Wesley W. Wendlandt
Texas Technological College lubbock,Texas
Vacuum and Controlled Atmosphere Differential Thermal Analysis Apparatus
Although a number of diierential thermal analysis (DTA) units have been described in THIS JOURNAL (1-3) and elsewhere ( 4 4 , there is a definite need for a simple apparatus which will allow the determination of the DTA thermogrrtm of a substance under conditions of vacuum, or controlled atmospheres other than air. The recent excellent review by Gordon (7) shows that this type of apparatus is available commercially hut that they are all fairly expensive ($7000 to $10,000). This article describes a simple apparatus which can he assembled easily and can he used under conditions of vacuum or controlled atmospheres from ambient temperature up to 1000°C. The furnace and sample holder is schematically illustrated in Figure 1.
the furnace zone by means of 3.0 mm diameter twoholed ceramic insulating tubing. The sample and reference cups were made of Inconel, 7 mm diameter and 10 mm long, obtained from the Robert L. Stone Co., Austin, Tex. The cups fit snuggly on the insulator tube and were in intimate contact with the thermocouple junctions. Construction of the furnace heating element and furnace temperature controller was the same as previously descrihed (3). Connection of the furnace and sample holder to the vacuum system is illustrated in Figure 2. I t was attached to the system through the "0" ring joint and clamp to provide a vacuum tight seal. Provision was also made for the sampling of the decomposition gases by use of the glass bulb, G, attached to the system by a standard taper glass joint. The gas content of the bulb could he analyzed by use of either a gas chromatograph or a mass spectrometer. To operate the unit under a static gas atmosphere other than air, the system was evacuated, then tilled with the desired gas through stopcock S. For vacuum studies, the system could be continuously evacuated during the heating cycle.
To Vac System
Figure 2.
Figure 1.
Schemoticillusfrotionof DTA furnace ond sample holder
The furnace and sample holder were essentially similar to that described by Lodding and Hammell (8). The furnace tube was 19 cm long and 2.5 cm diameter and was constructed partly of quartz. The lower end of the tube contained two Pyrex glass "0" ring joints, 25 mm id, which was attached to the quartz tube by a quartz-to-Pyrex graded seal. The lower "0" ring joint was enclosed by a nylon seal, machined to dimensions identical to that of the glass "0" ring joint and attached to it by a clamp. Provision was made for electrical connections through the seal by use of small bolts, sealed on the outside by Apiezon "Q" wax. The Chromel-Alumel thermocouple wires were brought into 428
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Journol of Chemical Educofion
Shematic ill".trofion of vocuum system.
The diientntial thermocouple signal was amplified by use of a Leeds and Northrup microvolt dc amplifier, Model No. 9835-B, and then recorded on the y-axis of a Houston Incdmments, Model HR-95, X-Y recorder. The temperature of the furnace, as monitored by the reference thermocouple, was recorded on the x-axis. The use of the apparatus, under conditions of vacuum and static a h atmospheres, is illustrated in Figures 3 and 4. Results from two types of compounds are illustrated. The first type is one in which the thermal dissociation reaction evolves a volatile substance and hence is pressure dependent. The second type is a compound which undergoes a phase transition and is not pressure dependent under the conditions descrihed here. Two
compounds of the first type are illustrated: they are CuSO4.5RzO and Cu(NHa)@04)2.6HzO. The DTA thermogram for CuSOn.5Hn0,in air, has previously been described and the thermal transition assigned (9,10). The curve for CuSOa.5H20obtained with this apparatus is similar to that previously described (9) except that the peak temperature maximas are all shifted to slightly higher temperatures. Maximum peak temperatures obtained were a t 130, 170, 280,755, and 850°C, respectively. The first three peaks are due to the evolution of hydrate-bound water while the last two are caused by the dissociation of the anhydrous CuS04to give a residue of CuO.
tion of CuS04 in vacuum gave CulO; while in air, CuO was formed. The thermograms for sodium nitrate, as illustrated in Figure 4, show that the peak maxima temperat,ilrc? are not changed by the use of different atmospheres. The first peak, a t about 280°C, is a crystaUiue phase transition which hrw previously been described by Kracek (11) and others (18). Accurate measurerncnts give the ::ansition temperature as 275.5'C. '1.1~ second peak, beginning a t 300°C, is that due to the fusion of the eom~ound. The re~ortcdrnclting point of sodium nitrate & 306.8'C (13).
Figure 4. DTA thermogram under rtotis air I1 mm Hg) otmospherer.
The variable furnace atmosphere inherently increases the applications to which the technique of DTA may be applied. Not only can oxygen or moisture sensitive substances be studied but by controlling the pressure, heats of sublimation (14) as well as other heats of dissociation data can be obtained. Acknowledgment. This work was sponsored by the Robert M. Welch Foundation of Houston, Texas. The glass fabrication ability of Fred Kennedy of Texas Instru-ments, Inc., is gratefully acknowledged.
Figure 3. DTA thermogromr under italic air 1680 mm Hg) a n d vacuum mm Hgl atmorphertr. Heating rate 10°C mi"-'.
(-1
The DTA curve for CuSOn.5Ha0, in vacuum, is similar to that obtained in air except that the peak temperatures are somewhat lower and there is a little better resolution of the peaks. Peak temperatures were 100, 145, 240, 685, and 780°C, respectively. There is a different mechanism operative for the dissociation of the anhydrous CuSOl because the residue was Cu20 instead of CuO, as was found in air. The Cu20 was identified by X-ray powder diffraction of the residue. Apparently, the CuO that is formed by the reactions: 2CuS0,
-
c u o .CuSO,
and c u o .c u s o ,
-
2cuo
Literature Cited (1) BORCRARDT, H. J., J. CHEM.EDUC.,33,103 (1956). W. W., J. CHEM.EDUC.,37, 94 (1960). (2) WENDLANDT, (3) WENDLANDT, W. W., J. CHEM.EDUC.,38, 571 (1961). W. J., AND CHIANG,Y., "Differential Thermal (4) SMOTHERS, Analysis," Chemical Publishing Co., New York, 1958. S. B., in "Analytical (5) KISSINGER,H. E., AND NEWMAN, Chemistry of Polymers," G. M. KLINE,ed., Interseience Publishers, New York, 1962, Vol. 12, part 2, chap. 4. (6) WENDLANDT, W. W., in "Techniques of Inorganic Chemistry," H. B. JONASSEN, ed., Interscience Publishers, to be published in 1963. (7) GORDON, S., J. CHEM.EDUC.,40, A87 (1963). W., AND HAMMELL, L., Rev. Seient. In'nstru., 30, (8) LODDINQ, 885 (1959). H. J., AND DANIELS,F., J. Phys. C h . , 61, (9) BORCRARDT, 917 11957). A., AND XARLAK,J., J. Am. Chem. Soc., 80, (10) REISMAN, 6500 (1958). F . J. Phw. Chem., 34,225 (1930). (11) K R A C E ~C., A. C., Rev. Pure Appl. Chem. 12, 54 (1962). (12) MCLAREN, C. D., "Handbook of Chemistry and Physics," (13) HODGMAN, 44th ed., The Chemical Rubber Publishing Co., Cleve land, 1962, p. 656. M. M., AND BORYTA, D . A., J. Phys. Chem., (14) MARKOWITE, 66,1477 (1962).
+ SO.
+ SO1
is reduced by the SO?formed by the equilibrium:
so, * SO* + '/*02
because CuO does not evolve oxyxen under the above .experimental conditions. The two curves for CU(NH&(SO~)~.GHZO illustrate essentiallv the same tvne of behavior between air and vacuum at.mospheres. I n vacuum, the peaks were shifted to lower temperatures with a slight increase in resolution of the peaks. This was quite pronounced for the evolution of (NH4)i304from the compound in vacuum. 'Two peaks were observed, a t 355 and 390°C, respectively, while in air, there was only a peak and shoulder peak a t about 450°C. Again, the decomposi" A
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1680 mm Hgl and vacuum
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