!387
Ana/. Chem. 1983, 55, 987-988
technique as is discussed below. In order to demonstrate the role of silica glass powder, various mixtures of B&04 and Vz05with or without SiOz were heated at 1000 OC and the rate of evolution of SO2 was monitored by recording the output of the Pirani gauge placed between the reaction tube and trap 1 (Figure 1). Without silica glass powder, the rate of SOz evolution increased with increasing VZO5 However, the evolution of SO2did not proceed smoothly, showing several peak rates at different temperatures. The evolution a t low temperature was incomplete, whereas at highLer temperature, SO2 evolved with vigorous bumping or boiling of the molten mixture. On the other hand, the mixtures containing SiOz with a Si02to V205ratio of 1 or greater evolved SO2 completely and smoothly at lower temperatures. This difference in reaction is also reflected in the behavior of the reaction products. They differed in viscosity and affinity for the silica glass wall as schematically illustrated in Figure 2. Without Si02, the molten mixture displayed high fluidity. It crept up the glass wall and also reacted with the silica glass wool (Figure 2B). In contrast, the mixture with: SiOzdecreased in volume forming a droplet which adhered tlo the overlying silica glass wool (Figure 2C). The droplet, when cooled, was porous and brittle. However, excessive Si02lowered the SO2 yield, probably by preventing melting. On the other hand, excessive V205also lowered the yield, probably because it boiled off and reacted with the copper wire as illustrated in Figure 2B. For these reasons, the Vz05/Si02weight ratio of unity is considered to be the most appropriate. Whereas a ratio of (V205+ SiOz)/BaS04 of 6:l gave a good yield of SO2 at a low temperature, a ratio of 20:l was preferred to assure uniformity of the oxygen isotope composition of the SO2 as previously discussed. The presence of BaVI2Omor Ba0.V204.5V205in the reaction product indicates that the partial pressure of oxygen a t the site of BaSOI decomposition was controlled by V205-V@4 equilibrium. The PO,of this system is close to that of Cu20-
CuO and some SO3 should have formed. However it must have been totally reduced to SO2by the Cu-Cu20 system,,as metal copper remained after the reaction was over. The L!O, of CU-CUZOat 600 to 1000 “C ranges from to lo4 atm and the equilibrium S03/SOzratio under such Po, would be as low as 10” to (8). Therefore, the presence of excess copper is essential to maintain the low Po, and consequently the high yield of SOz. In conclusion, the present technique is very suitable for the preparation of SO2from the small amounts of BaS04 (1-5 mg) obtained typically from various geologic and laboratory specimens. The distinct advantage of the present technique over other methods is that the SO2 product is pure enough to be directly subjected to mass spectrometricanalyses. Thus, the technique seems particularly suitable for an automated SO2 preparation and isotope ratio measurement system currently under development in this laboratory.
ACKNOWLEDGMENT We are indebted to H. R. Krouse, University of Calgary, for his valuable discussion and advice in the course of preparation of this paper. We thank N. Kishima and H. Chiba for their help. LITERATURE CITED (1) (2) (3) (4) (5)
Claypool, J., private communication, 1970. Holt, B. D.; Engelkemelr, A. G. Anal. Chem. 1970, 4 2 , 1451. Bailey, S. A.; Smith, J. W. Anal. Chem. 1972, 4 4 , 1542. Haur, A.; Hladlkovi, J.; Smejkal, V. lsotopenpraxis 1873, 9 , 329. Kamada, E.; Sakal, H.; Klshima, N., Pap. Inst. Therm. Spring Res., Okayama Unlv. 1980, 50, 1. (6) Haias, S.; Woiacewlcz, W. P. Anal. Chem. 1981, 5 3 , 686. (7) Coleman, M. L.; Moore, M. P. Anal. Chem. 1978, 50, 1594. (8) Robinson, B. W.; Kusskabe, M. Anal. Chem. 1975, 4 7 , 1179.
RECEIVED for review July 30, 1982. Accepted December 27, 1982. The present research was supported by the Grant in Aid for Scientific Research No. 57430010 by the Ministry of Education.
Thermometric Calibration In Thermogravimetric Analysis Andrew R. McGhie Laboratory for Research on the Structure of Matter, Universjty of Pennsylvania, 323 1 Walnut Street, Philadelphia, Pennsylvanla
A problem exists in accuratelydetermining the temperature of a sample undergoing programmed heating during thermogravimetric analysis. The currently accepted method of determining temperature is to position a thermocouple close to, but not touching, the sample boat. This procedure does not necessarily measure the temperature of the boat as it is very susceptible to temperature variation with the thermocouple position and flow rate of the purge gas. At least two methods of temperature calibration have been tried. First, small metal riders have been placed on the balance beam which fall off when they reach their melting point. This method has several disadvantages: (1)it does not measure the boat bmperature, (2) surface tension of the liquid may prevent the rider from falling off exactly at the melting point, (3) the metal may oxidize, (4) the total sample weight on the beam will change. A more recent attempt has been made to use the Curie point of magnetic materials (I). In this case a small magnetic sample is placed in the boat alone or with an actual sample and a small permanent magnet is
19 104
positioned near the boat (either inside or outside the balance) to cause an apparent weight change of a few percent. When the reference undergoes the Curie point transition this magnetic force disappears and a sharp apparent weight change is observed. Disadvantages in this case include (1) obtaining standard reference materials with well-characterized Curie points over a wide temperature range, (2) the abrupt base line change, (3) the relatively low precision, k3.6 “C, of the method (2).
EXPERIMENTAL SECTION In this paper we wish to describe a variant of the first method in which most of the disadvantages have been eliminated. The technique is best described with reference to Figure 1. A standard platinum boat has a thin, horizontal platinum wire spot-welded to it near the top of the pan. The wire is bent at right angles so that it lies direct1:y above the bottom of the pan. To this wire a hook of some material with a well-characterized melting point, e.g., a metal, is attached and from this hook a platinum weight (approximately30 mg) is hung. The hook should be of low mass
0003-2700!83/0355-0987$01.50/0 0 1963 American Chemical Soclety
Anal. Chem. 1983, 55, 988-989
100 90 -
BALANCE B E A M
Pt BOAT
M E T A L HOOK
SPOT W E L D E D Pt. W I R E
Pt WEIGHT
1
Flgure 1. Schematic view of the calibration technique applled to a Du Pont TGA balance.
I
2 w
;
/I ,
,Pb,
0.1mg
-t 200
250
300
I
,
-
-E +
=
2 w
:
100.
IIIIII
-
5'C/min
go-'
100, -
I1IIII
IO°C/min
3 90-
u
100 150 2 0 0 2 5 0 T E M P E R A T U R E ("C) Flgure 3. Effect on heatlng rate on calibration with tin during decom0
50
positlon of calcium oxalate. Note that the variation in decomposition rate with heating rate does not affect the calibration.
TEMPERATURE ("C)
Flgure 2. Signal obtained on melting pure reference samples In an empty boat, using tin and lead. Note that an impure lead wire was used which resulted in a lower melting point.
and be of a material having low surfacetension in the liquid phase. On heating, the hook melts and the platinum weight drops to the bottom of the boat. This causes an action and reaction which is immediately sensed by the balance creating an instantaneous mark on the chart. Since the platinum weight remains in the boat, no weight change takes place and a trace similar to that shown in Figure 2 is obtained. The hook on melting will either remain attached to the platinum wire by surface tension or fall to the bottom of the boat and will not affect the total weight of the boat. If there is any chance that the hook material might react with the sample, a partition of thin platinum foil may be placed in the boat to prevent contact. If necessary the hook can be made of materials other than metal and a variety of methods of suspending a platinum weight on a pure reference material can be easily conceived. In this technique action and reaction occur rapidly and it has been noted that, using microprocessor-controlledDSC equipment in which data is collected intermittently, the signal may go unrecorded. This can be overcome by using more rapid data collection. In addition, it is possible that the derivative mode can be used at high sensitivity to detect the transition for cases in which a large sample weight requires a low sensitivityweight range setting.
RESULTS AND DISCUSSION This technique has been tried and shown to work with hooks of tin (mp 232 "C), lead (mp 328 "C), zinc (mp 419 "C), and silver (mp 961 "C). Multiple samples have been used during a single run to give two reference points bracketing
the temperature range of interest during a TGA run. The reproducibility of melting point as a function of heating rate during actual decomposition of calcium oxalate is shown in Figure 3. The temperature displacement to slightly higher temperature in the 2 "C/min run is due to the fact that the sample was rapidly heated to 200 "C at 20 "C/min and then switched to 2 "C/min resulting in a transient in the heat flow pattern. The other three runs at 5,10, and 20 "C/min were reproducible to within fl "C. The fact that there is no base line change using this technique allows it to be used while actual samples are being run with little or no loss in the accuracy of the data obtained. More extensive studies of the accuracy and reproducibility of this technique have been made covering the temperature range up to 1200 "C. These will be reported in detail elsewhere (3). The simplicity and reproducibility of this method should make it a useful calibration technique which will complement the use of magnetic standards. It should be applicable to almost every design of the thermogravimetric boat.
LITERATURE CITED (1) Norem, S.D.; O'Nelll, M. J.; Gray, A. P. Thermochim.Acta 1970, 7 , 29-38. _. (2) Garn, P. D.; Menls, 0.; Weideman, H. G. J . Therm. Anal. 1981, 2 0 , 185-204. (3) McGhie, A. R.; Chiu, J.; Fair, P. G.; Blaine, R. L. Thermochim. Acta, in press.
RECEIVED for review December 20,1982. Accepted January 24, 1983. This work was supported by the National Science Foundation, MRL Program, under Grant No. DMR-7923647.
Preparation of Tetrabutylammonium Hydroxide for Atmospheric Sampling of Acidic Halogen Gases 6. W. Gandrud" and A. L. Lazrus National Center for Atmospheric Research, P. 0. Box 3000, Boulder, Colorado 80307
Recently tetrabutylammonium hydroxide (TBAH) has been applied to filter material to adsorb acidic vapors drawn through a filter (1-6). The primary emphasis in using TBAH has been placed on the capture of acidic halogen vapors in
the stratosphere and troposphere. The collection of acidic halogen vapors, such as HC1 and HF, is of interest because these gases are the most stable decomposition products of chlorofluorocarbons in the stratosphere. This ultraviolet-
0003-2700/83/0355-0988$01.50/0 0 1983 American Chemical Society
