Edward G. Malawer' and Eric R. Allen Atmospheric Sciences Research Center State University of New York at Albany Albany. New York 12222
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A Differential Thermal Analysis system for the Teaching ~abaatory
Differential thermal analysis (DTA) is a technique whereby the thermal effects, associated with physical or chemical changes, are recorded as a function of temperature or time as the material under study is heated a t a uniform ~ a t e . ~ InJ practice, the differential temperature between the sample and an inert reference (a substance undergoing no phase transitions in the temperature region of interest with a specific heat similar to that of the . , is monitored as a function of sample temperature. A typical melting point thermogram is shown in Figure 1. Normally the differential temperature is zero which is indicated by a recorder as a flat baseline. When the temperature of a phase transition has been achieved, the sample temperature will commence to lag behind the reference temperature and a displacement of the recorder pen is observed. Upon completion of the phase transition, the sample temperature "catches up" to the reference temperature, the recorder pen returns to the baseline, and a "peak" is thus recorded. Note that the differential temperature plot is much more sensitive to the thermal properties of the phase transition than a sample temperature scan. In the latter case, only a minor inflection in the temperature-time curve is observed a t the transition point. The area unde; this peak is proportional to the enthalpy of the phase transition and the sample mass. The thermal pn)poriionality constant for the apparatus is determined by measuring peak areas corresponding to phase transitions of materials whose enthalpy values are well known. Once the enthalpy is known, the latent heat of the transition can he calculated by dividing the enthalpy by the sample mass. The shape of the peak is also of interest since it is very sensitive to imouritv and sunercooline effects. The DTA technioue mav he'usedin the study of phase diagrams, solid-statk ~ h a s k transitions, solid-state reaction kinetics, flash points of explosive materials, as well as to determine the level of impurities and the extent of su~ercoolinein melts.3 A differential thermai a n a l y s i ~ ~ ~ s thas e mbeen designed and constmded mainlv from commerciallv available modular units. While this system was originally intended for routine research, its minimal cost, simplicity, and ease of operation make it ideal for the introduction of the DTA technique in undergraduate physical or analytical chemistry laboratory courses. This system has incorporated many important fea'Present Address: Phelps Dodge Cable and Wire Co., PO Bm 391, Younkers, N.Y. 10702. 2Wendlandt,Wesley W., 'Y.hermd Methods of Analysis," 2nd. Ed., J. Wiley and Sons. 1964. Chapter V. 3Daniels, T., "Thermal Analysis," Halsted Press (Wiley), 1973, Chapter 4. Vaughan, H. P. and Elder, J. P., "Advances in Quantitative DifferentialThermal Analysis," Amer. Lab., 6 (1),53 (1974).Cassel, Bruce, "Recent Developments in Quantitative Thermal Analysis," Amer. Lab., 7 (I),9 (1975,)
582 1 Journal of Chemlcal Education
tures of commercially available pre-packaged units a t a fraction of the cost. It is readily assembled and disassembled so that i t will not be just a "black box" to the average student. Assembly the DTA System A schematic diagram of the DTA system showing the arrangement major comPonentsis presentedin 2. The the system was a home-made,thermally insulated calorimeter bath whose con-
Figure
1.
Melting point thermogram
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sodium thiosuifate pemahydrate
(Na2S2O3.SHP).
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Figure 2. Black diagram of the differeotiaiUwrmal analysis (DTA) system. The them~oupleleads are depicted as single wires fw simplicity.
struetian is described in the following section. Unlike conventional DTA systems in which heating of the ssmple and reference is aceomnlished hv an electrical resistance heatine element mounted in a metallic hlnck. this svrtem made use of a fluid consistine,. of a 25% , h y volume mixturr ofethylenr glycol in water which uns heatedand tirrulnlrd hy a Ncslnh Instruments, Inc. modelTEL3 circulntiny bath. The bath fluid wss forced into and removed from the calorimeter bath hy means of a combination force and suction pumpsystem incorporated into the circulating hath. Linear heating and moling rates were controlled by the combination and a Neslah model of a Neslah model TP-2temoerature uroerammer " CT-150 thermoreeulator ranee was ' 0 t o 150°C. ~ ~ -whose ~ oueratine ~ ~ cooling of the ha$ was achieved h;a&d&er coil a t t e m p e & r e s above 50°C and by a Neslah model PBC-4 Freon-hased, immersion probe hath cooler below that temperature. (While an additional financial savings could be realized by omitting the Freon-based hath cooler, cooling rates achieved using the cold water coil alone below 50°C become increasingly non-linear and bath temperatures approach room temperature asymptotically.) Scanning rates from as little as several tenths of 1°C/min to approximately SDC/minwere easily abtained using a l-gal bath container. The heat sensors used in conjunction with a Bailey Instruments Co. model BAT-8 digital amplifying thermometer were four Bailey model I T - l Teflon-coated, copper-constantan thermocouple probes with a response time of less than 1sec. The amplifying thermometer, which incorporated a factow modification (Bailey type D), provided high resolution differential temperature measurement capability (to iO.Ol°C). This unit was used t o amplify the weak differential temperature signal generated by thermocouple probes placed inside both the sample and reference containers. It should he noted that in Figure 2, the thermocouple leads have been depicted asa single wire for the sake of simplicity. A voltage divider was used to reduce the output from the amplifying thermometer (of the order several volts1 t o a millivolt scale t o be w m p a t ~ h ~with c the ~ r a p h l rrecorder rmphryed. 'The sample trmperatuw u m rewrdrd dirwtly {witbwt an).arnplificnrton~by rnerlns o f w a d d ~ t i m nthrrrnoroupleprubn l placed in thr samplerontainrr and connected to a fourth reference probe placed in an iee-bath. Because the voltage output characteristics of all thermocouples are noticeably non-linear over a wide temperature span, if it is desired t o linearly interpolate the absolute temperature over a wide range on the chart paper, then it is necessary to use either asecond amplifying thermometer or similar compensating device. A Linear Instruments Corp. model 282 dual-pen, integrating strip chart recorder has been used to display the sample temperature, differential temperature, and the integral under the differential signal, simultaneously. ~
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Figure 3. Inner section of the calorimeter bath
only the inner jar and its contents are shown in Figure 3 for the sake of clarity. The primary requirement contributing t o the design of the calorimeter bath was that the sample and reference containers must always receive the same heat flux. This was achieved by allowing the incamine fluid to enter from the bottom of the ,~ iar and he removed frwm the top "low the ryl~ndrlralaxis ot'symrnetrv. Thp starnplr nnd rrferencr wntsmrrs wrre placed on uppouitesidrs of and equidmant frwn the'&-I".
