E. C. Ashbv Georgia Institute of Technology Atlanta. 30332 Pierre Claudy, J. Bousquet, and J. Etienne INSA Chimie Mineraie Laboratoire Associe au C.N.R.S. No. 116 Lyon. France
I High Vacuum DTA-TGA Instrumentation for Air-Sensitive Compounds
Thermal Gravimetric Analysis (TGA) allows one to follow the change of the mass of a compound with temperature. Differential thermal analysis makes a comparison hetween the temperature of the sample and that of an inert standard and gives the sign of the enthalpy change occurring during a physical reaction (structure change, melting) or chemical change (decomposition). These two techniques are complementary and very often can be utilized simultaneously with the same instrumentation. In practice, there'are a number of problems involved in using DTA-TGA a t atmospheric pressure, especially with air sensitive compounds such as main group organometallic compounds or main group metal hydrides. If the experiments are carried out under gas flow, there are apparent changes of weight which arise from a change in gas density with temperature (below 500°C), convection currents, or changes of the flow rate, however small, resulting in poor reproducibility. I t is difficult, if not impossible, to avoid oxidation of the product by the small quantities of water or oxygen introduced either by the carrier gas or that which remains after degassing the apparatus. In addition, mass changes due to hydrogen loss are small and the precision of the experiment is not as great as desired. Many organometallic compounds or metal hydrides contain variahle amounts of ether of solvation. Unfortunately, this presents a problem in that the ether can he cleaved re-
sulting in a non-desirable side reaction. In order to avoid ether cleavage problems, the DTA-TGA experiments must he carried out under vacuum (10-2-10-3 torr). In this way ether will he removed a t a lower temoerature. thus lessening the possihility of ether cleavage. In addition other oroblems are avoided. such as weieht chanee with temnerature (below 500°C) and hydrolysis, as well as oxidation which is alwavs . a nroblem when DTA-TGA is carried out in the presence of a carrier gas. Also by operating under hieh vacuum i t should also he poasihle to determine the nature and quantity of the gas kvolved a t the conclusion of the experiment. Descrlptlon of the Apparatus Gauges (Fig. 1)
The most difficult problem is t o have vacuum gauges whose tension-Pressure response ia the same. With ibis-in mind, chromatorraphic filaments "COW Mac Instrument" have been u s e d . ' ~ h e ~are supplied with a constant current (21.3 mA) and the tension which appears depends on the pressure and on the thermal conductivity of the gas. The hydrogen peak will be spread out and that is advantageous. Empirically, an adjustment has been made to get the same pressure on both gauges with a non-condensable gas (argon). Description of the Vacuum Line (Fig. 22)
The vacuum line has heen constructed to allow the use of the TGA under various conditions: vacuum, static pressure,
JI
J2
12
I
Figwe 1. Elecmcal diagram fw vacuum gauge. (a) Zener 1N5231 (b) Diode IN914 (c)TransRCA40362.
618 / Journalof ChemicalEducation
Figure 2. Diagram fw vacuum line.
or gas flow. Wheo the sample is run under gas flow, the gas enters the TGA by V and leaves through rl or rz and rs; Rz open, R1 closed. For vacuum runs, V is closed and R1 and RP are open and rl and rz are closed. Between R1 and the pump is the analytical device. I t is a simple U-shaped tube (diameter 16 mm). One hranch has a copper sleeve, the shape of which must ohey the following: (1) slow warming after removing the liquid nitrogen because the quantity of product to analyze can he greater (more than 2 mg) and (2) vertical thermal gradient to assure a good separation of the trapped products. On the top of the U-shaped cold trap is the analysis gauge, J1. The second cold trap is as near as possible to the exit of the analysis device. The efficiency of the pump must he good because the gas transfer during the analysis is by cold pumping. Prlnclple of the Apparatus
A Mettler Thermoanalyzer I1 and Welch Pump were used to construct the modified DTA-TGA instrumentation. The dynamic pressure in the vacuum line depends only on the amount of gas evolved since the flow rate of the pump is fixed. If the mass loss of the compound is known, i t is qualitatively possible to know the nature of the gas. For example, if 10 mg of ether or hydrogen are lost, the volumes of gas are greatly different and hence, the pressure peaks during the evolution are different. However, for a mixture of two (or more) gases it is more difficult to evaluate the pressure peaks. However, an easy separation is possible if one of the two gases is trapped in liquid nitrogen. Under these conditions, the pressure before the cold trap will he higher than after the cold trap, and thus a precise determination is possible. The gas condensed in the U tube (Fig. 2) can he analyzed. Under vacuum the trap is slowly warmed and the solids sublimed (under the experimental conditions the compounds are below the triple point) when the temperature of the trap is greater than the solid sublimation temperature under the experimental conditions. The vapor pressure is recorded by use of a vacuum gauge. By standardization it is possible to construct a curve which relates the area of the peaks (S) versus the quantity of product. The warming is ;eproducihle and thusthe measure of zero time and the appearance of a compound is characteristir of the compound: For hydrides, ethers and any carbon compounds with three atoms or more are trapped in liquid nitrogen.
Figure 4. Bottom curve: TGA for Mg(AIH4)~.0.595 THF (heating rate 2'CImin) Top curve: pressure curve from gauges J, and J2.
Results Test
The complete apparatus must he tested with a compound which evolves different gases, both condensable and non-condensable, at different intervals. Calcium oxalate, CaCZOa.Hz0, was chosen and the TGA carried out a t a 2OCJmin heating rate with the product in a platinum crucihle. The thermal decomposition occurs in three separate steps CaCI0,,H20
+ - + R T to 110°C
CaC20+ CaCO,
4wr
500-650°C
CaC,04
CaCO,
CaO
+
H?O
(1)
CO
(2)
CO,
(3)
In Figure 3 are shown: (1) the TGA (AW.IW,) %, (2) the DTG (AWJAT) mgldeg calculated from the TGA, and (3) the pressure curves of both gauges J I and Jz. For reactions (1)and (3) J z records a pressure; however, J1 stays to the limit of the vacuum. On the other hand, during reaction (2) J1 and J z give a similar pressure. The results are consistent with what is expected; Hz0 and Con are condensed and CO is not. The DTG gives the same indication as the pressure curves; only the amplitudes of the peaks are different. The maximum of the DTG and pressure curve exhibits a small difference with temperature, which comes from the damping of the TG apparatus and the change of the weight due to the gas evolution. There is no weight shift with temperature. Thermal Decomposition of Mg(AIH4)2
Figure 3. Bottom figure: TGA far CaC204.H20 (34.8 mg at Z°C/min). Middle figure: pressure curve from gauges J1 and Jz. Top figure: DTG curve.
For hydrides a special crucible is used (Fig. 2). Experiments have shown that when Hz is evolved, very often product particles are ejected from the crucible. T o avoid this. the aluminum crucible is enclosed bv a fritted elass disk through which gas ran escape hut no;solid. The temnerature is limited to 500°C which is sufficient for most hvdrides. The TGA of Mg(AIH&0.595 T H F is shown in Figure 4. The T H F is evolved in two stages between 40 and 140°C. During the gas evolution a reaction between T H F and the hydride is noticed because a small amount of non-condensable gas is recorded; then the decomposition begins according to eqn. (4) Volume 52, Number 9, September 1975 / 619
It was interesting to study in more detail the latter reaction, because it is known that LiAlHa and NaAIH4 decompose in two stages
-
3 MAlH,
M,AlH,
+
+ 2 A1 + 3 H, + Al + 312 H,
MAH, 3 MH
Isothermal decomposition was also carried out. At llO°C T H F was rapidly evolved (2 hr) without any decomposition. A white solid was ohtained; the X-ray analysis is reported in the tahle, column 1. At T = 11SoC, the decomposition is slow, and when the hydFogen evolution was half that expected, the TGA was stopped and the gray residue containing the same white crystals was examined (the tahle, column 2). Finally, the pyrolysis was carried out a t 200°C and the X-rav diwram showed onlv - MaH2 - - and Al (the tahle, column 3): The identitv of 1 and 2 where onlv the X-rav lines of aluminum are visible, gives the proof that the same compound is present, namely, Mg(AIH4)z. Therefore, the thermal decomposition is according t o eqn. (4) (the expected MgH2 spectra is not ohtained (the tahle, column 2) probably because its intensity is too weak to he observed).
Figure 5. Pressure versus time analysis of condensable gases evolved during decomposition of Mg(Aln,)2+?THF.
X-Ray Diffraction Data
Figure 6. Calibration curve far THF to be used when THF is evolved simultaneously with other condensable compounds.
1.29
1.29
THF(rng) S
Analysis A record made a t the end of an experiment with Mg(AlH&2THF is represented in Figure 5. This is done by closing valve R1 and removing the liquid nitrogen from the U tube. The pressure readout is then recorded as the trap warms to room temperature. Three initial small peaks could not be identified. Then the peak for T H F appeared and one for Hz0 which is preceeded by two other peaks. The first one is C4HgOH coming from the T H F cleavage. The various experiments with different quantities of M ~ ( A I H ~ ) T x T Hgave F the standardization curve S = ~ ( ~ T H(Fig. F ) 6 ) where S = area under curve and m = mg of T H F condensed. From the experimental data in the 5-15 mg range the precision is 2~3%.
620 / Journal of ChemicalEducation
6.95 15
9.3 222
9.5 214
10.6 239
13.2
295
14.4 298
14.4 304
26.5 446
If a hetter curve is needed, particularly for small quantities, i t is easy to use the volume u between r l and r2. A known pressure of T H F is introduced and condensed into the trap. From the u, P, and T of the sample, the mass of the T H F is calculated and then the curve can be calibrated. Conclusion
The modified apparatus described makes possible a qualitative analysis of gas evolved during DTA-TGA of an air-sensitive compound such as a main group metal hydride. At the end of an experiment, a quantitative analysis of the condensed gas can be made with an accuracy of 3%. It is possihle, on stopping the TGA a t various steps of the decomposition and carrying out the gas analysis, to know the nature and the amount of condensable gases evolved.
