Allen J. %ism
Southern Illinois University Carbondale, Illinois 62901
1
I
The Assembly of a Differential Thermal Analysis Apparatus
Students of analytical chemistry are faced with the task of becoming familiar with many different types of instrumental techniques. The theory on which such methods are based can be presented to the student, but in order for him to gain practical experience, expensive instrumentation is usually necessary. Differential t,hermal analysis (DTA) is a case in point. Expensive commercial instruments are available, hut because the principle on which the technique of DTA is based is relatively simple, i t is possible to construct DTA iustrumentation from equipment available to most laboratories. There are several interesting reports in the literature which describe the construction of DTA instrumentation (1-5). This report also deals with the construction of an apparatus which can be used to illustrate the technique of differential thermal analysis. The basic parts of a DTA apparatus are the sample holder, heat source, and detection system. Wendlandt (5) and Smothers and Chiang (6) list other necessary parts of the system such as signal amplifiers and furnace temperature controlling devices. No temperature programming or amplification devices were used in this work, yet thermograms were obtained for various materials which were comparable to those obtained by the use of more sophisticated instrumentation. The need for amplification was eliminated by the use of a relatively large sample (4, 5). Instrumentation
Sample Holder. The sample and reference materials were contained in 10 X 75-mm test tubes which were placed in the various heat sources used in t,his work. Alumina was used as reference material for all DTA trials unless otherwise indicated.
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Heat Sources. For one series of experiments a metal cylinder was used to hold the sample and reference test tubes. The cylinder used was of stainless steel with a diameter of 2.5 in., a height of 4.0 in. and was drilled to accommodate the test tubes. The cylinder was placed in a furnace and in this way both sample and reference materials were subjected to the same heat environment. The furnace used was an Enamel Amaco Kiln, Type 26 E, manufactured by the American Art Clay Company of Indianapolis, Ind. The furnace has "Hi" and "Lo" temperature ranges with a maximum attainable temperature of about 1090°C. The "Lo" setting which was used for all the trials reported in this work gave a heating rate of about 3'C/miu, although this varied somewhat from trial to trial. I n one series of trials, the heat environment was provided by immersing the sample and reference tubes in a beaker of silicone oil (General Electric's SF-96 (1000)) which was heated and stirred by a Thermolyne Corporation Stir-Plate, Model SP-A1 025B. This system eliminates the need for the metal block and furnace and also allows the operator to note any visual changes in the sample. Use of the silicone oil, however, places a restriction on the temperature to which the sample may be raised. The temperature range reported for this particular silicone oil is from -60 to 500°F. Detection System. DTA instrumentation of t,his type usually employs thermocouples whose signal is fed to an X-Y recorder or some ot,her type of detecting device. The thermocouples used in these experiments were chromel-alumel, 28-gauge glass-on-asbestos insulation, manufactured by Leeds and Northrup. Two-hole clay protecting tubes were used as a guide for the thermo-
couple wires and also helped to cent,er the thermocouple junction in the test tubes. The wires were inserted into the clay tubes so that about 1-in. lengt,hs extended beyond the end of the clay tube. Insulation was stripped from the wires in this area so that a junction could he formed by twisting the bared wires together. I n one series of experiments a Moseley Autograf X-Y recorder, Model 2D-2AM, was used t,o detect the signals from the thermocouples. This recorder has a range of 0.2 mV/cm to 20 V/cm in both the X and Y functions. The sensitivity employed in these trials was 0.5 mV/cm on the X axis and 0.2 mV/cm on the Y axis. A diagram of the wiring used in the assembly of this DTA apparatus is shown in Figure 1. ICE BATH
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REFERENOE
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Figure 1.
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SAMPLE
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FUNOTION
X'Y
adjusted to some convenient starting position on the chart. The recorder pen should also he adjusted with regard to the different i d tempemlure zero point. Follow the same procedure when using the two recorders for detection. Once the recorder(s) have been zeroed, replace the wire to the Y- terminal sand start the thermogrrtm by turning on the furnace and recarder(s). When the silicone oil is used, the temperature must be increaqed by periodically adjusting the heat control of the Stir-Plate. The heating rate under these conditions was found to he somewhat variable, but acceptable thermograms were obtained. The stirring of the silicone ail was also variable due to decreased viscosity of the oil with increased temperature.
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40 80 120 160 TEMPERATURE, DEGREES CENTIGRADE
Wiring diogrom of DTA appmratw. Figure 2.
An X-Y recorder is the ideal detection system for DTA experiments since i t can be used to simultaneously display the temperature of the reference material as a function of the temperature difference between sample and reference material. However, X-Y recorders are somewhat more expensive than single-function recorders and may not be available to all laboratories interested in performing DTA experiments. For this reason, a series of trials was conducted to determine the feasibility of using two single-function recorders for detection rather than the X-Y recorder. Two Sargent SR recorders were used; one to detect the signal normally displayed on the X axis (temperature increase), the other to detect the signal normally displayed on the Y axis (differential temperature). Two charts were obtained for each trial, and it was necessary to refer to the chart of temperature increase in order to determine the temperature at which a particular transition took place in the sample. This detection method was used with the furnace-block heating system and with the silicone oil heating system. The wiring was the same for this system as for the system utilizing the X-Y recorder. Operating Procedure Prepare an ice water bath in a Dewar Bask to serve as the temperature reference (0°C) and place i t in the circuit as indioated in Figure 1. Fill the test tubes approximately half full of sample and reference materials, respectively. Place the appropriate thermocouples in the sample and reference materials and place the test tubes in the hertt source to be used. Set the X-Y recorder to O0C by disconnecting the wire from the Y- terminal and connecting i t to the X + terminal. This maintains the X input a t 0°C and the recorder pen may then be
Benzoie acid thsrrnogrornr
Thermogram
Results
Figure 2 shows the thermograms recorded for samples of benzoic acid. Curve A was obtained using the furnace-block heating system and the X-Y recorder. Curve B was obtained using the silicone oil-hot plate heating system and the two single-function recorders for detection. It will be noted that curve B has a more uneven baseline than curve A, probably due to inadequate stirring of the silicone oil bath. The endothermic peak of curve B "leans" toward higher temperatures, indicating a heating rate which was somewhat too rapid. In spite of this, the initial temperatures of melting indicated by both curves are close to t,he reported value of 122°C. The melting point from curve A was 121°C and that from curve B was 122'C. Figure 3 shows the thermograms recorded for two samples of ammonium nitrate (C and D) and for two samples of copper sulfate pentahydrate ( E and F). Curves C and E were obtained using the furnace-block heating system and the X-Y recorder. Curve D was obtained with the furnace-block heating system but with the two-recorder system of detection. Curve F was recorded using the silicone oil-hot plate system for heating and the X-Y recorder for detection. Curve F again shows the somewhat uneven baseline associated with the use of the silicone oil. This does not interfere to any great extent with the location of the initial temperatures at which thermal changes take place in the sample, and the effect could probably be diminished by giving more attention to the stirring of the oil. Volume
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Copper su1fat.e pentahydrate is reported to lose its waters of hydration in at least three steps. The first two moles of water are released at about 10O0C,then two more are lost at about 120°C and the fifth mole of water is lost at about 250°C. (Various sources are not in agreement on t.he exact temperatures of these dehydrations.) Endotherms were obtained in these three regions for the sample identified as curve E although the endotherm which occurred a t 250°C is not shown in Figure 3. The trial from which curve F was recorded was terminated at about 220°C because the silicone oil began to smoke. Boiling points may also be obtained with this apparatus as was determined using a sample of water with n-butyl phthalate as reference material. The apparat,us described in this work has been used to show thermal changes in samples due to phase changes and loss of waters of hydration. It is felt that most laboratories probably have sufficient equipment, to assemble an apparatus of the type described here and that, it coulcl be used to effectively demonstrate the technique of differential thermal analysis.
C
Literature Cited
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Figure 3. hydrote.
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Thermogmmr of ommonium nitrote and copper rvlfoC pento-
Journal of Chemical
Education
(1) (2) (3) (41 I51
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