Thermobarogravimetric (TBGA) Technique to Characterize Sublimation Processes Nitronium Perchlorate J. N. Maycock and V. R. Pai Verneker Research Institute for Adcanced Studies (RIAS), Martin Marietta Corporation, Baltimore, Md. 21227 A thermobarogravimetric technique, using a Mettler thermoanalyzer, is discussed as an approach to obtaining the fundamental parameters of sublimation processes for solids which sublime and decompose simultaneously. The technique has been applied to nitronium perchlorate, and the resulting data has been kinetically analyzed using the contracting volume model for sublimation. These data are consistent with current models of the irreversible thermal decomposition of nitronium perchlorate. The activation energy of sublimation of nitronium perchlorate below 100 OC is 18.7 kcal mole-' and 14.4 kcal mole-' between 110 and 150 "C.
tically affect the rate and activation energy parameters of the sublimation process. Bancroft and Gesser (2) have applied, quite successfully, a TBGA technique to a qualitative study of the thermal decompositions of several bromates which can decompose by two competing processes giving two different gases of decomposition. This paper describes the application of a TBGA technique, where the weight loss of the sample and decomposition gas pressure are measured simultaneously, to determine the fundamental parameters of the sublimation of nitronium perchlorate which also undergoes an irreversible decomposition into gaseous products in the same temperature range.
COMPOUNDS WHICH UNDERGO DECOMPOSITION and sublimation simultaneously are common among explosives and solid oxidizers. Characterization of the parameters of these two competing processes is important for a complete understanding of the role of these compounds as explosives or as components of a solid propellant. Sublimation parameters cannot be deduced from thermal decomposition studies using either a weight loss technique (TG) or the normal accumulatory pressure method, due to the unknown amount of thermal decomposition taking place to produce the gaseous products. Jacobs and RussellJones ( I ) have recently reported on the sublimation of ammonium perchlorate by using only thermogravimetric techniques, with the assumption that the normal irreversible thermal decomposition was not present in the temperature range being studied. Assumptions of this nature can be quite inaccurate for many meta-stable materials and will dras-
Apparatus. The simultaneous measurement of the weight loss and decomposition gas pressure of the test sample under isothermal conditions as a function of time can be conveniently performed by using a Mettler thermoanalyzer. Figure 1 is a diagrammatic section through the balance and the vacuum system. The test sample, about 150 mg of nitronium perchlorate, is contained in the platinum thermogravimetric (TG) cup, A , and attached to the micro balance by the TG stick. After securing the T G stick, the furnace, B, is fastened securely in place. The entire unit, balance chamber and furnace, is then evacuated with the mechanical pump down to about 30 X Torr, further evacuation to 5 x 10-5 Torr was possible with the two oil diffusion pumps. After this evacuation, valves L, H , D, G, and E were closed to form a closed volume decomposition chamber of 16.0
(1) P. W. M. Jacobs and A. Russell-Jones, J. Phys. Chem., 72, 202 (1968).
( 2 ) G. M. Bancroft and H. D. Gesser, J. Znorg. Nucl. Clzem., 27, 1537 (1965).
EXPERIMENTAL
B
Figure 1. Diagrammatic section through balance and vacuums system of the Mettler thermoanalyzer
N
Mechanical Pump
Diffusion Pumps VOL. 40, NO. 13, NOVEMBER 1968
1935
5
TORR,
5 min. ~
Figure 2. Hell jar assembly over TC stick-used hygroscopicity problems of test samples
to eliminate
loss, pressure increase, and temperature profile for nitronium perchlorateat 150 "C
liters. A leak check with the ionization gauge K and thermocouple gauge M showed a leak rate of 5 x 10-4 Torr per minute. After establishing the leak rate the sample was heated at 25 "C per minute to the desired temperature for the isothermal runs with the constancy of the selected temperature being 1 "C. The decomposition gas pressure was measured either by the thermocouple gauge M or a M.K.S. Baratron differential pressure gauge on the gas outlet system. Comparison of the readouts of these two gauges showed good agreement between them. The sensitivity of the Baratron, of 1 X Torr readability for an overpressure of 760 Torr, was the only advantage of using the Baratron. The weight loss was automatically recorded with the pressure gain. A sensitivity of 10 mg per inch was usually used although for more accurate data a sensitivity of 1 mg per inch was also used. The sublimate tended to condense on the cool portion of the quartz furnace tube. Whenever an overpressure of helium was used for determining the pressure dependence of the rate constant, the Baratron was used both as the pressure monitoring system and for setting up the initial overpressure. Materials. Commercial grade nitronium perchlorate (Callery Chemical Co.) was used initially in the as-received form for some exploratory runs. The irreproducihility of the isothermal data with this material, however, necessitated the standardization of all future material. The problem was undoubtedly the extreme hygroscopicity of the nitronium perchlorate. To eliminate this problem all transference operations of material from the stock hatch to thesample containers were performed in a drybox having a stream of pure, dry nitrogen passing through it. Even with this precaution it was found by differential thermal analysis (DTA) that all samples invariably absorbed a certain amount of water. But by using DTA and mass spectrometric analyses it was found that complete removal of water and products of hydrolysis could he accomplished by pumping on each individual sample for a
period of 18 hours in a vacuum line maintained at 10-4 Torr. Samples undergoing this treatment then gave reproducible data. All samples for TBGA were weighed into the platinum reaction vessels in the dry box and then transferred, also in the dry box, to carrying jars containing dry nitrogen. These carrying boxes were opened and the platinum reaction vessels were inserted into the "head" of the TG stick on the thermoanalyzer inside a bell jar fitted with glove ports (Figure 2). Prior to opening the carrying boxes, the balance chamber and the bell jar over the top loading TG stick were pumped down to 10-5 Torr and then filled with dry helium. After loading the reaction cups into the TG head, the quartz furnace was firmly positioned over it before removal of the bell jar. By this transference technique the nitronium perchlorate was never exposed to normal atmospheric conditions. The use of the bell jar proved to be very effective in combating the extreme hygroscopicity problems associated with nitronium perchlorate.
*
1936
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION
The simultaneous measurement of sample weight loss and accumulatory decomposition gas pressure was measured at 10" intervals in the temperature range 80 to 150 "C. These data were accumulated for isothermal decomposition studies. Figure 3 is a plot of the p us. f and w us. f data for 150 "C. The pressure was interpolated from the recorder display by using the millivolt to pressure conversion term. The thermal decomposition of nitronium perchlorate has been established ( 3 , 4 as being composed of two regions : (3) J. N. Mayccck and V. R. Pai Verneker, J. Pkys. Chem., 4077 (1967). (4) J. N. Mayccck and V. R. Pai Verneker, ibid.,in press.
n,
/ I
I
TIME (rnins)
Figure 4. Test for applicability of Equation 1 for sublimation of nitronium perchlorate at 120 "C
+ '/?O, Above 100 "C; 2NOC104 NOzC104 + ClOz + NO2 N02ClOa NO2 + Oe + C1Oe Below 100 "C; NO+2104
+
NOC104
+
+
(1)
(2) (3)
Below 100 "C, the decomposition of nitronium perchlorate involves only reaction ( I ) with the products of decomposition being nitrosonium perchlorate (NOC10,) and oxygen. Above 100 "C completely gaseous products are formed through the intermediary nitrosonium perchlorate. In this particular temperature range the nitrosonium perchlorate does not accumulate. Thus, two pressure-to-weight conversion factors can be calculated, one above 100 "C and the other describing the decomposition below 100 "C. The pressure to mass conversion is accomplished by the gas law :
Figure 5. Dependence of log k on T-' for sublimation of nitronium perchlorate Figure 4 shows that this equation presents a good fit to the data over the a range of 0.2 to 0.9. Calculation of the rate constants, k , from these plots are replotted as log k against T-' in Figure 5. These data show, very clearly, a break at about 110 "C in accordance with the normal thermal decomposition data. The activation energies are 18.7 kcal mole-' below 110 "C and 14.4 kcal mole-l above 110 "C. It is interesting to note that the trend of the activation energies for sublimation of nitronium and nitrosonium perchlorate, Le. 18.7 and 14.4 kcal mole-', follow the trend of the molecular weights, Le. 145 and 129. Trends of a similar nature have been reported for the ammonium halides (5). The sublimation rate equation can be expressed as a linear surface regression rate, A , exp (--E/RT) cmjsec
(5)
A , ( p / l M ) exp ( - E / R T ) moles/cm2 sec,
(6)
B
=
or where V I = 16.0 liters (the constant volume decomposition chamber), PI = the recorded pressure, TI = 300 "K the average temperature of the constant volume, Ti= 273 OK, Pz = 1 atmosphere, and V , the volume the gas would occupy under STP conditions which is the unknown. Since 1 gm mole occupies 22.4 liters at STP and the fractional partial pressures are known from the earlier work, the conversion factor for 27.4 pressure into weight is readily calculated to be 1 Torr mg. A check on this conversion factor was possible from the decomposition of nitronium perchlorate at 130 "C under 450 Torr of helium overpressure. Under these conditions where there was no visual sign of sublimation, a T G weight loss of 149 mg with an associated pressure increase of 5.1 Torr gives a conversion factor of 1 Torr 29 mg. In consideration of the possibility of small errors in the constant volume value and a small amount of sublimation under 450 Torr, the agreement between these two values is quite good. Subtraction of the calculated decomposition mass term from the recorded mass loss data gives the weight of sublimation as a function of time for given temperature conditions. These sublimation data were then converted into fractional decomposition units a , which when plotted against time gave the usual decomposition curves. A process, such as sublimation, which can be assumed to be the evaporation of an ion pair (molecule pair) and occurs on the surface at a rate proportional to the surface area has kinetics that should obey the contracting volume relationship 1
- (1 -
= kt
(4)
u
=
where p is the density and M the molecular weight. On this basis it is not clear why the activation energy should be dependent on the molecular weight of the subliming solid. The recent studies ( 4 ) of the thermal decomposition of nitronium perchlorate under an overpressure of He have shown that the associated activation energies are 25 kcal mole-' ( 5 ) R. F. Chaiken, D. J. Sibbett, J. E. Sutherland, D. K. Van de Mark, and A. Wheeler, J. Chem. Phys., 37,2311 (1962).
0025
-
0.020
r-'o-3
-
-'2 0.015
1
0.010
0.005 l
l
0
n
l
100
,
l
,
200 300 OVERPRESSURE- TORR He
l
,
400
~
,
5
Figure 6. Dependence of k on overpressure of dry helium at 130 "C VOL. 40, NO. 13, NOVEMBER 1968
1937
below 100 "C and 15 kcal mole-' in the range 100 to 150 "C. By comparison of these values with the activation energies for sublimation it is very probable that the rate determining process in the thermal decomposition of nitronium perchlorate takes place in the solid rather than in the gas phase or sublimation process. On the other hand, it is probable that the rate determining processes in the decomposition of nitrosonium perchlorate are gas phase processes. These same activation energies for the normal decomposition were obtained by also reducing the pressure-time data of the TBGA. The pressure dependence of the sublimation rate was measured at 130 "C and 90 "C. The data show (Figure 6, which is the data for 130 "C) a drastic reduction in the rate constant k (calculated from Equation 1) as the pressure increases from Torr to 5 Torr but that further increases in pressure to 450 Torr produce little further reduction in the k value.
CONCLUSIONS The TBGA method of calculating sublimation parameters appears to be sound and applicable t o any material that sublimes and simultaneously decomposes into gaseous products. One disadvantage of the present experimental arrangement is the relatively slow (25 "C min-l) heating rate to the desired temperature. Ideal conditions would be for a step function in the temperature profile. Nitronium perchlorate as the test sample has been good, for the TBGA data shows two sublimation processes, for nitronium perchlorate and for the nitrosonium perchlorate formed below 100 "C. The sensitivity of the sublimation to a n overpressure of an inert gas has also been demonstrated by this technique.
RECEIVED for review June 6, 1968. Accepted August 28, 1968.
Reaction of 2-Methyl-8-Quinolinolwith Aluminum(lll) in Nonaqueous Media Paul R. Scherer and Quintus Fernando Department of Chemistry, Uniaersity of Arizona, Tucson, Ariz. Aluminum(lll) reacted with 2-methyl-8-quinolinol in the presence of diethylamine in chloroform solution to give hydroxy complexes of bis(2-rnethyl-8-quinoIinolato)aluminum(lll). When the reaction was carried out in dimethyl sulfoxide, a 1:l adduct of tris(2-methyl8- q uinoI i no Iato)a Iu m in u m(I I I) a nd d imet hy I s u Ifo x id e was obtained. These compounds have been isolated in crystalline form, their stoichiometry confirmed by elemental analyses, gravimetric and titrimetric determinations, and the compounds have been characterized by single crystal X-ray diffraction data, proton nuclear magnetic resonance, infrared and mass spectra. On the basis of the available experimental evidence, probable structures for these compounds have been postulated and discussed.
IN 1944, Merritt and Walker ( I ) reported that 2-methyl-8quinolinol did not form a precipitate with aluminum(III), whereas 8-quinolinol readily formed an insoluble precipitate of tris(8-quinolinolato)aluminum(III). Since then, 2-methyl8-quinolinol has proved to be a n extremely useful reagent for the separation and determination of many metal ions in the presence of aluminum(II1). The reasons for this anomalous behavior of 2-methyl-8-quinolinol however, have remained obscure. One of the first explanations given by Irving, Butler, and Ring in 1949 (2), was that steric effects involving the 2-methyl groups prevented the formation of the tris complex with aluminum(II1). If complex formation is sterically hindered, the relative ease with which bis(2-methyl-8-quinolinolato)beryllium(II) and tris(2-methyl8-quinolinolato)gallium(III) are formed cannot be understood unless appropriate values for the metal-oxygen and metal-nitrogen distances are arbitrarily assumed. Any explanation of the anomaly on the basis of small differences in the bond lengths in these molecules is questionable in (1) L. L. Merritt, Jr., and J. K. Walker, IND.ENG.CHEM., ANAL ED., 16, 387 (1944). (2) H. Irving, E. J. Butler, and M. F. Ring, J. Ckern. SOC.,1489 (1949).
1938
ANALYTICAL CHEMISTRY
view of the almost complete lack of X-ray crystallographic data on aluminum(II1) and gallium(II1) chelates of 8-quinolin01s. If steric interactions of the 2-methyl group prevent the formation of only the 1:3 complex, it is reasonable to expect to find evidence for the formation of 1 : 2 and 1: 1 complexes of aluminum(II1) with 2-methyl-8-quinolinol. No experimental evidence for the existence of such complexes in aqueous solutions has yet been found. Thus, almost 20 years after the introduction of 2-methyl-8-quinolinol as a selective analytical reagent, its nonreactivity with aluminum(II1) was anomalous and remained unexplained (3). An alternative approach in arriving at a n explanation of the anomaly is to attribute the apparent nonreactivity of the aluminum(II1) with 2-methyl-8-quinolinol to peculiarities in the behavior of the aluminum(II1) ion in aqueous solutions. Evidence for the existence of the aluminum(II1) complexes with 2-methyl-8-quinolinol might be sought therefore, in nonaqueous solvents. In 1962 Ohnesorge and Burlingame (4, on the basis of a series of fluorescence measurements, demonstrated the presence of a 1 : 1 complex of aluminum(II1) and 2-methyl-8-quinolino1, in absolute ethanol. In view of these results, it is of interest to determine whether the 1 :2 and the 1 : 3 complexes can be obtained in nonaqueous media. One such attempt has been made recently by Horton (5) ; the infrared spectra of melts containing 2-methyl-8quinolinol and aluminum(II1) indicated the presence of metal complexes. However, no compounds with a clearly defined stoichiometry were isolated and characterized. In a preliminary report, we have shown that it is possible to synthesize 1 :2 complexes of aluminum(II1) and 2-methyl-8(3) H. Irving and D. L. Pettit, in "Proceedings of the International Symposium," Birmingham University (1962), P. W. West, A. M. G. MacDonald and T. S. West, Eds., Elsevier, Amsterdam, 1963. (4) W. E. Ohnesorge and A. L. Burlingame, ANAL.CHEM., 34, 1080 (1962). (5) G. R. Horton, ibid.,39, 1036 (1967).
