Papers presented in the ~ m p o e l s m on Thermal ethods of Analysis, Dlvision of Anal tical Chemistry, 1 3 7 t 9 Meeting, American Chemical Society, Cleveland, Ohio, Apr 11960.
Thermal Methods of Analysis Thermogravimetric Measurements ARTHUR E. NEWKIRK General Eledric Research laboratory, Schenectady, N. Y.
b Interest in thermogravimetry has increased in recent years because of the commercial availability of automatic, continuously recording thermobalances which are rugged, reliable, and accurate. Although isothermal or static thermogravimetry is an old art, the new instruments facilitate studies by dynamic thermogravimetry in which the sample i s heated at a uniform rate. Dynamic thermogravimetry is, however, subject to errors which are often overlooked. These include the effect of changing air buoyancy and convection, the measurement of temperature, and the effects of atmosphere, heating rate, and heat of reaction. The present limitations of dynamic thermogravimetry would seem to indicate that its chief value is for the determination of the approximate quantitative behavior of a system but not, by itself, for the determination of precise constants.
T
HERMOGRAVIMETRY is the science and art of weighing substances while they are being heated. While modern thermogravimetry appears to offer the possibility of studying the complete thermal decomposition behavior of a substance with a simple, automatic apparatus which in a few hours plots the weight-temperature relation directly, it requires the use of a sensitive balance under conditions which sometimes seem designed to make it as difficult as possible t o obtain the correct result. I n addition to the obvious sources of error, further limitations are inherent in almost any reaction system, and particular systems often present additional unusual difficulties. Despite these problems, thermogravimetric results can be surprisingly good, and many of the apparent limitat,ions of thermograviiietric measurements can be effectively overcome.
1558
ANALYTICAL CHEMISTRY
Modern thermobalances facilitate studies in which a sample is subjected to conditions of continuous increase in temperature, usually linear with time. This may be called dynamic thermogravimetry, and is relatively recent (1923) compared to isothermal or static thermogravimetry, which has a long history. Dynamic thermogravimetry is particularly attractive in the early stages of an investigation because it is a way to survey rapidly the complete thermal behavior of many systems. Its value was pointed out by Guichard (11), who recommended continuous weighing at linearly increasing temperatures for the study of dehydration reactions. Within a few years he recommended its extension as a first method to be used in the exploration of all phenomena which take place with a change in weight. As he pointed out, it gives the principal results very easily, and for the delicate cases i t often indicates the zone of temperature in which it is necessary to multiply one's observations. Dynamic thermogravimetric measurements are subject to errors and limitations which do not seem to be readily recognized. Those discussed in this paper apply most directly to thermobalances, such as the Chevenard, in which the sample is exposed to a gas a t 1-atm. pressure. Vacuum thermobalances have their own peculiarities and will not be considered.
EFFECT OF C H A N G I N G AIR BUOYANCY AND CONVECTION
First we will consider the effects of air buoyancy, convection, and related effects on an inert sample supported from below in the center of a cylindrical, capped tube furnace. This arrangement is favored for thermobalances
because it minimizes both heating of the balance mechanism, and its disturbance by thermal convection currents. Under these conditions an inert sample may show a n apparent weight gain of up to 10 mg., and correction for this is the most important correction that may have to be applied to the recorded weight change. The several causes of this change are known, but the relative magnitudes due to each are not, and even the need for applying the correction has sometimes gone unrecognized. Duval, for example ( 5 ) , did not mention the effect for reasons which have become clear only recently (8)*
The three causes of the apparent weight gain, decreased air buoyancy, increased convection, and the effect of heat on the balance mechanism have been discussed in detail (1, 12, 18). The apparent weight gain increases with increasing volume of the load, thereby showing its dependency on decreased air buoyancy. The calculated magnitude of this effect is a fuaction of the gas, being for hydrogen 0.1 mg., for air 1.4 mg., and for argon 1.9 mg., for a temperature change from 25' to 1000° C., and a load of one No. 4-0 porcelain crucible. The apparent weight gain varies with heating rate, thereby showing its dependency on convection. Recently in this laboratory the apparent weight gain has been observed to increase markedly if the balance mechanism was shielded to prevent interference from stray air currents. This shielding also reduced heat dissipation, thereby shoning the effect of heat on the balance mechanism which in the Chevenard thermobalance may be attributed to deereased buoyancy of warm oil in the dash pots. Inasmuch as the relative magnitude of these effects depends on the design of
4r
4t
3-
7
NlTR06EN FLOW, CC/MM.
Figure 2. Effect of gas velocity on apparent weight gain of one porcelain crucible at room temperature
2o
100 Figure 1.
t
400 500 600 700 800 900 1000 TEMPERATURE - 'C. EfFect of lop opening on apparent weight gain 200
300
Chevenard thermobalance, heating rote 300°/hour
-One parcelo,incrucible, Coors 230-000 (about 4 grams)
--- Two crucibles
-
5 I-
"
I 1
0 ' 0
V
1 200
I
1500/HOUR I
400
600
I
800
I
1000
TEMPERATURE OF THERMOCOUPLE WELL,'C.
the balance and its environment, different apparent weight gain correction curves are obtained by different operators. I t has recently been suggested by Duval (8) that the cause of these differences is the extent to which the top cap of the furnace permits air to vent through the furnace tube, and he concludes that by proper choice of the size and position of the vents it is possible t o obtain no apparent weight gain on heating. It is unlikely that this is correct in the general case. If the upward flow of air through a vented thermobalance furnace will by chance just compensate for the buoyant and turbulent effects on an empty crucible, it would be surprising that i t should do so equally well for small and large loads in the same crucible. The apparent weight loss caused by the upfloning stream of air on the crucible and the apparent weight gain due to turbulence must be determined largely by the crucible size and shape, whereas the apparent weight gain due to decreased air buoyancy will depend not only on the crucible but on the volume of its contents. However, for small loads of relatively the same size and density, the differences would be negligible, and these were the conditions used by Duvnl in his study of analytical precipitates. He also advises using cruciblcs having the same form, and in partkular the same large 3 r d small diameters. Apparent x-eight change curves as a function of top opening are shown in Figure 1. These were obtained nith
a single empty crucible as described ( I S ) . Three characteristics are worth
noting: Except when there is a very large opening, there is always an initial weight gain, even when, on further heating, there is an over-all weight loss; most of the curves can best be represented by a straight line in the temperature range 200" to 1000" C., and the lines shown are a least-squares fit to observations made a t 100" intervals in this range; it is not possible to choose an opening which will give no apparent weight gain on heating over the whole temperature range, although the effect can be greatly reduced, and for many purposes rendered negligible. The effect of doubling the load from one to two crucibles with an 8-mm. opening is shown by the dashed line. The choice of a top opening to minimize the apparent weight gain is also a function of the rate of heating. With the top closed, a marked difference is observed a t heating rates of 150" and 300' per hour (18). With a n 8-mm. top opening the differences are greatly reduced, but still apparent. When samples are examined in the thermobalance using a gas tube to control the atmosphere, an additional apparent weight gain is caused by the gas flow, Figure 2. I t s magnitude is affected by the molecular weight of the gas used. Changes in air buoyancy and convection can cause apparent weight changes of sufficient magnitude so that for careful work correction curves should be obtained on inert sample4 under the same conditions used for the
Figure 3. Effect of heating rate on crucible temperature
active samples. The magnitude of the apparent weight changes may be reduced considerably by proper venting. MEASUREMENT OF TEMPERATURE
Thermobalances are regularly used for determining the drying ranges of precipitates and are becoming increasingly popular for the determination of kinetic constants of thermal decomposition reactions of technical importance; a warning about the significance of such measurements seems in order. Although some thermobalances have been described in which the sample temperature is measured directly, the more usual practice is to measure the temperature in the furnace near the sample. These measurements are subjec,t to error in a manner which seems often to have been overlooked. The error is caused by the use of a heating program in which the temperature of the furnace is continually increased at a constant rate with the result that reaction temperatures so determined are usually higher than those determined by the more common process o€ heating a sample for a fixed length of time at each of a series of constantly increasing temperatures. Part of this difference is merely thermal lag. and part, is due to the finite time required t o m u s e a detretable weigilt change. The effpct of thermal lag is ::horsn for threc different htating rates in Figure 3 . V O i . 32, KO. 12, NOVEMBER ! 9 6 0
* 1559
500'
______
?\
NO VENT VENT 7mm. DlA.
8004
w
900'
Figure 4.
Effect of venting on thermogram of CaC20d.H20
In these experiments a second thermocouple was located in a thin quartz sheath directly in tli- empty crucible, and the figure shows the extent to a hich the tc.mperature of this therniocouple lagged behind the furnace temperature. The lag is significant, 3" to 14", and is roughly proportional to the heating rate. The effect of reaction time may be demonstrated by a very simple experiment. When a drop of nater wighing 0.10 gram !$as heated in a porcelain cruciblc in a Chevenard thermobalance a t a heating rate of 300" per hour, constant weight was attained a t 122" C. It is obvious that the sample did not achieve this temperature, that it is a higher temperature than that necessary to dry the crucible (were longer drying times used a t loner tempera-
tures), and that the observed value of the temperature will be influenced by the amount of n.ater and the rate of heating. One ~ o u l d not think of spccifying thc Iiighw tcmpc~mture as 3 necessary l o w r limit for drying in this case, y1.t this conrlusion is usually madv about analytical prwipitatr>sand occasionally leads to confusion. A good examplc is the \vork on the use of zinc inonosalic~~laldosiiie for the determination of zinc. D u d ( 7 ) followed thc procedure suggested by Flagg and Furman (9)
80 -
for the drying and deconiposition of zinc inonosalicylaldoxiiiie, but, since his thermogram had no plateau for anhydrous zinc monosalicylaldoxime, he proposcd abandoning this method. Interestingly enough. in neither of his papers (2, 7 ) does the therrnogram indicated to bc that of zinc nionosalicylaldoxime correspond to the dcscription given for it. A morr detaikd study by Rynasiewicz and Flagg (26) showed that zinc nionosalicylaldoxime was stab]? from room temperature to 285" C., but that the plateau rwordrd by the thermohalance depended upon the initial water coiitcwt. l'he thermolysis curve of a sample air-dried a t room trmperaturc to zcro \vater rontcnt had a platrau from 25" to 2%" C., and a part,ially dried sainplr \vith a water content of 50YG gave a platisau from 135" to 250" C. Each of thme samples was heated a t a rate of 300' C. pcr hour, but held for 1 hour at' u constant tempcraturc, belo\\- the tleromposition teniperature, 100" and 1 3 2 C., rcspwtiwly. A saniplr initially w t , 630% \vntc,r! gave a plateau in the rang(' 243" to 315" C. Inasmuch 3 s Duviil used a highcr hcating ratc, on thc ord('r of 380" por hour. in thia rtyjon, it is perhaps not surprising that he failcd to find a plateau. 'I'he rejection of tliis prccipitatc as an anal\-tical mctliocl for thc drtcriniii:ition of zinc on thcy grounds that it does not give a stnblr horizontal in the thcmiiobalnncc :it on(' p:irti(sular
I 30
I
70 f:.
-
28 26
I
\
A
I
g
60-
ca W
B
I
5
I
0 I
30-
b-
5
40-
2 30
24
22 20
-
20 IO -
I
1
100
200
300
400
600
500
700
800
900
0-
IO00
TEMPERATURE, 'C. TEMPERATURE,'C.
Figure 5.
Na2C03;
Effect of atmosphere on decomposition of reaction Na2C03 WOJ in COn
+
A. Sodium carbonate in air B. Dried sodium carbonate in Con; gas flow 2 5 0 ml./min. C. Sodium carbonate tungsten oxide in COz; gas flow 2 5 0 ml./min. Weight change: calculated 0.0996 gram, observed 0.0995 gram
+
1560
0
ANALYTICAL CHEMISTRY
Figure 6. Dehydration of sodium (5 to 28-hydrate
12) tungstate
Atmosphere and heating rates A. Humidified air, 300°/hr. B. Humidified oir, 1 50°/hr. C. Humidified air, 1 Oo/hr. D. Room air, 10°/hr. n = moles HIO per 5 N a 2 0 . 1 2 W O { ; sample weight 0 . 5 0 0 0 gram
u Figure 7.
Effect of heating rate on the thermogram of CaC204.-
H20
__ 300°/hr. ---- 1 50°/hr.
corrected for differences in scale and apparent weight gain
rate of h a t i n g is clearly unwarranted, alt’hough it may be reasonable to reject it for a program of automatic analysis of the kind developed by Duval. The results suggest that when the thermobalanco is used to study the drying of bulky prccipitatrs that contain considrr:iblc water, it would be well to u s o vcry sloir. rates of hrating. Another source of error, when a controlled atmosphere is used, is the effcct of thr gas stream in lowering the tht\rmocouplc temprrature relative to th(5 s:iniple tempcmturc~. In the author’s c.spcriencc this difference amounts to a fcn. tlrgrees and is unimportant a t high rates of heating wherr tht, truc, s:tmplc tcnipwaturcs cannot hc known to any bettor than this, since gradirnts of this order of magnitude mist in thc sampl(~itself. However, whrn the thermobalance is uscd as a constant temprrature device, this effect must be taken into account. Furthcr csamples of the difficulties ivhich inttlrfc,re with knon.ing the sample
temperature will be given later in discussing the effect of rate of heating and heat of reaction, but the problem may be suminarized as follows. Although the relation between the tcrnperature in tlic tlierniobalance furnace (which may be measurtd precisely) and the temperature of the sample undergoing reaction (which is not mcasurcd) is indeterminate, crrtain qualitative generalizations may he inatlc about the direttion and niagnitude of the diffcrence since, for a givvn inatrumcmt, the geometry of thc, systcni and the procedure uscd arr usually thc’ snmc’. l’he true tempmituw of a suhstancc which does not changc w i g h t on hrating will tend to br I o w r than thc recorded temperaturc, tlic iiiagnitudc of the deviation depending on thc mass of the sampk, its heat capacity, thermal conductivity and dcgrcc of fineness, thc rate of gas flo~vin the furnaco, and the rate of heating. During :in cndotherniic change in t h r samplc, tht. deviation will of course be greater, while during
‘r
an exothermic change the true temperature may range from only slightly less than the recorded temperature to much greater depending on the amount of heat produced. However, there appears to be no simple way to estimate a priori the magnitude of the difference for a given sample. The establishment of temperature ranges for drying (or the temperature range of stability of a compound) from thermobalance curves is, therefore, not simply a matter of reading the temperatures a t the beginning and end of a level portion of a single thermogram. As a general rule this range will be approximately correct, but will have been shifted upward in temprraturc> by the effect of the rate of heating. The statement of Iluval ( 6 ) that when thc pyrolysis curve of an analytical precipitate affords a horizontal, all dryinga must be effected a t a temperature situated between the two ends of the, Icvel, while correct for his apparatus and procedure, will not he corrwt in gcneral. Fortunatrly, even though the tcmprraturc to bc assigned to a point on a thermogravimctric curve may bc indeterminate, the curves obt:tincd by heating equal \wights of the samc mnterial untlrr the sanie conditions in separate runs arc> usually idmtical. A desvriptive trwn t o appl to such t,t’mpcraturcs should be u term “procrdural dcconiposition tcmperatiire” or “p.d.t.,” which was coinrd by Doylr in reporting on his extcnsive and thorough studics of the application of thc thermobalancc to polymrr dccompositions (.$), is suggested. 7
EFFECT OF ATMOSPHERE
\Then a samplv is dried or dcconiposcd in a thermobalance in ambient air, the atmosphere ncar the samplc is continriously modified by addition of gi~seous decomposition products or the loss of part of thr original gas by reaction with
I
1
0.9
07
9
’
E 06’-
HEATING RATE ‘C/hr
02 0,
-
I
0 0-~ I00
2 C d 7 b O 340
360
3b
4W
+I
‘2
TIME (TEMP.)
Figure 8. Pyrolysis of polystyrene in nitrogen at different heating rates
Figure 9. Determination of reaction rate and extent of reaction from a thermogram VOL. 32, NO. 12, NOVEMBER 1960
1561
€
lo
35t
i-d RUN 0 0
A
HEATING RATE CONSTANT T.
T-274 T-269 T-275
25
I50 C I hr. 300 'Clhr.
4 ;\
E
I \
t f
!O
2;o
460
6b0
860
lob0
TEMPERATURE OF THERMOCOUPLE WELL,DC.
Figure 1 1 . Temperature lag .due to sample. Decomposition of CaC204.
-
H20 Heating rate 600"jhr. A. Crucible only 8. Crucible 0.2 gram CaCrOd. H,O C. Crucible f 0.6 gram of CaCz04.H20
+
I
0.00I I 1.4
1.5
16 1000 / T O K
1.7
1.8
Figure 10. Rate of weight loss of Du Pont Mylar at three heating rates
the sample. Even small changes in the composition of this atmosphere can affect the thermogram as shown by Figure 4. The dashed curve represents the decomposition of calcium oxalate monohydrate in a closed top furnace. By venting the top, the apparent weight gain correction is successfully reducedon the plateaus the dashed curve lies above the solid curve and the difference increases with increase in temperaturebut the course of the decomposition varies, as is particularly noticeable for the second step. A second example is the decomposition of sodium carbonate Figure 5 (14). I n air a t a heating rate of 300" C. per hour, decomposition sets in above 850' C., curve A , but in COz it does not occur even at 1050" C., curve B. By using a platinum crucible and a C 0 2 atmosphere, sodium carbollate may be used as a thermogravimetric reagent without interference from the decomposition reaction. This compound has been used successfully in this laboratory to study the composition of complex polytungstates as indicated by curve C. A third example illustrates the importance of both atmosphere control and heating rate. Sodium (5:12) tungstate 28-hydrateJ when decomposed in ambient air at a heating rate as low as loo per hour, giys no evidence of stable regions for either the 28? 562
a
ANALYTICAL CHEMISTRY
hydrate or Whydrate, Figure 6, curve D. I n humidified air good plateaus are obtained a t this heating rate, curve C, but not a t higher rates, curves B and
A. Further examples of the effect of atmosphere on the thermogram are given by Garn (IO). EFFECT O F HEATING RATE
The effect of heating rate on the thermogram obtained for a given sample of CaC204.H20is illustrated more clearly in Figure 7. The two curves are plotted to the same temperature scale and corrected for differences in the apparent weight gain. For each of the three reactions-dehydration, loss of CO, and loss of COz-the temperatpre of apparent onset of decomposition is lower at the lower heating rate, and the decomposition is completed at a lower temperature, the lower the heating rate. By going to heating rates lower than 150' per hour, the thermogram would be shifted to lower temperatures. In this case there is independent evidence to show that the first and third plateaus, those for CaCZ04.HZ0 and CaC03, actually represent stable compounds in the lower parts of the indicated stability ranges, but careful work by Peters and Wiedemann (15) has shown that anhydrous calcium oxalat,e is de-
composed slowly at least as low as 300" C. Similar results are obtained with polymer decompositions as shown for polystyrene by Figure 8. The effect of the different heating rates may be most readily appreciated by comparing the temperature a t which a fixed amount of decomposition has occurred, and in this case for 10% decomposition the temperatures are 357" C. for a heating rate of 60" per hour, and 394' C. for a heating rate of 300" peF hour, or a differcnce of 37" C. Sabatier (17) has observed a similar effect in the dehydration of mica. For samples heated a t 12' to 30" per hour, the observed temperatures for a corresponding stage in weight loss ranged from 736" to 815", a difference of 80" C. The effect of heating rate is important if the thermogram is to be used for kinetic analysis. We can obtain from a thermogram for a series of temperatures both the sample weight remaining, a - xi and a - X Z , and the reaction rate, and shown in Figure 9 by the hngents to the curve a t points 1 and 2 for the temperatures 2'1 and 2'2. I n the case of Mylar the decomposition is first order, and the logarithm of the reaction rate constant k , dz/dt = k ( a - z), may be plotted against 1/T to yield a straight line. The results of measurements at constant temperature and a t two heating rates are shown in Figure 10. The lines obtained are approximately paraliel and, using the data obtained under isothermal conditions as a standard, we can calculate that, the apparent temper:itiire is de-
(g), (2)zJ
creased by 12’ for a heating rate of 150’ per hour, and by 19’ for a heating rate of 300” per hour. The magnitude of these differences varies with the material and the form of the sample, but in any event, kinetic analyses made from dynamic thermograms should consider such effects. Doyle (3) has made extensive studies of polymer decompositions using dynamic therm&* gravimetry. EFFECT OF HEAT
ON
REACTION
The heat of reaction will affect the difference between sample temperature and furnace temperature, causing the sample temperature to lead or lag the furnace temperature depending on whether the reaction is exothermic or endothermic. The method of differential thermal analysis is possible because this temperature difference exists, and i t is common practice in DTA to use relatively high rates of heating, 600’ per hour, to accentuate the differential temperature. Since such differential temperatures may range well above 10’ at this heating rate, kinetic constants calculated from thermograms made at such high heating rates may be unavoidably and significantly in error. When the reaction is endothermic, the effect of temperature lag and differential temperature will be additive, but when the reaction is exothermic, the effects will tend to compensate each other. T h a t the effect of t,he heat of reaction can be large is illustrated strikingly in the case of the Oxidation of tungsten carbide in air ( I S ) . As
shown by the thermograms, approximately the same rate of reaction was observed with the thermobalance furnace heated to 527’ C. and held constant, as was observed when the furnace was heated continuously and uniformly. The effect is perhaps more clearly shown in Figure 11 where the differential temperature between the furnace and sample wa9 measured directly for the decomposition of calcium oxalate monohydrate at a high heating rate and for two different sample sizes. Curve A for the crucible alone shows a 10’ to 14’ lag in the range 100’ to 1OOO” C. When a 0.2-gram sample is used, the endothermic loss of water results in a 25’ lag a t 200’ C., the exothermic loss of CO brings the sample nearly back t o the difference observed with the crucible alone, but the lag again increases during the endothermic loss of CO,. With a 0.6-gram sample, these effects are accentuated and a t one point the sample and furnace temperature are nearly the same.
(2) DeClerp, Monique, Duval, C., Anal. Chzm. Acta 5, 282 (1951). (3) . . Doyle, C. D., “Evaluation of Experi-
mencal Polymers-Aging Tests Conducted in the Thermobalance and Empirical Tests in Dry Nitrogen,” Progress Rept. No. 6,Sept. 1 to Dec. 1, 1959. Contract AF 33(616)-5576,Proj. ect do. 7340. (4) Doyle,.C. D., Hill, J. A., “Evaluation of Experimental Polymers for Dielectric Applications,” Second Quarterly Progress Rept., July 5 to Oct. 5, 1958, Contract AF 33(616)-5576, Project N n XJ8-7371). \-
(5) Duvrtl, C., “Inorganic Thermogravimetric Analysis,” p. 26, Elscvier, New York, 1953. ( 6 ) Ibid., p. 34. (7)Ibid., p. 283. (8) Duval, C., Mikrochim. Acta 1958, 705 I V”.
LITERATURE CITED
(9) Flagg, J. F., Furman, N. H., IND. ENG.CHEM.,ANAL.ED. 12, 663 (1940). (10) Garn, P. D., Kessler, J. E., ANAL. CHEW32,1563(1960). (11) Guichard, Marcel, Bull. SOC. chim. France 37,62 (1925). (12) Mielenz, R. C., Schieltz, N. C., King, M. E., “Clays and Clay Minerals,” Natl. Acad. Sci.-Natl. Rcsearch Council Publ. 327, Washington, D. C., 1954,p. 285. (13) Newkirk, A. E.,J. A m . Chem. SOC. 77,4521 (1955). (14) Newkirk, A. E., Aliferis, I., ANAL. CHEM.30,982 (1958). (15) Peters, H., Wiedemann, H. G., 2. anorg. u allgem. Chem. 300, 142 (1959). (16) Rynasicwice, J., Flagg, J. F., ANAL. CHEM.26,1506(1954). (17) Sabatier, G., J. chim. phys. 52, 60 (1955). (18) Simons, E. L., Newkirk, A. E., Aliferis, I., ANAL.CHEM.29, 48 (1957).
(1) Claisse, F. East, F., Abesque, F., “Use of the ”rhermobalance in Analytical Chemistry,’’ Department of Mines, Province of Quebec, Canada, 1954.
RECEIVEDfor review May 26, 1960. Accepted September 1, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
ACKNOWLEDGMENT
The author thanks M. A. Bonanno and H. W. Middleton for assistance with the thermogravimetric measurements, and E. L. Simons for helpful discussions.
Thermogravimetry in Self-Generated Atmospheres PAUL D. G A R N and J O H N E. KESSLER Bell Telephone laboratories, Inc., Murray Hill, N. 1.
b The customary lack of clarity of thermogravimetric curves inspired consideration of the environment in which samples are ordinarily heated. Restriction of effluence to the immediate vicinity of the sample, permitting escape of gas but preventing exchange between the sample cavity and the ambient atmosphere, affords better definition of the weight losses. The major portion of the decomposition occurs under a pressure of ca. 1 atmosphere of the gas evolved. This work demonstrates that the uncertain composition of the atmosphere within the traditional crucible may b e avoided. The technique proposed has most of the advantages of con-
trolled atmosphere thermogravimetry with no additional apparatus except the sample holder.
I
N THERMOGRAVIMETRIC studies the sample is generally heated in a n open vessel-such as a cup-to facilitate the escape of decomposition products. The atmosphere surrounding the cup is generally air, although with inverted furnaces (1) or special chambers (4) some control of the atmosphere is possible. Only recently have controlled atmosphere thermobalances been available commercially. The controlled atmosphere is usually a neutral atmosphere-that is, an atmosphere which prevents some undesirable reaction but
does not affect the reactions under study. Thermogravimetry depends on the driving off of vapor, but this vapor may have been bound to the residue in a number of ways. Carbon dioxide, for example, may have been: adsorbed on a surface; a part of an inorganic carbonate or bicarbonate; an organic carboxyl group; or the oxidation product of carbon which had been otherwise bonded. Similar possibilities could be advanced for water. I n these two cases some of the reactions would require a supply of oxygen but a number could proceed simply on heating. Of these latter, some are irreversible and some are reversible. The decomposiVOL. 32, NO. 12, NOVEMBER 1960
1563
