Wesley W . Wendlandt Texas Technological College Lubbock, Texas
An Inexpensive Diff erentid Thermal Analysis Apparatus
Although differential thermal analysis (DTA) has been known since 1887 (3,only recently has it assumed much importance to the analytical chemist. By use of this technique, the qualitative identification of a compound is possible by determining the temperatures a t which endothermic (endotherm) or exothermic (exotherm) reactions take place as the substance is heated. Previous studies of this type, to mention but a few, consisted of the identification of metal nitrates and perchlorates (B), lubricating greases ( I ) , polyglucosans (6), and starches and other organic compounds (4, 6). However, much fruitful work lies ahead in this interesting area of investigation. Many authors have described the instrumentation for DTA (8). The apparatus usually consists of a furnace and sample holder, a furnace temperature coutroller, an electronic or other type of recording device, and a low-level dc amplifier. To the small laboratory, the cost of such an apparatus consisting of the above components is usually prohibitive. In an effort to reduce this financial barrier, an inexpensive DTA apparatus that performs reasonably well in the temperature range from ambient to 800°C was constructed. The DTA apparatus consists of an easily built furnace and stainless steel sample holder; a motor-driven variable transformer to control the temperature of the furnace; and an inexpensive strip-chart microvolt recorder made from an illuminated D'Arsonval type galvanometer. The operation of the apparatus is completely automatic. Recorder. A schematic diagram of the recorder is given in Figure 1. The recorder consists of a Leeds and Northrup D'Arsonval type galvanometer, Cat. No. 2430-C, 0.0029 p amp/mm, in which the reflected beam of light falls on a Type 920 twin-cathode photocell. The unbalance from the photocell cathodes is fed into two Type 2051 thyratrou tubes which are resistance coupled to the Barber-Coleman, Type OYAZ433, reversible pen drive motor. The thyratron circuit used in the recorder has been described by Wilkie (10) and is a modification of the original circuit proposed by Pompeo and Penther (7). The chart drive assembly was built from '/s-in. thick aluminum sheet while the chart gear drive mechanism was cut from an appropriate size aluminum rod. The chart drive motor was a 2-rph synchronous motor made by the Cramer Controls Corp., Centerbrook, Conn. It was equipped with a two-way friction clutch which allowed manual movement of the paper to be made in either direction. Using a motor of this speed, the chart paper travel was approximately 6 in. per hour. Other chart speeds could be obtained by changing the inexpensive drive motor. Standard 5-in. wide Varian 94
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Journal of Chemical Education
Figure 1.
P. C.
Schematic diogmm of the recorder;
G.
Galvanometer;
Type 920 photocell; MI. Pen drive motor; M2. Chart drive motor; Chart paper roll; L. Leroy Type 000 lettering pen.
Associates chart paper was used in the recorder. The input to the galvanometer was connected to a ten-turn, 100-ohm Micropot which acted as a voltage divider from the differential thermocouples. Since the galvanometer was quite sensitive, an expensive dc amplifier was eliminated because for most purposes, the galvanometer response was adequate. Actually, in most cases, the Micropot was adjusted to take off 80% of the emf generated by the differential thermocouples. The entire recorder unit, except for the chart drive assembly, was enclosed in a light-tight wooden box. Furnace. The furnace is illustrated in Figure 2. The sample holder, A, as well as the furnace tube, were made from Type 303 stainless steel. Dimensions of the sample holder were 3.0 in. X 1.2 in, with the 10.0-in. long furnace tube of a slightly larger inner diameter to permit the holder to fit snugly. The furnace tube was insulated with several layers of asbestos paper and wound with 10 ft of 0.020-in. Nichrome wire (1.62 ohms per ft). A 4-in. aluminum tube formed the outer case of the furnace, using asbee tos wool as the internal insulation. The thermocouples were made from 28-gauge platinum and 28-gauge 90y0 platinum-10% rhodium alloy wire. Their location in the sample holder is shown in B of Figure 2. The thermocouple leads were held in
Figvre 2. Furnace assembly: A. Somple holder, ride view; 8. holder, end view; C. Furnace. ride view.
Sample
porcelain insulators and cemented into the two lower holes in the sample holder with Sauereisen No. 1 cement. The upper hole in the sample block contained an iron-Constantan thermocouple which was used to measure the furnace temperature. The differential thermocouple heads terminated in the sample chambers which were 0.25 in. in diameter X 0.625 in. deep. The samples were heated using a "sandwich" type packing which consisted of a layer of ignited alumina, a layer of sample (in intimate contact with the thermocouple bead), and another layer of alumina. Using this method, adequate recorder response was obtained from samples ranging in weight from 100-350 mg. Furnace Temperature Controller. The temperature controller consisted of a 0-135-v Powerstat rotated by a '/rrph synchronous motor containing a two-way friction clutch. Starting a t an initial Powerstat voltage of 40 v, the furnace heating rate curve in Figur* 3 was obtained. As can be seen, the heating rate was linear from about 50 to 750°C, a t about 10.5-C per minute. Results. To illustrate the operation of the apparatus, the thermograms of magnesium nitrate 6-hydrate and
barium nitrate are given in Figure 4. These two compounds were chosen because the magnesium salt is decomposed in the low temperature range (up to 500°C) while the barium salt decomposes in the high temperature range (above 500°C). Gordon and Campbell (2) have previously studied these two compounds but a rigorous comparison cannot be made because of the diierent type of furnace sample holder and recorder that was used. Magnesium nitrate 6-hydrate exhibited four endotherms, the temperature of the curve maxirnas being 90°, 110°, 190°, and 450°C, respectively. Fusion of the salt is revealed by the llO°C endotherm while immediately following this is the dehydration endotherm a t 190°C. The decomposition of anhydrous magnesium nitrate to magnesium oxide occurred a t the 450' endotherm. Origin of the 90°C endotherm is not known; i t could possibly represent a crystalline phase transition.
I
I
I
100 200 300 400 500 600 700 TEMP 'C.
800
Figure 4. Thermogroms of magnesium nitrate 6-hydrate and barium niSomple rirer 0.281 g and 0.094 g, respectively. Sensitivity of 0.80. trate.
The DTA thermogram agrees fairly well with the reactions found from a weight loss study using a thermobalance (9). Again, it is not possible to make a rigorous com~~rison~bec&se of the different conditions in the furnace chamber and also the differentheating rates employed. The high temperature thermal decomposition of barium nitrate gave two endotherms; one a t 670°C, the other a t 710°C. The endotherms are due to the decomposition of nitrate to the metal oxide. Agreement with previous studies (8) is excellent for the 710°C endotherm but the 670°C peak is somewhat high. The apparatus, as described, has been in operation in this laboratory for several months. Entire cost of the unit, excluding labor, was about $250. Acknowledgment
It is a pleasure to acknowledge the assistance of Warner Kendall and John L. Bear in construction and assembly of the apparatus. Literature Cited
Figvre 3.
Heating rote curve of furnace.
(1) Cox, D. B., (1957). (2) Gomo~,S., (1955).
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MCGLYNN, J. F., Anal. Chem., 29, 960
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CAMPBELL, C., And. Chem., 27, 1102
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(3) LE CIDLTELIER, H., Bull.
8 0 ~ .franc.
mindrd., 10, 204
(1887). (4) (5) (6) (7)
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MORITA, H., AND RICE, H. M., And. Chem., 27,336 (1955). MORITA,H., And. Chem., 28, 64 (1956). MORITA,H., Anal. Chem., 29, 1095 (1957). POMPEO, D. J., AND PENTHER, C . J., Rev. Sci. Instr., 13,
Journol of Chemicol Educofion
218 (1942). (8) SMOTHERS, W. J., AND CHIANO,Y., "Differential Thermal
h l y s i s : Theory and Practice," Chemical F'ublishing Co., New York, 1958. ( 9 ) WENDUNDT,W. W., Tez. J. Sci., X, 392 (1958). (10) WILHIE,J. B., Rev. Sci. Instr., 16, 97 (1945).
