An Experiment for the Physical Chemistry Laboratory HANS J. BORCHARDT University of Wisconsin, Madison, Wisconsin
D ~ F ~ E N T I A thermal L analysis (DTA) is a method for observing the transitions and react,ions that a substanoc nndergoes on heating. The substance under investigation, together with a sample of inert material, is placed in a furnace. The furnace temperature is raised a t a constant rate. The diflerence in temperature between the active and inert materials ( A T ) is observed as a function of furnace temperature. The measurement is usually accomplished by means of a differential thermocouple. If no reaction is taking place, both samples will be a t the same temperature. If the active material undergoes an endothermic reaction, such as loss of water of hydration, its temperatnre will lag behind that of the inert material. When the chemical reaction is completed, no more heat is absorbed and a steady state is again attained where AT is zero. Hence, a curve going from zero, rising to a maximum and dropping again to zero, is expected. Typical differential thermographs are shown in Figure 4. DTA apparently first was used in 1887 by Le Chatelier ( 1 ) in an investigation of the constitution of clays. It has since been widely used in mineralogy and soil studies. Peaks due to processes such as dehydration, the decomposition of carbonates, and the oxidation of organic matter permit the identification and differentiation of the complex mixtures which constitute these samples. In receut years increasing use has been made of this tool in a variety of chemical problems. The thermal
of organic compounds has been studied by Pirisi and Mattu ( 2 ) . The possibility of characterizing complex organir substances by their differential thermo~rawhs has been considered by 6Icrita and Rice (5). A considerable amount of data is accumulating on the d e composition of inorganic compounds, p a r t i c u l a r l y nitrates, carbonates, and perchlorates (4, 5, 6, 7 ) . Intermediate phases in the magnesia-silica-water sys-
ten1 were detected by Kalousek and Mui (8). Guth and co-workers (9)studied the stepwise oxidation and reduction of praseodymium and terbium. Krishna Prasad and Patel (10) investigated the composition of synthetic manganese dioxides. Ob~ervationsof chemical reactions betm-een solids havebeen made by Smothers and Chiang (11) and by Audrieth, Mills, and Netherton (12). Martin and Jaffray (19) studied transitions in anhydrous sodium carbonate. In many rases DTA is used as a scanning tool t,o ohtain rapidly an over-all view of the proccsscs which occur on heating. Once the region of interest has been established, a detailed investigation is made using chemical analysis, X-ray diffraction, et,r. Differential heating curves are by far t,he more romman in the literature, although some use is being made of differential cooling rurves for the determination of phase diagrams (14). The advantage here over ordinary cooling curves is that peaks rather than breaks in the cooling curve are observed. On the other hand, difficulties may he encountered with supercooling (15). A booklet discussing some of t,he aspects of DTA and giving an exhaustive bibliography of the literature prior to 1951 was written by Smot,hers, Chiang, and Wilson (16). An important development in DTA occurred in 1914 when Spiel (17) derived equations showing that the area under a peak in the DTA thermograph is proportional to the heat of reaction if certain ronditions are met. These conditions are (1) t,hat the sample hold-
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JOURNAL OF CHEMICAL EDUCATION
ers containing the inert and aclive materials, respectively, have the same shape; ( 2 ) that the temperature of their surroundings be the same; (3) that the thermal conductivity and heat capacity of the material in the sample holders be alike; and (4) that there be no thermal gradients in the samples. Conditions (1) and (2) can
he met by proper design of apparatus. Condition (3) is satisfied if the active substance is mixed with snfficient inert material so that its thermal properties are essentially those of the inert material. A thermal gradient in thesample cannot he avoided but apparently it has little influence on the results if it remains constant. This is evident from the work of Wittels (IS),who experimentally verified Spiel's equation. Barshad (19) critically discusses the method of calibration. Morehead and Daniels (20) and Knrath (21) have used this technique to measure the energy stored in certain minerals by alpha-particle bombardment. The growing interest in DTA, together with the fact that the necessary apparatus can be easily and inexpensively constrncted, suggests that a laboratory experiment might be welcome. The following apparatus and experiments have been prepared for use in the physical chemistry laboratory at the University of Wisronsin. APPARATUS Apparatus uhich is suitable for work to 360°C. is shown in Figures 1 and 2. It consists of two aluminum blocks (brass would also be suitable), the lower one A containing the differential thermocouple and the upper one B serving as sample holder and furnace. In this manner the two junctions of the thermocouple are imbedded in the active and inert materials, respectively. The thermocouple is made of 28-gage platinum and platinum-10 per cent rhodium wire. I t is important that the thermocouple junctions be as small as possible since the sensitivity is very much dependent on their heat capacity and thermal conductivity. They are insulated from the block by thermocouple beads (not shown). The base of block A is a transite disc. Block B has three holes in the center. The outer ones, 8 and S', are the sample holders while the center one T accommodates a 360" thermometer. By means of pin P the sample holders are lined up above the thermocouple junctions, TC and TC'. The vertical rod of A is 0.015 inches longer than the hole in B which accommodates it. In this way, a snug fit a t the base of the sample holders is assured. The outer casing of B is cut from transite pipe. The heating coil consists of 7'/%feet of 24-gage nichrome wire to give a total resistance of 12 ohms. The leads from the coil run to the transite wall where they are connected to the Variac leads by means of jack and plug contacts. The coil is insulated from the block by asbestos stripping. The thermocouple junctions are connected to a Leeds and Northrnp model 2430 galvanometer G, or other suie able galvanometer, as indicated in Figure 3 where r and v are 50-ohm variable resistors, v serving as a voltage divider to control the sensitivity and r serving to maintain the proper damping resistance. The use of a continuously-recording potentiometer in place of the galvanometer gives considerably better results. A circuit used a t this university for research purposes consists of a Liston-Becker model 14 d.-e. breaker-amplifier together with a Brown recording potentiometer, having a scale of from 0 to 20 millivolts.
VOLUME 33, NO. 3, MARCH, 1956
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An approximately constant temperature rise is obtained by periodically advancing the Variac. The precise schedule for advancing the Variac is supplied to the student prior to the experiment. EXPERIMENTAL PROCEDURES
It has been mentioned previously that dilution of the active substance with inert material is necessary if the areas under the peaks are to be used for calculations. There are two other reasons why this procedure should always he followed. One is that an undiluted substance may stick to the thermocouple junction as a result of fusion. Damage to the thermocouple junction may result during an attempt to remove it. The other reason involves baseline drift. I n the absence of a reaction one would expect the differential thermograph to consist of a straight horizontal line, since the galvanometer reading should remain constant with temperature. This line is referred to as the baseline. If the heat capacities and thermal conductivities of the two samples are not the same the baseline will be displaced. This displacement appears to be directly proportional to the rate of temperature rise. It is a conKHPhtholme stant displacement if the rate of temperature rise is constant; its magnitude depends upon this rate. I t will vary if the rate of temperature rise is varying. Tempermure Thus, baseline drift will be particularly noticeable when Figurr 4. Differontill Th.rm.mraph. the Variac is first turned on or advanced. A baseline disolacement is freauencv observed after a reaction has occk-ed. This ha; beenattributed to the heating rate is used. Thus, too high a heating rate change in heat capacity and thermal conductivity of may have the effect of causing closely adjacent peaks the material as a result of the reaction. The baseline to appear as a single one (23). Too low a heating rate drift is sometimes observed even if the above precau- on the other hand, may cause a peak to become so tions are taken. This is believed to be due to irregu- flattened out that it cannot be distinguished from the larities in the packing of the samples. baseline scatter. This occurs in the case of the 260" The inert material commonly used is calcined alu- copper sulfate peak (shown in Figure 4) when a heatmina or silica. These may not be inert at elevated tem- ing rate of five degrees/minute is used. An excellent peratures if the active material is highly basic or acidic, discussion of sDme of the experimental factors involved but no trouble should be encountered in the tempera- is given by Grim (25). ture range of the apparatus described here. Both the I n all cases, a mixture containing 50 per cent of acinert and active substances should be ground to approxi- tive material is used. About 0.12 g. of this mixture and mately 100-200 mesh. Some substances are available the same amount of silica powder constitute the samonly in the form of extremely fine powders. I n these ples. They are firmly packed down by means of a small cases, correspondingly fine inert material is recom- metal rod. The galvanometer sensitivity should be mended. adjusted to accommodate all readings. An initial If the apparatus is to be used for calorimetric deter- setting of 3 on a scale running from 0 to 10 is advisable minations, it must be calibrated with substances of where increasing readings indicate an endothermic reknown heats of reaction. Calibration must be per- action. The prsper sensitivity setting is determined by formed over the complete range of temperature since trial and error since i t depends very much on the size the sensitivity varies with temperature. This is proh- of the thermocouple junctions. This information ably due to variation of the thermal e. m. f . of the should be supplied to the student. Galvanometer thermocouple and the variation of the thermal gradient readings are recorded for every five-degree rise on the in the sample with temperature. Barshad (19) recom- thermometer until the reaction occurs, then a t one- or mends using the heats of fusion of the following com- two-degree intervals. pounds for calibration: m-dinitrobenzene (m. p. 90°C., BaClz.2H20, CdClr2'/e HzO, and CuS01.5H20 have A H 24.7 cal./g.); o-dinitrobenzene (m. p. 117'C., been chosen as substances to illustrate stepwise deAH,.. 32.3 cal./g.); benzoic acid (m. p. 12Z°C., AH,,. hydration, although many other hydrates would he 33.9 cal./g.); AgNOs (m. p. 212"C., AH,.. 16.7 cal./g.); equally suitable. The decomposition of KHCOI to and NaNOa (m. p. 307'C., AH,.. 45.3 cal./g.). &COI is an example of an inorganic decomposition, Certain peaks may not appear unless the proper and that of potassium acid phthalate an organic de-
e
JOURNAL OF CHEMICAL EDUCATION Summary of Peak Data Comvound
Number o i veaks
T
If a known quantity of CuSOa5HzO is used and the peak areas mezsured, an approximate calibration can be obtained. The total heat absorbed by the sample is calculated from the known heat of reaction. This together with the measured peak area gives the area per calorie absorbed. The first two copper sulfate peaks, corresponding jointly to the reaction,
should be used. This is because complete resolution between the peaks is not obtained and because a larger area can be measured more accurately. With this calibration the heats of reaction of the other compounds composition. An experiment using inert material in studied can be obtained. Once again a known quanboth sample holders is carried out to determine how tity of sample is used and the peak area measured. much of the pattern is due to spurious fluctuations. A From the peak area and the calibration, the total heat heating rate of five degrees/miuute is used except in absorbed is determined. This together with the weight the cases of C U S O C ~ H ~and O BaC12.2H,0 where the of sample used gives the heat of reaction per mole. rate is ten degrees/minute. All the above reactions are This is compared to the known heats of reaction (26). Agreement within 15 per cent is expected. endothermic. Potassium acid phthalate gives additional peaks a t RESULTS OF ,TYPICAL EXPERIMENTS elevated temperatures. Exothermic peaks were obThe resulting curves are shown in Figure 4. By cou- served a t 337' and 475" in this laboratory with other ventiou, an endothermic peak is drawn below the base- apparatus. Since the 337' peak extends into the reline and an exothermic peak above. The table sum- gion beyond 360°, the limit of the ordinary mercury marizes the data concerning the number of peaks and thermometer, it is recommended that the student tennithe temperatures at which they first become noticeable, T . nate this experiment a t 330". The CdC1,.21/a H1O sample should be freshly preThis is referred to as the "reaction temperature." T is the temperature a t which the reaction begins to pared for each experiment. It was found that samples proceed at an appreciable rate. It is not the equilibrium which were allowed to stand open for two days lost decomposition temperature which, of course, is a their first peak. This is presumably due to efflorescence thermodynamic property. It is the temperature (at a to the monohydrate. The experiment with BaCl3.2H20shows how DTA given pressure) a t which AF is zero. Above this temperature AF is negative and the reaction proceeds may be used to detect and aid in the establishment of spontaneously, but there is no information concerning new phases. Until recently the existence of the monothe rate of decomposition. I n most cases one must go hydrate had been open t o question. Thus, Sidgmick t o a temperature considerably above the equilibrinm- (27) was prompted to say that the dihydrate was stable decomposition temperature before the decomposition to at least 60°C., and that "the nature of the solid phase actually occurs a t a noticeable rate. What one obtains above this temperature is uncertain." The differential by DTA is the temperature at which the reaction takes thermograph clearly shows the two-stage decomposition. It also gives the temperature region in which the place rapidly. Reaction temperatures, T, are mmewhat dependent intermediate phase might be found if BaClz.2Hr0 is upon the apparatus and procedures followed. A com- heated under conditions similar to those in differential parison of DTA curves on standard samples was made thermal analysis. Thus, the compound may be isoin 31 laboratories under the auspices of the ComitB lated, its characteristic X-ray pattern determined, and International pour L'Btude des Argiles (24). Consider- its composition established by chemical analysis. able variation in the reaction temperatures was reported. Under certain circumstances, the first CuSOa5H,0 ACKNOWLEDGMENT peak may appear as a closely spaced doublet. This This work was carried out a t the suggestion of Prowas first observed by Taylor and IUug (26) and cited fessor Farrington Daniels whose advice and encourageby them as evidence for the existence of a tetrahydrate. ment is gratefully acknowledged. The beats of reaction for the formation of the tri- and monohydrates of copper sulfate are as follows (26): LITERRTURE CITED
Thus, one would expect the areas of the 95' and 130" peaks to he approximately equal, as is observed.
(1) LE CHATELIER,H., Bull. m e . franc. Mineral., 10, 204 (1887). (2) P I ~ I S I , R., AND F . MATTU,Rend. serninm fac. sei. univ. Cugliari, 22, 81 (1952); ibid., 163 (1052); Ann. ehim. (Rome),43, 574 (1953); Chimiea (Milan), 8, 188 (1953); ibid., 28.3 (1953). H., .4XD H. M. RICE, Anal. chew?., 27, 336 (1955). (3) MORITA,
VOLUME 33, NO. 3, MARCH, 1956 (4) SHARGORODSKII, S. D., A N D 0.I. SHOE, Ukrain. Khim. Zhur., 16, 426 (1950). (5) RoDE,T.V., Doklady Akad. AraukS. S.S.R.,91,313(1953). (6) KEENAN.A. G.. J. Am. Chem. Soe.. 77. 1379 (1955).
kansas Inst.Sn'. Z'echnol.,Resea~ehSer.,No. 21,l-14(1951). (17) SPIEL, S., Rept. Inv. NO. 3764, Bureau of Mines, U. S. Dept. of Interior, 1944. M., A n . Mineralogist, 36, 615 (1951). (18) WIT~ELS, I., Am. Minemlogist, 37, 667 (1952). (19) BARSHAD, F. F., AND F. DANIELS, J . Phw. Chem., 56,546 (20) MOREHEAD, \----,. (19.52) (8) KALOUSEK, G. L., A N D D. MUI,J. Am. Cemm. Soe., 37, 38 \----,. (21) KURIITH, S. F.,Thesis, University of Wisconsin, 1953. (1954). G. A,, A N D G. A. SWAN,Tran8. Can. 11~91.hcining (9) GUTH,E. D., ET AL.,J. Am. Chem. Soc., 76, 5239 (1954); (22) COLLINS, Met., 57,331 (1954). ibid., 5249 (1954). PRASAD, N. S., AND C. C. PATEL,J. Indian Inst. (23) GRIM,R. E., Ann. N. Y. Aead. Sei., 53, 1031 (1951). (10) KRI~ANA R. C., AND K. R. FARQUHARSON, Cmy?. (24) MACKENZIE, Sei., 36A, 23 (1954). (11). SMOTHERS. W. J.. AND Y. CHIANG. Pmc. Intern. S~,mvosium geol. intern. Compt. rend. 19th SQSS.Algiem, 1952, No. 18, ' ~eactiuiiyof ~ b l i k~othenbu7g'l952, , Pt.. 5014: 183 (1953). T. I., AND H. P. KLUG,J. Phw. Chem., 4 , 601 (12) AUDRIETH, L. F., J. R. MILLS,A N D L. E. NETHERTON, J . (25) TAYLOR, Phys. Chem., 58, 482 (1954). (1936). , AND J . JAFFRAY, C m p t . rend., 236,1755 (1953). (26) Rossm~,F. D., ET AL.,"Selected Values of Thermodynamic (13) M ~ T I NP., Properties," National Bureau of Standards Circular 500 A., AND F. HOLTZBERG, J. Am. Chem. Soc., 77, (14) REISMAN, (1952). 2115 (1955). N. V., "Chemical Elements and their Com(27) SIDGWICK, (15) KARAN,C., J. Am. Chem. Phys., 22,957 (1954). pounds," Clarendon Press, Oxford. 1950, p. 252. W. J., Y. CHIANG,AND A. WILSON,Unit.. Ar(16) SMOTHERS,
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