a movable core transformer which will be discussed in a later publication. With alteration of the programmer slide wire voltage and with a suitable block and thermocouple system, thc temperature range can be extended to 1200O c. LITERATURE CITED
(1) Barrall, E. M., 11,Rogers, L. B., ANAL. CHEM.34, 1101 (1962).
(2) Borchardt, H. J., Daniels, F., J . Am. C h a . SOC.79, 41 (1957). (3) Fujii, C. T., Carpenter, C. D., Meuesner. R. A.. Rev. Sci. lnstr. 33.362 (1962). ( 4 ) Hay, A. W., 144th National Meeting, ACS, Lo8 Angeles, Calif., April 1963. (5) Ke, Bacon, “Organic Analysis,” Vol. I V , J. Mitchell, Jr., Ed., Interscience, New York, 1960. (6) MacKenzie, R. C., “The Differential Thermal Investigation of Clays,” Mineralogical Society, London, 1957. (7) hfuller, Ralph H., ANAL. CHEM.35,
No. 4, 103A (1963). (8) Stone, R. L., Zbid., 32, 1582 (1960). (9) Vassallo, D. A., Harden, J. C., Zbid., 34, 132 (1962). (10)Vold, M. J., Zbid., 21, 683 (1949). (11) Watson, E. S., O’Neill, M. J.,
Justin, J., Brenner, N., Pittsburgh Conference for Analytical Chemiat,ry and Applied Spectroecopy, March 19b3.
RECEIVEDfor review June 25, 1963. Accepted September 16, 1963. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963.
Simu I tu neous DifferentiaI The rma I Ana lysis a nd Thermog ravimetric Analysis Using the Open-Pan Type of Sample Holder H. G. McADlE Ontario Research Foundation, 43 Queen’s Park, Toronto 5, Ontario, Canada
b The correlation of information obtained from differential thermal and thermogravimetric analyses is facilitated by performing both operations simultaneously. Tra ditionaI sample holder designs have complicated results by preventing adequate control of the sample environment, and thermocouple positioning has often led to erroneous sample temperatures and uncertain kinetic data. A new instrument is described capable of the simultaneous differential thermal and thermogravirnetric analysis of samples spread in a thin layer on an open pan, the temperature of which can be closely measured. A controlled atmosphere furnace is used to esfublish the sample environment and details of the sample holder design are described. A close correlation of enthalpy and weight changes has been found in a variety of inorganic and organic systems in different atmospheres. The abbreviation DATA is suggested for this technique: differential and thermogravimetric analysis.
T
HE TECHNIQUES of
differential thermal analysis (DTA) and thermogravimetric analysis (TGA) are proving to have rapidly increasing application, both individually and in combination with each other and with other modern analytical methods such as gas chromatography. Several methods for combining DTA and TGA have been described in the literature (6, 8, 9) but nearly all suffer from an inherent weakness in the sample holder geometry in that, traditionally, samples have been packed in cylinders or placed as com1840
ANALYTKAL CHEMISTRY
paratively thick layers in open crucibles. Under both these geometrical conditions, there is considerable impedance to the changing ambient atmosphere within the sample, hence the sample environment cannot be known with any degree of certainty. Lack of an appreciation of the sample environment has been one of the niajor factors complicating the correlation of enthalpy change with weight change. One obvious way to improve this correlation is to perform both techniques simultaneously, so that all the data are obtained from the same sample heated a t one rate under one condition of sample geometry, packing, and ambient atmosphere. Perhaps the most suitable sample geometry for establishing rapid exchange with the environment is a thin layer spread on a flat pan. This is also a very flexible arrangement in that no special packing techniques are required and sample fusion need not be a problem. In the present work this geometry has been adapted to permit study of thermal decompositions simultaneously by DTA and TGA. A further problem in existing a p paratus for thermogravimetry is that sample temperatures are usually estimated by a thermocouple located at some point beside, beneath, or above the sample holder. Rarely does this produce an accurate record of the sample temperature, particularly during transitions. For instance, in Figure 1 is shown the temperature of a flat-pan sample holder with sample, programmed a t 1’ C. per minute, showing the deviations in actual sample heating rate during two thermal transitions. The furnace, meanwhile, has continued to increase a t :t constant rstr, indicated by the broken
line. This deviation depends upon the masses of the sample and sample holder, the enthalpy change of the process, and the relative rates of heating and transition, among other factors. Since the actual heating rate changes during a thermal transition, it is desirable to report DTA and TGA results in terms of the wtual sample temperature rather than in terms of furnace temperature. Otherwise, depending upon the time interval associated with the transition and the heating rate of the furnace, the furnace temperature prevailing a t a particular instant during the transition may bear little relation to the actual sample temperature a t that instant. EXPERIMENTAL
The deviations from linearity constitute endotherms, and it remained only to devise a means for comparing sample holder temperature with an equal mass and shape of reference material under as nearly identical conditions as possible. The manual apparatus adopted for this purpose is shown in Figure 2. The sample holder is the lower of two No. 304 polished stainless steel pans with raised edges, weighing 10.300 f 0.002 grams and machined to identical dimensions, each pan having two square-ended wells 0.036 inch in diameter and 0,050inch deep. The pans are separated 11/2 inches on a 4-hole refractory thermocouple insulating tube, and are held in place by small spring clips seated in shallow grooves scored in the circumference of the tube. The holes through the pans are machined to give as close to a sliding fit as the refractory dimensions permit. The top end of the refractory tube is held by a pair of O-rings pressed into a well in a Lucite terminal block
carrying the heavy-gauge external leads. The sample temperature is measured by a No. 30 Fiberglas insulated ironconstantan thermocouple, M, placed in another channel with the bead a t exactly the level of the sample pan. The bottom of the channels is sealed with Alundum cement to prevent convection along the leads. Connection to the main terminal block is made by means of No. 40 iron and constantan leads. It was found that if a loop of about &inch diameter was kept in the flexible leads, the effect on the balance sensitivity was negligible. To eliminate any static effects on the Lucite terminal block the underside was covered with a thin grounded copper sheet. The thermocouple bead is separated from the sample pan by approximately 0.5 mm. of refractory wall and
the temperature difference between the measuring bead and the sample pan is less than 0 . 5 O C. The detection of thermal transitions occurring on the sample pan by this thermocouple was illustrated in Figure 1. The sample and reference pans are enclosed in a tight-fitting cylinder of 100-mesh stainless steel gauze, having a removable top. This serves as a satisfactory shield against convection within the furnace core which, otherwise, causes serious drifting of the differential temperature base line. In practice the weight of empty pans plus shield is noted under the gas flow conditions to be used. Dry nitrogen is used within the furnace unless otherwise indicated. The balance is raised, the convection shield removed, and 0.2 to 0.5 gram of -100-mesh sample
Figure 1. Deviation of heating rate during thermal transition
suspended from one arm of a standard analytical balance. In the original model of this apparatus the balance was mounted on a heavy-duty laboratory jack above the furnace, the balance being raised to remove the sample holder for loadding. A movable furnace, mounted above the balance, is a more satisfactory arrangement. The cylindrical furnace has a 2l/r inch i.d. ceramic core wound noninductively with No. 16 Chrome1 A wire to a cold resiatance of approximately 11 ohms. Coaxial with the ceramic core is an aluminum core of la/,-inches i.d. and ‘/,-inch wall thickness. Since the prenent work involves temperatures below 500’ C. it is possible to use such Bn aluminum core, chosen for its thermal conductivity and high reflectivity over a wide range of wavelengths. The furnace is closed at the bottom by a silicone rubber stopper containing an inverted No. 7740 Pyrex gss diffusing disk, through which the desired atmosphere is admitted to the furnace. At the top, the core is closed by a split shield containing a */rinch diameter hole. The furnace is mounted in a frame which can be leveled independently of the balance 60 thai, vertical alignment and centering of the furnace core with the thermocouple tube can be achieved. The furnace is controlled manually by twin Variacs in series, generally a t a heating rate of 1’ C. per minute. The differential thermocouples are 0.010-inch Pt, Pt/13 % Rh, slots being cut in the circumference of the thermocouple refractory tube to allow the junctions to be formed. These are welded until the bead size approximates the diameter of the well in the stainless steel pans, enabling a tight fit when pressed into the wells. The leads, D, are carried up separate channels in the refractory tube to i,he terminal block and connected to No. 40 copper leads a t nickel-silver terminals. These flexible leads, in turn, (ire connected to a second identical set of terminals mounted on the bahnce column and
f-A
View A A
‘LGAS IN Figure 2.
--*
Sketch of apparatus for simultaneous DTA and TGA VOL. 35,
NO.
12, NOVEMBER 1963
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1841
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Figure 3. Thermal decomposition of ammonium nitrate -50
spread evenly to a depth of 1 to 2 mm. on the sample pan, the reference pan remaining empty. The convection shield is then replaced and the balance returned to its operating position. A short time is usually necessary for the temperature of sample and reference pans to equalize after handling before beginning the heating program. In the present work the abscissa temperatures are t.hose of the thermocouple, M . Weighings were made at 0.5' to 5.0" C. intervals in this temperature. All materials studied were reagent grade used without further purification.
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TEMPERATURE
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RESULTS
The thermal behavior of ammonium nitrate in a dry nitrogen atmosphere is shown in Figure 3. The initial deviation of the DTA base line is a characteristic of the instrument, possibly associated with the establishment of a uniform rate of temperature rise within the furnace. The endotherm a t 55' C. is associated with the direct transition from the
I
I 50
200
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Figure 4. Thermal decomposition ane inclusion compound
rhombic to the tetragonal form (4) which occurs in samples previously heated above the tetragonal to cubic transition a t 125' C. Otherwise, two transitions would be expected: rhombic to monoclinic at 32.1' C., followed by monoclinic to tetragonal a t 84.2' C. The tetragonal to cubic transition was observed a t 130.0" C. and the fusion endotherm a t 172.5" C. In all cases the peak temperatures were 3' to 4" C. above the incipient temperatures, the latter being within 1' C. of reported values for these transitions. The comparatively slow return to the differential temperature base line is characteristic of systems in which the sample and reference are more or less thermally isolated. No detectable weight change was observed until 140' C. when there was a
I
40
of urea-n-dodec-
small weight loss, the rate of which decreased rapidly following melting of the sample. This is possibly associsted with liberation of interstitial water, and the material was not studied beyond the melting point. When dealing with samples which melt it is important to base the initial sample weight upon the volume of liquid sample which can be contained by the edges of the pan. Otherwise there is the possibility of mechanical loss following melting. Another system in which the melting phenomenon was observed is that of the inclusion compounds of urea with n-paraffins, a detailed study of which has been presented elsewhere (7). Figure 4 shows the thermal decomposition of the urea-n-dodecane complex in a nitrogen atmosphere. The lowtemperature endotherm corresponds to
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Figure 5. Thermal decomposition of copper sulfate pentahydrate
1842
ANALYTICAL CHEMISTRY
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Figure 6. dihydrate
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Thermal decomposition of cobalt(l1) oxalate
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Figure 8. bonate
TEMPERATURE :TI
Figure 7. Thermal decomposition of ammonium carbonate Uneqiial s a m p l e weights
the destruction of thl. hexagonal urea lattice and the liberati in of the included hydrocarbon from the canals in a state energetically equivalent to that of the liquid. Because n-dodecane a t these temperatures has an :tppreciable vapor pressure, it subsequently volatilizes relatively easily. A weight loss of 22.60y0 was predicted ( I S ) and 22.45y0 found. Subsequently, the fusion of the residual tetragonal urea occurs with a gradual decomposition of the melt, and the loss of ammonia and other products. The thermal decomposition of copper sulfate pentahydrate is illustrated in Figure 5 where the $eight loss curve clearly dirtinguishes the trihydrate, monohydrate, and anhydrous forms, each weight loss being closely associated with an endotherm. The measured weight loss steps were not quite stoichiometric, corresponding to 1.93, 1.95, and 1.12 moles of wat?r, respectively. The differential tfrermogram agrees well with others reported in the literature for this materid (2, IO). Since there is little restriction placed on the sample in this instrumLent,impedance to the removal of 1ibera.ted water by the dry nitrogen purge is due almost entirely to the layer of next lower hydrate h i l t up on the surface of the decomposing crystal. Hence, any formation of liquid water af, a first step in the production of the next lower hydrate, such as discussed by Borchardt and Daniels ( I ) , is not rc:solved. The observed reaction tempcratures are somewhat lower than reaorted previously because of a number of factors: the dehydration is carried out under essentially anhydrous conditions, heating rate is slow, and a fine particle size of freshly ground ma;erial was used.
Further, the actual sample temperature is more closely approximated because of the positioning of the measuring thermocouple. The decomposition of cobaltous oxalate dihydrate is another system which has been reported frequently in the literature ( 2 , s ) showing both formation and decomposition processes. The decomposition of this material in a closed sample holder under its own atmosphere produces the cobalt(I1) oxide, while in an open sample holder the mixed oxide, CosOa, is formed shortly after the loss of water of hydration. In Figure 6 examination of this system in a dry nitrogen atmosphere shows a separation of about 60" C. between the end of the dehydration process and the decomposition of the anhydrous oxalate. The latter reaction was a violent one which appeared to give rise to a doublet exotherm. It should be noted that the ordinate for this exotherm is 10 times that for the endotherm. The final reaction product was confirmed to be Co804 by x-ray diffraction. This doublet exotherm suggests a two-step process may be involved in the decomposition of the anhydrous oxalate, the first step being decomposition to an oxide of cobalt in a single valence state, involving the weight loss, followed by rearrangement to the mixed oxide in which the cobalt ions are both divalent and tetravalent (11). Alternatively, it is possible that the violent evolution of CO and CO, from the decomposing oxalate changes the ambient nitrogen atmosphere in this vicinity so markedly that the reaction rate is temporarily reduced, causing the first peak. As the reaction products are dissipated in the nitrogen atmosphere
I00
15C TE11'FE*17 9 E
200
'C)
Thermal decomposition of sodium bicarUnequal s a m p l e weights
within the furnace the reaction rate again increases giving rise to the second peak. With sufficiently rapid diffusion of the reaction products, only one esotherm should have been obtained with a peak height somewhat greater than observed. Effect of Ambient Atmosphere. The decomposition of ammonium carbonate is shown in Figure 7 in which, since this material i s unstable ( I f ? ) , evidence of decomposition was apparent from the lowest temperatures. In a nitrogen atmosphere there is a broad endothermal decomposition, mithout discontinuity, which is complete by 80" C. giving a weight loss curve similar t o that reported by Garn and Kessler (3) for decomposition in an open cylinder sample holder. I n the presence of a carbon dioxide atmosphere, both DTA and TGA curves are shifted to higher temperatures. The endotherm area is slightly different due to different sample size. The instrumental shoulder now appears on the low-temperature side of the endotherm and the main portion of this peak is sharpened slightly. The two stages of weight loss reported (3)when the sample decomposes under its own evolved atmosphere are not resolved here, since the sample is decomposing in an atmosphere of only one of its product gases and not the equilibrium mixture of ammonia, water vapor, and carbon dioxide. In view of the unpredictable composition of reagent grade ammonium carbonates ( l a ) , little more can be said beyond the fact that the sample is completely decomposed to gaseous products a t about 75" C. Ammonium carbonate illustrates both an advantage and a disadvantage of the open-pan type of sample holder, compared with the piston-cylinder type. The advantage is that the decomposition VOL. 35, NO. 12, NOVEMBER 1963
1843
may be carried out in any desired atmosphere, not merely that of the decomposition products; the disadvantage is that for best resolution it may be necessary to feed an equilibrium mixture of the product gases into the furnace. While this requires additional external equipment, in the long run it may provide the most flexible arrangement. The decomposition of sodium bicarbonate in atmospheres of nitrogen and carbon dioxide is seen in Figure 8. Again the sample weights are not identical, hence the difference in size of the endotherms. The effect of the carbon dioxide atmosphere in this case is not only t o raise the decomposition temperature, but also to reduce the temperature range over which the decomposition occurs. This results in an increased rate of decomposition, from 5.8 mg. per degree to 7.8 mg. per degree, and a considerable sharpening of the point of incipient decomposition. In both cases the weight loss curves were single step and the final weight loss was within 0.1% of theoretical. Figure 9 shows the thermal decomposition of calcium sulfate dihydrate in nitrogen and in steam atmospheres. It is well established that a two-stage transition takes place, first with the loss of 1.5 moles of water to produce the hemihydrate, then loss of the remaining 0.5 mole to produce anhydrous calcium sulfate. The resolution of the two stages is very suspectible to water vapor pressure, so that in a dry nitrogen atmosphere with a finely divided and well exposed sample this resolution is poor. However, when a steam atmosphere is introduced as the sample reaches 100’ C., the two processes are clearly distinguished. This enhanced resolution has been reported as the basis for determining dihydrate in hemihydrate mixtures by DTA (6). The present work indicates a similar analysis could easily be performed by TGA, with
1844
ANALYTICAL CHEMISTRY
Figure 9.
Thermal decomposition of calcium sulfate dihydrate Unequal sample weights
the added advantage of being able to calculate composition directly from weight loss rather than from peak areas. Nomenclature. In the variety of concurrent or consecutive thermal analysis techniques presently evolving, i t is advantageous to have clear and concise abbreviations for otherwise cumbersome descriptions. In the case of simultaneous DTA and TGA a term such as “differential thermogravimetric analysis” has been used, but is confusing in the use of the word “differential” and also reduces to the same initials, DTGA, as derivative thermogravimetric analysis. As a possible solution to this case the abbreviation DATA is proposed: differential and thermogravimetric analysis. LITERATURE CITED
(1) Borchardt, H. J., Daniels, F., J . Phys. Chem. 61,917 (1957). (2) Gam, P. D., ANAL. CHEM.33, 1247 (1961).
(3) Garn, P. D., Kessler, J. E., Zbid., 32, 1563 (1960). (4) Hendricks, 5. B., Posnjak, E., Kracke, F., J. Am. Chem. SOC.54,2766 (1932). (5) Kriiger, J. E., Bryden, J. G., J. Sci. h t r . 40, 178 (1963). (6) Kuntze, R. A., Mat. Res. Std. 2, 646 (1962). (7) McAdie, H. G., Can. J. Chem. 40, 2195 (1962). (8) Paulik, F., Paulik, J., Erdey, L., Acta. Chim. Hung. 26, 143 (1961). (9) Reisman, A., ANAL.CEEM.32, 1566 ( 1960). (10) Rsisman, A., Karl&, J., J . Am. Chcm. SOC.80,6500 (1958). (11) Remy, H., “Treatiee on Inorganic Chemistry,” p. 294, Elsevier, Amsterdam, 1956. (12) Sclar, C. B., Carrison, L. C., Science 140, 1205 (1963). (13) Smith, A. E., Acta Cryst. 5 , 224 (1952). RECEIWD for review June 5, 1963. Accepted July 11, 1963. Division of Analytical Chemistry 144th Meeting, ACS, h e Angeles, dalif., Aprd 1963.
Work supported by a grant to the Ontario Research Foundation from the Province of Ontario received through the Department of Economice and Development.
