Differential Thermal Analysis Using Self-Generated Atmospheres at Sub- and Supra-Atmospheric Pressures PAUL D. GARN Department o f Chemistry, The University o f Akron, Akron 4, Ohio
b When a material decomposes to yield a gaseous product, the product gas can b e detained within the sample holder by limiting diffusion interchange with the atmosphere in the furnace chamber. These self-generated atmospheres at various externally applied pressures will have the same effect as deliberately supplied atmospheres of the product gas. The technique provides a convenient means of study of pressure-temperature relations, particularly of hydrated salts.
A
article (3) described the advantages to be gained from confinenicnt of atinospheres generated by a thri~iiio~ravimetric sample to t,he imincdiatc vicinity of that sample and, a later papcr ( 1 ) extended the apI)lic~aliility to differential thermal analy&. 1)ointing o u t in particular t8he niciit.: of it> use in obtaining good and meaningful agreement between thermogravimPtric and differential thermal data. In essence, the sample drcwiii)osed in an. atmosphere containing almost no,ihing but the decwnil)osition product ga.5 a t the ambient prwsure. The self-generated atmosphere quite naturally eliminates the need for dynamic or controlled atmospheres for general 1)url)osesso that these relatively tedious techniques can be reserved for more detailed studies on specimens of ['articular interest.. The physical chemistry involved is reviewed below. In the case of a reversible decomposition of a carbonate or hydrate PRE:VIOU;
MCOS
?=?
!\IO
that is, for example, the possibility that the oxide and carbonate will coexist at some given temperature depends only on the pressure of carbon dioxide over the system. If the pressure actually existing is not the equilibrium pressure, the reaction will proceed in the direction which will tend to bring about this condition. IR the earlier paper (3) the effects of inhibiting the escape of decomposition products were elucidated-Le., the material would decompose only slightly until the equilibrium pressure of product gas was equal to the ambient total pressure. Further supply of heat allowed rapid decomposition over a short temperature span because the decomposition was occurring under a fixed (by the outside-) pressure of the product gas. Kote that the product gases are confined not just to the furnace chamber but to the sample chamber. The result is that the composition of the several microliters of space among the particles can be of great difference from that in the furnace chamber even though only a negligible amount of decomposition has taken place. The procedure need not be limited to ambient pressure conditions. By varying the external (to the sample holder) pressure, the equilibrium pressure which the material must attain for rapid
EXPERIMENTAL
Apparatus. T h e powdered samples (ea. 100 mg.) were pressed into t h e closed-end tube of Figure 2 and the tube was slipped over the ceramic rod carrying the thermocouple. (For samples t h a t do not compact well, the furnace base assembly can be tilted.) T h e furnace tube is p u t in place, the desired gas pressure is supplied from a cylinder, and the heating program is started. Subatmospheric pressures were obtained using a vacuum p u m p for the kaolin study. RESULTS
+ coz
or
I n either case, the reaction is governed by both rate and equilibrium constants but if it is assumed that at t'he temperatures of interest bot,h t'he decomposition and recomposition proceed very rapidly, rate constants can then be ignored and the more important problem of equilibrium can be considered. The assumption is valid for most hydrates and carbonates. The equilibrium constant for such a reaction is expressed as
decomposition is changed. As a consequence, the temperature of this rapid decomposition is also changed. As a result of this change, besides the simple study of the effect of pressure, reactions may be separated in temperature, heats of decomposition may be calculated, or even new reactions or steps may be observed. The separation of reactions can be deduced from Figure 1. The equilibrium vapor pressures that dictate close-lying decomposition temperatures at some particular pressure also prescribe that this temperature gap will change with pressure-except if the heats of decomposition were very similar. Calculation of heats of reaction is simplified by use of geometrically increasing pressure, since this yields nearly equal temperature intervals.
Figure 1. Arbitrary representation of equilibrium pressure-temperature relationships for two similar materialse.g., hydrates
The relation of this technique to that of supplying an atmosphere of the product gas is thoroughly demonstrated in the results presented hertlin. The product gas in each case is water vapor. Figure 3 shows the effect of water vapor pressure (the water vapor supplied by the specimen and the presssure from a cylinder of oxygen) on the dehydroxylation of kaolin. The results are precisely analogous to Stone's (4) data on other kaolins obtained by use of a dynamic atmosphere of water vapor. Of special interest is the tendency of the exotherm to occur less sharply and a t lower temperatures a t the higher pressures of water vapor. This variation in the appearance of the exotherm results from the variation in conditions during the dehydroxylation. The presence of water vapor at VOL. 37, NO. 1, JANUARY 1965
77
THERMOCOUPLE
--CERAMIC
TUBE
A 4 Figure 2.
DTA furnace assembly (courtesy of Apparatus Manufacturers, Inc.) Pressurizing the furnace chamber fixes decomposition temperature b y establishing the pressure which must b e overcome for r a p i d escape o f product gos from closed-chamber sample holder (shown enlarged)
JJ the various pressures alt,ers the course of t'his earlier reaction enough to permit the lat'er rearrangement at lower and lower temperatures ( 2 ) . I t is most, essential t'o note that xhile the effects produced are analogous t,o the dynamic atmosphere technique a static atmosphere of water vapor in the furnace chambers would also have t,he same effect. The experimental t,ech-
nique used herein is extremely simple and yet produces t'he same effect' as the more elaborate and tedious techniques. This technique does not re1)lace t'he others completely, but, for those cases in which products other than water vapor need not be removed this technique yields the same results and is experiment,ally easier. -4 clear-cut observat,ion of a new reaction occurs with barium chloride dihydrate. The system is under more extended study; only one curve is presented here (Figure 4 ) . The dihydrate decomposes in two steps) seldom well e the first separated. At one a t dehydration and the point, of water are close toget'her in temperature so that the reaction is BaClz 2H20 + BaCl,
I
+ HQO(~)
5 8 0 648
ANALYTICAL CHEMISTRY
100
PO0 TEMPERATURE "C.
Figure 4. DTA curve of barium chloride dihydrate under 4 atm. of selfgenerated water vapor
more complicated. - i t four and eight atmospheres, tor example, this decomposition occurs in two and three steps, respectively. Thib is currently being investigated further. LITERATURE CITED
assuming the first product' to be the monohydrat,e. .it pressures of two or more at,mospheres this part of the total process occurs in two st'eps :
(1) G_arn, Paul I)., 1241-51 (1961).
13aC1, . 2 H 2 0 + 13aCh . H 2 0
( 3 ) Gam, P. D., Kessler, J . E., ANAL. CHEM.32, 1563-5 (1960). (4) Stone, K. Id., Howland, R . .4., .Yatl. Acad. Sci.-.Vatl. Res. Coiincil Pzibl. NO. 395, 103-17 ( 1 9 X )
+ H20(1)
(1)
and subsequently
A N A L . CHEM. 33,
(2) Gam, Paul U., "Thermoanalytical Methods of Investigation," Academic
Press, Xew York, 1964.
806 080
Figure 3. DTA of kaolin in self-generated water vapor under various pressures
78
H20
c
HzO(i1
+
HzO(0)
('4
.It these higher pressures, however, the second dehydration step has become
RECEIVED for revieu July 23, 1964 Accepted October 9, 1964 Pittsburgh Conference on Bnalj tical Chemistry and Applied Spectroscopy, AIarch 1964.
