May, 1963
DISSOCIATION PRESSURE OF AMMONIUM PERCHLORATE
does not constitute an important part of the total. There is obviously considerable scrambling of the hydrogens and deuteriums and for the most part they appear to be statistically arranged. However, the observed values and the statistical values arie small so that a close comparison is not warranted. Acknowledgment.-Miss Mary Sawyer ran the mass spectra and tabulated the data. ‘Dr. It. E. Lundin
1077
performed the n.m.r. analyses which were so necessary in correcting the spectra for the isotopic impurities. Dr. C. Wagner very kindly made available the unpublished data on 3-C13-propene. Reference to a company or product name does not imply approval or recommendation of the product by the U. 5. Department of Agriculture to the exclusion of others that may be suitable,
DISSOCIATIOK PRESSURE OF AMRIONIURI PERC’HLORATEI BY S. HENRY IKAMI, WILLISA. ROSSER,AND HENRYWISE Stanford Research Institute, Menlo Park, California: Received November 8, 1968 The dissociation pressure of ammonium perchlorate has been measured in the temperature range of 520-620’ K. by the transpiration method. The data indicate that ammonium perchlorate sublimes by the dissociation process NH4C10Qs) = NHa(g) HC104(g). The heat of dissociation has been found to be 58 f 2 kcal./mole in the cited temperature range.
+
Introduction During the past several years some efforts have been directed to elucidating the mechanism of the decomposition2-’ and combustion*~9of ammonium perchlorate (AP). Bircumshaw and Newman2 found that the decomposition of AP below 513OK. in vacuo left a residue after 20-30% of the salt had decomposed. This residue had a different crystalline appearance from the original material. However, it was confirmed to be AP by analytical and X-ray methods. Friedman and Levylo in their preliminary study of the reaction between NH3 and HC104 reported that solid formation was observed in the mixing chamber under certain experimental conditions which gave a crude measure of the dissociation pressure of 4P. This observation suggests that a dissociative process is involved in the sublimation of AP. It is apparent that the dissociation pressure of AP represents an important parameter in the anal.ysis of the combustion mechanism of solid propellants based on this oxidizer.
composition and dissociation products of AP were carried away in a helium gas stream passing in an upward direction through the glass frit and the powdered sample. The volumetric flow rate of helium was determined by suitable calibrated flowmeters. All experimental measurements were carried out on that portion of AP which remains after decomposition of 20 to 30% of the original sample.2 Before the dissociation pressure measurement, the sample was heated for approximately an hour. During the initial stage of decomposition a rapid exothermic reaction occurs which may elevate the temperature of the powder bed as much as 100°K. above the bath temperature with the evolution of brownish-yellow vapors. The residual material was a porous, amorphous solid, which crumbled easily. At this time a water-cooled cold finger was quickly placed in position in the transpiration cell (Fig. 1). The distance between the cold finger and the sample was about 2 cm. in most experiments; in some cases it was increased to 4 cm. The temperature of the water in the cold finger was maintained a t approximately 340 f IO’K. to prevent the condensation of H20. After a suitable reaction time, the finger was removed carefully and the white sublimate washed into a flask with about 50 ml. of distilled water. The solution was analyzed primarily for NH4+, Clod-, and C1- in some ca~es.12-1~The dissociation pressure of AP may be readily deduced from the flow rate of helium and the quantity of AP deposited on the finger.
Apparatus and Procedure.-The reagent grade AP from Matheson, Coleman and Bell Company was fractionated mechanically into two particle size ranges, 43-61 U, and > 61 p. I n EL typical experiment a weighed sample of fractionated AP was placed in a glass-fritted cell (Fig. 1) and brought to the desired temperature by immersing the cell in a bath of molten salt.ll The temperature of the molten bath was carefully regulated and measured with a glass-sheathed iron-constantan thermocouple. The de-
Results and Discussions I n a number of runs the following major parameters were varied: (1) fl~owrate of helium gas, (2) reaction temperature, (3) initial mass of AP, (4)initial particle size, and ( 5 ) partial pressure of NH3 added to carricr gas. As shown in Table I the sublimates analyzed were found to contain equimolar quantities of NH4+ and Clod-; therefore, the results may be interpreted in terms of an equilibrium djssociation of “&IO4
(1) This work was supported b y the Office of Naval Research, Department, of the Navy. (2) L. L. Bircumshaw and B. H. Kewman, Proc. Rov. SOC.(London), 8241,115 (1954); A221, 228 (1955). (3) L. L. Bircumshaw and T. R. Phillips, J . Chem. Sue., 4741 (1957). (4) A. K. Galwey and P. W. M. Jacobs, ibid., 837 (1959); 5031 (1960). ( 5 ) A. K. Galwey a n d P. W. M. Jacobs, Trans. Faraday Sac., 65, 1166 (1959); 66, 581 (1960). ( 6 ) A. K. Galwey and P. W. M. Jacobs, Proc. R o y . Soc. (London), 8254, 455 (1960). (7) P. W. M. Jacobs and A. R. Tariq Kureishy, “Eighth Symposium (International) on Combustion,’’ The Williams and Wilkins Company, Baltimore, Maryland, 1962, p. 672. (8) R . Friedman, R . G. Nugent, X. E . Rumbel, and A. S. Scurlock, “Sixth Symposium ([nternational) on Combustion,” Reinhold Publ. Corp. Kew York. N. Y., 1957, p. 612. (9) J. B. Levy and R. Friedman, “Eighth Symposium (International) on Combustion,” The Williams a n d Wilkins Company, Baltimore, Maryland, 1962, p. 663. (10) R. Friedman and J. B. Levy, Final Technical Report AFOSR 2005, Atlantic Research Corporation, Alexandria, Virginia (1961). (11) J. A. Beattie, Rev. Sci. Inrfr., 2, 458 (1931).
SH4C104(s)
=
NH3(g)
+ HC104(g)
(1)
Since equivalent amounts of KH3 and HClO, are praduced, the dissociation pressure P d is equal t o the sunn of P N Hand ~ P H C provided ~ O ~ there is no excess of either gas initially. The calculated dissociation pressures based on the analyses of NH4+ and C104- are in satisfactory agreement. The small variations observed are within the precision of the analytical techniques estimated to be &lo%:, for NH4+ and C104-. Based on (12) F. D. Snell a n d C. T. Snell, “Colorimetric Methods of Analysis,” 3rd. Ed., Vol. 11, D. van Noatrand Co., Inc., New York, N. Y., 1949, p. 816. (13) E. A. Burns a n d R. F. Muraca, A n a l . Chem., 82, 1316 (1980). (14) D. M. Coulson a n d L. A. Cavanagh, abod., 82, 1246 (1960).
S. H. INAMI, W. A. ROSSER,AND H. WISE
1078
Vol. 67
TABLE I EXPERIMENTAL CONDITIONS AND DATAON DISSOCIATION VAPORPRESSURE MEASUREMENTS Initial maas of A P
Collection time, min.
Total moles in sublimate, X loa--
Runs
(OK.)
(g.)
Flow rate, eo./ min.
1 2 3 4 5
523 523 525 525 528
1.00 2.00 2.00 6.00 2.00
152 152 152 152 152
90 135 105 90 105
0.012 .016 .014 .013 .021
6 7 8 9 10 11 12 13 14
537 538 547 548 551 551 551 561 561
2.00 2.00 6.00 2.00 1.00 1.00 2.00 2.00 2.00
152 152 1-52 152 293 152 152 152 1-52:
85 70 120 60 100 90 60 60 60
,024 .029 .047 .034 .I13 ,053 .038 .047 .057
15 16 17 18 19 20
561 561 562 577 575 573
2.00 6.00 2.00 2.00 2.00 2.00
293 152 293 152 293 152
63 60 60 60 60 60
.097 .051 .0045 .098 .I9 .090
21 22 23 24
588 588 600 600
2.00 2.00 2.00 2.00
152 152 152 152
63 60 60 30
.19 .20 .027 .15
Bath temp.
2.00 152 25 602 2.00 152 26 615 2.00 152 27 616 Initial particle size >61p, and,
r MANOMETER MERCURY
HELIUM TANK
-mole
N&+
c104-
c1-
.. ..
..
..
..
..
..
0.0017 .OOlO
0.024
.. ..
.. .. .. ..
..
.035
.. .. ." .. ..
.I9 .21
..
.19 .OS5 .16 .17 nil .15
.. ..
.. .. .0025
..
Clod-
..
..
.. ..
NH4 +
0.032 ,028 .032 ,035 .049
.070 .079 .098 .14 .I4 .14 .15 .20 .23
*. .. ..
..
POS
..
POS POS
.027
..
..
.. ..
0.057
.. ..
AP sample heated for more than 4 hr. before collection
..
.I4
.. ..
..
.. ..
..
43-61 43-61
p p
Distance between powder bed and cold Gnger is 4 cm.
*.
.. ..
.40 .41 .37
.40 .32
* 74 .81
.64 .70
1.26
1.23
..
Remarks"
43-61 /IL 43-61 p
..
20 cc./min. NHs added AP sample heated for more than 4 hr. before collection 20 cc./min. NH, added Distance between powder bed and cold finger is 4 cm.
.I5 1.19 .I3 .0087 30 1.09 .28 .. 2.29 30 .. *. .27 2.24 .28 30 2.26 .0018 distance between powder bed and cold finger 2 cm., except where noted.
FLOWMETER
TRANSPIRATION CELL DURING COLLECTION OF SUBLIMATE
SALT BATH
Fig. 1.-Schematic
Vapor pressure, mm., -based on--
diagram of apparatus.
these data the dissociation pressures are plotted against the reciprocal of the absolute temperature as shown in Fig. 2. The resulting line may be expressed by the equation
+ 10.56
log P (mm.) = - 6283'7
T
(2)
In our experiments the flow rate of the carrier gas was selected sufficiently high so that the mass transfer from the powder to the cold finger was not diffusionlimited. Yet the flow velocity was kept low enough to eliminate mechanical carry-over of particulate matter. The absence of mechanical transport of powdered AP particles onto the cold finger could be demonstrated by: (1) the agreement observed in experimental measurements of dissociation pressures in which the flow velocity was increased by a factor of two, and (2) the non-appearance of a deposit at temperatures a t which
dissociative sublimation is negligibly small. Under these experimental conditions there is no evidence of any serious cooling of the powder, since the results obtained are comparable within the experimental precision when the finger is placed 2 or 4 cm. from the bed (Table J, miis 14 and 24). I n addition, it can be shown that the decomposition of HCIOl is negligibly small in the temperature range studied from the rate constant for the decomposition of gaseous perchloric acid reported by Levy.15 To test the saturation of the carrier gas by the dissociation products, the flow rate of helium gas was varied by a factor of two and the initial mass of the sample by a factor of six. A decrease in the flow rate of helium and ai1 increase in the initial mass of the sample will tend to favor saturation of carrier gas since the residence time of the products is increased. Under most favorable conditions (helium flow of 152 cc./min. and a 6-g.sample) the results obtained were comparable to runs made under most unfavorable conditions (low high flow). mass Experimentally the dissociation pressure measurements were limited to a temperature range from 510 to 620OK. At lower temperatures the quantity of sublimate collected within a reasonable length of time was too small for analysis; a t temperatures above 62OOK. the decomposition of the perchloric acid can no longer be neglected.
+
(15) J. B. Levy, AFOSR TN 1555, Atlantic Research Corporation, Alexandria, Virginia (1961).
May, 1963
DISSOCIATION PRESSURE OF AMMONIUM PERCHLORATE
During the determination of the dissociation pressure of AP, it has been observed that dissociation (reaction 1) and thermal decomposition occur simultaneously. After the sample was heated for an hour a t 513OK. the pressure of the decomposition products calculated from the composition of the gas mixture as determined by thermal conductivity measurements was found to be roughly three times greater than the dissociation pressure. Experiments at 498OK. and a t 573OK. in which the sample was heated for more than four hours before starting collection of disociation products gave pressure data in good agreement with those runs in which the sample was heated for a much shorter time (Table I, runs 5 and 20). On the basis of these results it may be assumed that the thermal decomposition reaction does not affect the dissociative sublimation. As indicated in Table I, the sublimates were found to contain, besides NH4f and c104-, small amounts of C1- (2-3 mole %). I n the sublimates collected from experiments in which 20 cc./min. of NHa were introduced to the carrier gas, equimolar amounts of n " 4 + and C1' were found to be present; however, only traces of c104-were detected (see runs 17 and 23). A. simple calculation shows that when NH3 is added the pressure of HC10, is suppressed to nearly zero. From these results it is reasonable to assume (1) that the chloride ion is most likely present as SH4C1 arising from the interaction of Clz (thermal decomposition product) with NH,
-
60
60-
30 -
05 40
-
20
10-
:0 8 E
E
0 6 -
w
0 5 -
7
04-
I
Y)
03-
z
P
2
02-
U YI
z 01-
008
-
006
-
003 -
005
004
002
-
0.01
and (2) that the AP sublimes by the dissociation process (reaction 1). As indicated by reaction 3, for each mole of NH3 that reacts with Clz, "4 of a mole is recovered as KHa +. Since the chloride analyses were made in only a few runs and the precision of the analysis of C1- was probably flo%, no attempts were made to compensate for this small loss of NH4f. Due to this small loss the error in the calculated dissociation pressure based on the NH4+is estimated to be smaller than the analytical errors. On the other hand, the loss of KH3by reaction 3 reduces the amount of HClO4 on the cold finger. Consequently, the error in the calculated dissociation pressure based on the c104-analysis is somewhat laxger. The entropies of sublimation with dissociation a t pressure of 10 mm. were calculated for "4C1," ".4?j03," and XH4C104 and are listed in Table 11. The entropy of AP is consistent with the other ainmonium compounds which further suggests the equilibrium dissociation of ilP (reaction 1) rather than sublimation of a xH&104 molecule as an entity. The latter fact is further substantiated by the pronounced suppression of sublimation by ammonia gas in the carrier stream as observed in our experiments. A comparison of the experimental heat of dissociation with a theoretical value based on the heat of reaction 1 is made difficult because of lack of information concerning the heat of formation of gaseous HC104. An estimate of the heat of reaction a t 298OK. may be (16) W. H. Rodebush and J. C. Miohalek, J. Am. Chem. Soc., 5 1 , 748 (1929). (17) G . Feiok, abad., 7 6 , 5858 (1954).
10'79
1.4
I.5
I.7
I .6
Fig. 2.-Dissociation
f
1.8
1.9
2.0
x 103 P K I - ~
pressure of ammonium perchlorate.
TABLEI1 ENTROPY OF DISSOCIATION
Compounds
AH dissoo. (k?al./mole)
Temp. at which dissoo. press. = 10 mm. (OK.)
39.8 485 NH.&Oa( 1) 39.4 483 NH,Cl( 9) 657" n"dC104( S) 58 Based on extrapolation to 10 mm.
A S a t cited temp. (oal./deg. mole)
-82 433 -88
obtained by using the following values NH4ClO4= -70.7 kcal./mole; KH3(g),19(AH&98 (s),l*(AHf0)298 = -11.0 kcal./mole; HC104(1),19(AHf0)298 = -9 8 kcal./mole. The value of (AHf0)298 for HC104(g) of -0.4 kcal./mole is based on the assumption that the heat of evaporation of HC104(1) is the same as that of HNO3(l), Le., 9.4 kcal./mole. From these thermodynamic data, the heat of dissociation of NH4C104(s) is calculated to be 59.3 kcal./mole, a value only slightly different from that obtained experimentally at higher temperatures. Acknowledgment.-The authors are indebted to Mr. Oliver Smith and Miss Elizabeth McCarthy for the chemical analyses. (18) A. A. Gilliland and W. H. Johnson, J. Research Aratl. B u r . Standards, M A , 67 (1961). (19) National Bureau of Standards Report S o . 7437, "Preliminary Report on the Thermodynamic Properties of Selected Light-Elements and Some Related Compounds," Jan., 1962.
