[Reprinted from Journal of Chemical Education, Vol. 54, Page 582, September 1977.]Copyright 1977, by Division of Chemical Education, American Chemical Society, and reprinted by permission of the copyright owner

Edward G. Malawer 1and Eric R. Allen

Atmospheric Sciences Research CenterState University of New York at Albany

Albany, New York 12222

A Differential Thermal AnalysisSystem for the Teaching Laboratory

Differential thermal analysis (DTA) is a technique wherebythe thermal effects, associated with physical or chemicalchanges, are recorded as a function of temperature or time asthe material under study is heated at a uniform rate.23 Inpractice, the differential temperature between the sample andan inert reference (a substance undergoing no phase transi-tions in the temperature region of interest with a specific heatsimilar to that of the sample) is monitored as a function ofsample temperature. A typical melting point thermogram isshown in Figure 1. Normally the differential temperature iszero which is indicated by a recorder as a flat baseline. When-the temperature of a phase transition has been achieved, thesample temperature will commence to lag behind the referencetemperature and a displacement of the recorder pen is-ob-served. Upon completion ofthe phase transition, the sampletemperature "catches up" to the reference temperature, therecorder pen returns to the bas~line, and a "peak" is thus re-corded. Note that the differential temperature plot is muchmore sensitive to the thermal properties of the phase transi-tion than a sample temperature scan. In the latter case, onlya minor inflection in the temperature-time curve is observedat the transition point.

The area under this peak is proportional to the enthalpy ofthe phase transition and the sample mass. The thermal pro-portionality constant for the apparatus is determined by \measuring peak areas corresponding to phase transitions ofmaterials whose enthalpy values are well known. Once theenthalpy is known, the latent heat of the transition can becalculated by dividing the enthalpy by the sample mass. Theshape of the peak is also of interest since it is very sensitive toimpurity and supercooling effects. The DTA technique maybe used in the study of phase diagrams, solid-state phasetransitions, solid-state reaction kinetics, flash points of ex-plosive materials, as well as to determine the level of impuri-ties and the extent of supercooling in melts.f

A differential thermal analysis system has been designedand constructed mainly from commercially available modularunits. While this system was originally intended for routineresearch, its minimal cost, simplicity, and ease of operationmake it ideal for the introduction of the DTA technique inundergraduate physical or analytical chemistry laboratorycourses. This system has incorporated many important fea-

'Present Address: Phelps Dodge Cable and Wire Co., PO Box 391,Younkers, N.Y. 10702.

2Wendlandt, W-esleyW."Thermal 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 QuantitativeDifferential Thermal Analysis," Amer. Lab., 6 (1),53 (1974). Cassel,Bruce, "Recent Developments in Quantitative Thermal Analysis,"Amer. Lab., 7 (1),9 (1975;)

582 / Journal of Chemical Education

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. tures of commercially available pre-packaged units at a frac-tion of the cost. It is readily assembled and disassembled sothat it will not be just a "black box" to the average student.

Assembly of the DTA SystemA schematic diagram of the DTA system showing the arrangement

of major components is presented in Figure 2. The heart of the systemwas a home-made, thermally insulated ~alorimeter bath whose con-

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Figure 1. Melting point thermogram for sodium thiosulfate pentahydrate(Na2S2035H20).

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Figure 2. Block diagram of the differential thermal analysis (OTA) system. Thethermocouple leads are depicted as single wires for simplicity.

struction is described in the following section. Unlike conventionalDTA systems in which heating of the sample and reference is ac-complished by an electrical resistance heating element mounted ina metallic block, this system made use of a fluid consisting of a 25%by volume mixture of ethylene glycol in water which was heated andcirculated by a Neslab Instruments, Inc. model TEZ3 circulating bath.The bath fluid was forced into and removed from the calorimeter bathby means of a combination force and suction pump system incorpo-rated into the circulating bath.

Linear heating and cooling rates were controlled by the combinationof a Neslab model TP-2 temperature programmer and a Neslab modelCT-150 thermoregulator whose operating range was 0 to 150C.Cooling of the bath was achieved by a cold water coil at temperaturesabove 50C and by a Neslab model PBC-4 Freon-based, immersionprobe bath cooler below that temperature. (While an additional fi-nancial savings could be realized by omitting the Freon-based bathcooler, cooling rates achieved using the cold water coil alone below50C become increasingly non-linear and bath temperatures approachroom temperature asymptotically.) Scanning rates from as little asseveral tenths of 1C/min to approximately 5C/min were easily ob-tained using a I-gal bath container.The heat sensors used in conjunction with a Bailey Instruments Co.

model BAT-8 digital amplifying thermometer were four Bailey modelIT-l Teflon-coated, copper-constantan thermocouple probes witha response time ofless than 1 sec. The amplifying thermometer, whichincorporated a factory modification (Bailey type D), provided highresolution differential temperature measurement capability (toO.OIC). This unit was used to amplify the weak differential tem-perature signal generated by thermocouple probes placed inside boththe sample and reference containers. It should be noted that in Figure2, the thermocouple leads have been depicted as a single wire for thesake of simplicity.

A voltage divider was used to reduce the output from the amplifyingthermometer (of the order several volts) to a millivolt scale to becompatible with the graphic recorder employed. The sample tem-perature was recorded directly (without any amplification) by meansof an additional thermocouple probe placed in the sample containerand connected to a fourth reference probe placed in an ice-bath. Be-cause the voltage output characteri tics of all thermocouples are no-ticeably non-linear over a wide temperature span, if it is desired tolinearly interpolate the absolute tem~erature over a wide range onthe chart paper, then it is necessary to use either a second amplifyingthermometer or similar compensating device. A Linear InstrumentsCorp. model 282 dual-pen, integrating strip chart recorder has beenused to display the sample temperature, differential temperature, andthe integral under the differential signal, simultaneously.

Construction of the Calorimeter BathThe inner section of the calorimeter bath is depicted photograph-

ically in Figure 3. The identical sample and reference containersshown were constructed by fusing the necks of 25-ml round-bottomflask bulbs to I-ft lengths of I2-mm Pyrex tubing coaxially. The useof such bulbs as sample containers was adopted to allow considerableroom for the lateral expansion of solid samples upon heating (makingcylindrical containers impractical due to frequent breakage). It wasfound that a convenient sample mass was approximately 2 g. Thematerial chosen as a reference was neat ethylene glycol. It was selectedbecause it exhibits no phase transitions in the temperature range ofinterest (0-150C) and, being a liquid, maintains good thermalcontact with the heat sensor immersed in it.The bath was constructed from a I-gal Pyrex battery jar (6 inch o.d.

by 12 in. height) encased in a 4lh-gal jar (12 in. o.d. by 12 in. height)with glass wool as insulation. Corrugated cardboard was used toconstruct a small window in order to view the sample and referencecontainers (especially for proper thermocouple placement). Note that

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Figure3. Innersection of the calorimeter bath.

only the inner jar and its contents are shown in Figure 3 for the sakeof clarity.

The primary requirement contributing to the design of the calo-rimeter bath was that the sample and reference containers must al-ways receive the same heat flux. This was achieved by allowing theincoming fluid to enter from the bottom of the jar and be removedfrom the top along the cylindrical axis of symmetry. The sample andreference containers were placed on opposite sides of and equidistantfrom the %-in. o.d. brass outlet tube. Correct alignment of the sampleand reference containers in relation to the inlet and outlet tubes wasachieved by a machined aluminum cap fitted with a lh-in. layer ofTeflon. The latter was used to insulate the cap from the bath containerand- its contents. In addition, Teflon sleeves around the inlet andoutlet tubes served to isolate the cap from the fluid flowing throughthem. The centers of the sample and reference containers were posi-tioned 2lh-in. apart. The outlet tube was rigidly fixed in the cap whilethe inlet tube was allowed to slide freely so that the cap could easilybe removed in order to change samples.The inlet tube was in the shape of a "J" and was placed close to the

bath wall again equidistant from the sample and reference containers.Correct alignment of the bottom portion of the "J" tube was assuredby a round brass spacer plate with large holes punched in it to allowfor good dispersion and circulation of fluid above and below the plate.Rapid turbulent mixing was achieved by a small circular brass baffleplate placed just above the orifice of the inlet tube in the center of thebath. The bath circulator and calorimeter bath were connected bymeans of thick-walled rubber tubing resistant to corrosion, heat, andvibration. It should be noted that provided the basic design of thecalorimeter bath is followed, the absolute dimensions are quite flexibleand are mainly a function of the size of the bath container selected.

AcknowledgmentThe authors wish to thank Dr. G. Garland Lala for his helpful

suggestions, Mr. William Winters for his care in constructing com-ponents for the calorimeter bath, and the New York State EnergyResearch and Development Authority for financial support.

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Volume 54, Number 9, September 1977 / 583