H20.The thermogravimetric data indicate a loss in weight of 12.3% at 232°C. The theoretical loss in weight for two moles of water is 12.4%. The dehydrated complex is apparently stable between 232' and 318°C. It should be mentioned that the temperature range for the stability of a compound as determined from a thermogravimetrio curve is only approximate. Newkirks has recently pointed out that the establishment of the temperature range of stability of a compound
' The metal chelate samples were about 50 mg; muah larger samples, however, may be used. This arrangement permitted a weight loss of up to 10 mg to be conveniently measured. The sensitivity was constant s t 2.5 divisions per mg. 5 NEWKIRK, ARTHUR E., Anal. Chem., 32, 1558 (1960). Volume 38, Number 1 1 , November 1961
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is not simply a matter of reading the temperature a t beginning and at the end of the level portion of a single thermogravimetric curve. The temperature will be approximately correct; it will, however, have been shifted u ~ w a r din temuerature bv the effect of the rate of heating. Although samples were run in a nitrogen atmosphere, other gases, such as oxygen, carbon dioxide, argon, etc., may be used. The thermohalance cannot be operated under vacuum conditions. The cost of constrnction, excluding the analytical balance and potentiometer, was about $125; the most expensive item was the Temco input controller which cost about $85.
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