JUNE, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment This investigation was made under the auspices of Research Project 37 of the American Petroleum Institute. Financial support and cooperation from this institute are acknowledged. W. R. Mendenhall carried out the measurements of the JouleThomson coefficients reported. T. Vermeulen determined the directly measured heats of vaporization and the heat capacities of the liquid. The assistance of Louise M. Reaney and D. C . Webster is acknowledged in connection with the calculations.
Literature Cited (1) Burrell and Robertson, U. S. Bur. Mines, Tech. Paper 214 (1916). (2) Cahetet and Mathias, Coompt. rend., 102, 1202 (1886).
681
(3) Coffin and Maass, J . Am. Chem. SOC.,50,1427 (1928). (4) Dana, Jenkins, Burdick, and Timm, Refrig. Eng., 12, 387 (1926). (6) Daniel and Pierron, Bull. SOC. chim., 21,801(1899). ( 6 ) Osborne and Van Dusen, U. S. Bur. Standards, Sci. Paper 315 (1918). (7) Oudinoff, Bull. S O C . chim. Belg., 23, 266 (1909). (8) Parks, Shomate, Kennedy, and Crawford, J . Chem. Phys., 5, 369 (1937). (9) Sage, Kennedy, and Lacey, IND. ENQ.CHEM.,28,601 (1936). (IO) Sage and Lacey, Ibid., 27, 1484 (1935). (11) Sage, Webster, and Lacey, Ibid., 26, 1213 (1934). (12) Ibid., 29, 658 (1937). (13) Ibid., 29, 1188 (1937). (14) Ibid., 29, 1309 (1937). (15) Seibert and Burrell, J . Am. Chem. SOC.,37, 2683 (1915). (16) Young and Jasaitis, Ibid., 58,377 (1936). RECEIVED December 27. 1937.
Nitroglycerin and Ethylene
Glycol Dinitrate Vapor Pressures of Binary Solutions
S
INCE ethylene glycol dinitrate is now extensively used
in commercial explosives in solution with nitroglycerin (glycerin trinitrate) t o increase freezing resistance, vapor pressure data on binary solutions of these compounds are of industrial importance, particularly from the standpoint of volatility. A search of the literature revealed that no measurements of the vapor pressures of such solutions have been reported. As for the individual esters, an examination of the literature showed that vapor pressures of nitroglycerin have been reported by Marshall (4) and Peace (8), Chiaraviglio and Corbino (Z), Naoum and Meyer (IO), and Crater (W), and the vapor pressures of ethylene glycol dinitrate by Crater (2) and Rinkenbach (13). The values reported by the various authors are not in agreement with one another. Marshall (5) gave a review and discussion of previous work and selected from a diagram of all the published results those values of the vapor pressures which he considered most probable. Several papers of a polemical nature have been published by Marshall (6, 6, 7) and by Naoum and Meyer (9, 11, 22). This work was accordingly undertaken to determine the vapor pressures of solutions of nitroglycerin and ethylene glycol dinitrate as well as to check the values reported in the literature for the vapor pressures of the pure compounds.
Preparation of Nitric Ester Solutions ANALYSISOF MATERIALS BEFORE NITRATION. Ethylene glycol and glycerin samples of high purity were used in the preparation of the nitric esters. The following table shows the analysis of these raw materials, carried out according t o International Standard Specification Methods:
J. D. BRANDNER Atlas Powder Company, Tamaqua, Pa.
The vapor pressures of nitroglycerin were measured from 30" to 50" C., and of ethylene glycol dinitrate from 20" to 50" C., using a dynamic method. By extrapolation of the straight lines obtained by plotting the data as logarithm of vapor pressure us. reciprocal of the absolute temperatures, final values of the vapor pressures of the pure niitric esters were selected from 10" to 50" C. Measurements at 40" and 50" C. of the concentration of vapors from binary solutions of nitroglycerin and ethylene glycol dinitrate indicated them to be ideal. Equations were accordingly derived using the Clapeyron-Clausius and Raoult equations for calculating the vapor pressure of any binary solution at any temperature from 10" to 50" C.
Glycerin
Specific gravity Silver teat Analysis, % by wt.: Char Ash Chlorides Acidity (50 cc. required), cc. 1 N NaOH
Ethylene Glycol 1.1176 Light red-brown No color, no ppt. color, no ppt. 1.2620
0,029 0,003 0.0002
0.001 0.0004 0.0001
0.04
0.01
PUREXITRIC ESTERS.The mixed acid used in the nitrations had the following composition: total sulfuric, 49.99 per cent; total nitric, 52.44 per cent. Samples of the esters used were from laboratory nitrations of two glycerin and ethylene glycol samples and were given special treatment in purification and drying. After having been washed with a solution of sodium carbonate for the purpose of neutralizing
682
INDUSTRIAL AND ENGINEERING CHEMISTRY
any excess acid, the esters were washed thoroughly six times with distilled water to remove the lower nitrates. The prepared esters were then dried in desiccators over calcium chloride for several weeks. For h a 1 drying each ester was placed in the Geissler bulbs to be used in the vapor pressure determination, and pure dry air was bubbled through the ester for about 24 hours. Besides this drying, sample B of ethylene glycol dinitrate was further dried by placing Drierite (anhydrous calcium sulfate) in the ester for 4 to 5 hours; then it, was filtered through dry filter paper and put into Geissler bulbs for further drying. The vapor pressures obtained showed that both methods of drying gave comparable results. The stability of the ester was tested during the drying period by examining the air which had been bubbled through the sample. This was done by placing a strip of starchpotassium iodide paper, moistened with a narrow streak of a glycerin-water solution, in the outlet tube of the second Geissler bulb. After each vapor pressure determination, the nitric ester was tested again for products of decomposition. I n no case was any color developed on the test paper. This delicate test for nitrogen oxides showed the esters to be perfectly stable at the maximum temperature used in this work. The purity of the esters was checked by a determination of nitrogen on the du Pont type nitrometer; the results are as follows: Ester Nitroglycerin Ethylene glycol dinitrate
-Per Sample A 18.44 18.34
VOL. 30, NO. 6
Geissler bulb, and an emergent stem corre;tion was aFplied. Temperatu:e control below 30" C. was *0.02 ; above 30 C. it was 10.05 . CHECK AGAINST PUREWATER. The accuracg of the method of determining vapor pressures was checked by measuring the vapor pressure of freshly boiled distilled water. The results of two determinations of the vapor pressure of water a t 25.00' C. were 23.84 and 23.75 mm. of mercury (average 23.79). These results compare well with the value given in International Critical Tables (3) of 23.76 mm. of mercury. The difference between the average measured vapor pressure of water and the accepted value can be accounted for by an error of 0.02' C. in temperature or 0.2 mg. in weight of water vaporized, both of which are within the limits of experimental error.
0.6 04
Vapor Pressure Measurements APPARATUS.The vapor pressure measurements were conducted by a dynamic method in an apparatus somewhat similar to that used by Crater (a) : The air was purified by drawing it through a tube of 4-mesh soda lime, through an absorption bottle filled with lump (anhydrous) calcium chloride, through a U-tube of which one leg was filled with coarse oxalic acid and the other with 8-mesh soda lime, and through a U-tube of 10-mesh c. P. calcium chloride (anhydrous) which was in the thermostat. The air leaving the calcium chloride tube was filtered through glass wool to remove dust and was passed through a block tin coil in order to bring the air to the temperature of the bath. The air was led through two Geissler bulbs in series containing the sample, where it became saturated with nitric ester vapors at the temperature of the thermostat. Leaving the thermostat, the saturated air was through a U-tube of 8-mesh calcium chloride t o prevent dig%: of water vapor back into the bulbs, and the air was drawn into a calibrated 47-liter bottle containin distilled water. Suction for drawing air through t8e apparatus was obtained by siphoning water from this storage bottle. The storage bottle was equi ped with a thermometer and a mercury manometer, and anotier mercury manometer was located at the outlet of the second Geissler bulb. All joints were made with rubber tubing which had been cleaned with hot caustic and distilled water and had been dried in an oven. The rubber tubing was wired on all joints, and all joints except those on the Geissler bulbs were coated with lacquer to prevent leaks. Th,e temperature of the bath was determined t o the nearest 0.01 C. by means of a Bureau of Standards thermometer, located beside the first
! w
t 01 008
006
z
6
0
Cent NitrogenSample B Theoretical 18.46 18.51 18.34 18.42
SOLUTIONS. Solutions of the two esters were prepared by weighing accurately and as rapidly as possible the desired amount of each of the dried liquids into a weighing dish. The solution was mixed thoroughly with a rubber stirring rod, immediately transferred to the Geissler bulbs, and dried further since moisture was picked up during transfer. This drying was accomplished by bubbling pure dry air through the .bulbs. During this drying the bulbs lost a little weight as a result of evaporation of the liquid. This loss was assumed to be due entirely to the ethylene glycol dinitrate, and the slight change in composition was calculated on this basis. This is permissible since the vapor pressure of the glycol ester is about one hundred times as great as that of the glycerin ester.
I"
w
2t
0.01 W
0.008
&
0.006 IJ IT 0.004
2 v)
oz W 0.wz om01 O !!
I 40 1
45
I
35
30 I
1
25
-TEMPERATURE
20 1
-
1
r
10
2
J
5
'5C.
FIGURE 1. LOGARITHM OF VAPORPRESSURE vs. TEMPERATURE
VAPORCONCENTRATION. The ester whose vapor pressure was to be determined was weighed and dried in Geissler bulbs as described above and placed in position in the thermostat. After allowing sufficient time for temperature equilibrium, air was passed through the bulbs. The amount of air bubbled through the nitric ester depended on its vapor pressure. In the case of nitroglycerin from 20 to 25 liters of air were used because of the low vapor pressure, and in the case of ethylene glycol dinitrate from 10 to 15 liters were generally sufficient for accurate measurement. The air was bubbled through the nitric ester a t the approximate rate of 3 liters per hour. Increasing the rate to 4 liters per hour caused unsaturation of the air. When unsaturation occurred, it was easily noticeable by a considerable loss in weight of the second Geissler bulb. No results are included in this paper where the loss in weight of the second bulb exceeded one per cent of the total loss in weight. Readings were taken a t regular intervals of the barometer and the various thermometers and manometers. The technic of drying and weighing the Geissler bulbs was as follows: The bulbs were removed from the bath, and the ends of the tubes were wiped with clean cheesecloth and closed with short pieces of rubber tubing, fitted with glass rods. The bulbs were then wiped dry with cheesecloth and placed in a desiccator beside the balance in order to come to room temperature. After about 15 minutes the bulbs were removed from the desiccator one a t a time, the rubber tubing was removed, the ends of the tubes were wiped with cheese-
JUNE, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
cloth, and the bulbs were weighed. All weights were corrected to vacuum conditions. Vapor pressure data were obtained for pure nitroglycerin a t 30",35", 40", 45", and 50" C.; for pure ethyleneglycoldinitrate a t 20", 25", 30", 35", 40",45"' and 50" C.; and for 20-80, 40-60, 60-40, and 80-20 per cent (dry weight) solutions of ethylene glycol dinitrate and nitroglycerin a t 40' and 50" C.
Calculation of Vapor Pressure The calculation of the vapor pressure of a pure substance from data obtained by this dynamic method was outlined in detail by Crater ( 2 ) . The present method of calculation differs in one respect from that of Crater. I n calculating the vapor pressure of the liquid, Crater assumed that it was under barometric pressure, whereas actually the pressure in the Geissler bulbs is less than atmospheric because of the resistance of the drying train and the head of liquid in the bulbs. The error introduced by neglecting this pressure drop amounts to several per cent and has been taken into account in the present calculations. Thus
where pl = va or pressure of liquid, mm. Hg V , = vokrne of vapor formed at standard conditions, liters Ho = barometer pressure (corrected to 0' C.), mm. Hg - manometer pressure at Geissler bulbs, mm. Hg volume of air at standard conditions, liters
pa :
Table I gives the results of the vapor pressure determinations for pure nitroglycerin and ethylene glycol dinitrate. TABLEI. VAPORPRESSURES
-
T2mp.. Samc, plea 20 26 30 35 40 45
11
It
-Mg.
Run 1
...
Nitroglycerin
Vapor per Liter-
Run 2
...
Av.
...
-
Vapor pressure, mm. Hg
.... ....
Ethylene Glycol Dinitrate Mg. Vapor vapor pressure, per liter mm. Hg 0.43 0.70
0.048 0.078
1.12 0.125 o:oio o.'Oi2 o.'oi6 0.0012 A B 0.027 0.027 0.0020 1.74 0.196 A 0.037 0:042 0.040 0.0030 2.67 0.299 B 0.062 0.062 0.0047 4.02 0.448 0.108 5.79 0.648 50 A 0.104 o:ii2 o.0081 a Samples A and B refer to samples of nitroglycerin and ethylene glycol dinitrate prepared by separate nitrations.
683
The measured and calculated vapor concentrations are listed in Table 11. OF VAPORS FROM NITROGLYCERINTABLE11. CONCENTRATION ETHYLENE GLYCOL DINITRATE SOLUTIONS
Concn. Ethylene Glycol Dinitrate in Soln. Temp., % by wt. Mole fraction ' C. 20.13 0.270 40
Mg. Vapor per Liter Exptl. Calcd.
50
0.74 1.60
0.75 1.65
40,70
0.506
40
50
1.37 2.95
1.36 3.01
59,77
0.689
40 50
1.86 4.12
1.84 4.06
79.70
0.854
40 50
2.24 4.93
2.27 5.01
Results The results given in Table I are shown graphically in Figure 1 where the log of vapor pressure is plotted against reciprocal temperature. The data for both nitroglycerin and ethylene glycol dinitrate give excellent straight lines when plotted in this manner. The h a 1 vapor pressures were obtained by selecting values from an accurate plot of this sort. Since the data closely followed straight lines, it was considered permissible to extrapolate these lines to lower temperatures-i. e., to 10" C. This was done, and the final values selected are given in Table 111. Vapor pressures obtained by other investigators are also given for comparison. The results of Naoum and Meyer (IO)and of Crater ( 2 ) are not in agreement with those found by this investigation in the case of nitroglycerin; their values are in every case higher than those here reported. These high values may have been due to traces of moisture present in the samples of nitroglycerin. At the start of this investigation it was found that the drying procedure recommended by Crater ( 2 ) failed to remove the last trace of moisture, and high and erratic results were obtained until the drying procedure described was adopted. The results given here are in good agreement with those obtained by Marshall and Peace (8) for nitroglycerin, and with Rinkenbach's value for ethylene glycol dinitrate a t 15" C. found by interpolation of his results a t 0" and 22" C.
This method as used did not permit of a direct determination of the vapor pressure of solutions of ethylene glycol dinitrate and nitroglycerin, since, without knowing the composition of the vapor formed, it was impossible to calculate the volume of the vapor. However, the weight of vapor formed per liter could be determined experimentally, and this figure could be compared to that obtained by substituting concentrations of vapors from the pure liquids into an equivalent expression for Raoult's law. The weight of vapor per liter is calculated from experimental data by use of the relation: MOLE FRACTION O F ETHYLENE
where m, = weight of vapor per liter, mg. W = weight of vapor (loss in weight of Geissler bulbs), mg. The theoretical concentration of vapor is obtained from Raoult's law which may be expressed as
c, = C N Q X C, = CEQDY
(3) (4)
where X = mole fraction nitroglycerin in solution Y = mole fraction ethylene glycol dinitrate in solution C = concentration, mg./liter
GLYCOL
DINITRATE
FIGURE 2. VAPOR CONCENTRATIONS OF BINARY SOLUTIONS OF NITROGLYCERIN AND ETHYLENE GLYCOL DINITRATE
Figure 2 shows the good agreement which obtains between the experimentally determined data a t 40" and 50" C. far solutions of nitroglycerin and ethylene glycol dinitrate and the ideal, or Raoult's law values, since all the points lie well along straight lines joining the vapor concentrations of each pure component. The slight deviations from Raoult's law can be attributed to experimental error, especially when it is realized
INDUSTRIAL AND ENGINEERING CHEMISTRY
684
VOL. 30, NO. 6
TABLE111. COMPARISON OF VAPORPREBSURE DATA Nitrogl cerin Vapor Aeasure, Mm. Hg Naoum and Meyer Crater
Temp., Mg. vapor This Marshall C. per liter investigation and Peace . . . 10 0.002 0.00012 15 0.003 0.00021 ..... 20 0.005 0.00038 0:oio 0:00i5 0.00025 25 0,009 '0.00065 ..... 30 0.015 0.0011 0.0033 ..... 35 0.026 0.0019 0:036 0.041 0.0031 0.0065 0.0024 40 0.065 45 0.0050 ... ..... 50 0,108 0.0081 o:&iis a Obtained by interpolation of Rinkenbach's measurements at Oo and 22O C,
...
... ...
.... .... ....
..... .....
.....
that the composition of the solution after drying cannot be determined directly but must be calculated from the initial composition. The fact that nitroglycerin and ethylene glycol dinitrate form perfect solutions has also been confirmed by density measurements made in this laboratory. Although the measurements were not sufficiently precise to warrant publication, they showed clearly an additive relation of volume on mixing the two esters. Applying the Clapeyron-Clausius equation in the following form, (5)
to the curves given in Figure 1, the heats of vaporization of pure nitroglycerin and ethylene glycol dinitrate can be calculated in the temperature range studied: AH, = 19,170 cal,/mole AH, = 16,230 cal./mole
Nitroglycerin : Ethylene glycol dinitrate:
On integration of the Clapeyron-Clausius equation and substitution for po of its value from Raoult's equation, p = POX
(6)
the following equation is obtained: logp =
where p
=
AH - 2.303 RT
+ log x + constant
Evaluating the constant from the data in Table I11 and substituting the heat of vaporization, the partial pressure of nitroglycerin in solution can be expressed by the following equation : =
4y
- -+ log X
+ 10.860
The partial pressure of ethylene glycol dinitrate in solution can be expressed similarly: log p , =
3y
- -+ log Y
+ 10.785
(9)
I n deriving these equations, the absolute zero of temperature was assumed to be -273.2' C. Since the total vapor pressure of the solution is merely (p. p y ) , these equations are useful in calculating the vapor pressure of any binary solution in the temperature range from 10"to 50" c. Vapor pressures of solutions of nitroglycerin and ethylene glycol dinitrate of commercial importance were calculated by means of Equations 8 and 9, and are given in Table IV. The important feature of this table is that it shows clearly the
+
...
Rinkenbach 0:03055
.... ....
.... .... .... .... ....
rapid increase in volatility of solutions of these nitric esters with increase in temperature and ethylene glycol dinitrate content.
TABLE IV. CALCULATED VAPOR PRESSURES OF NITROGLYCERIN ETHYLENE GLYCOL DINITRATH SOLUTIONS % Ethylene Glycol Dinitrate 0 10 20 30 40
--
loo C.
0.0001 0.0027 0.0051 0.0073 0.0093
Total Vapor Pressure, 2OOC. 30° C. 0.0004 0.0011 0.0074 0.0186 0.0137 0.0345 0.0195 0.0490 0.0249 0.0624
Mm. Hg40° C. 0.0031 0.0433 0.0817 0.1159 0.1475
50° C. 0.0081 0,1002 0.1841 0.2608 0.3314
Conclusions I n the temperature range from 10" to 50" C . pure nitroglycerin has an extremely low vapor pressure, even lower than that of mercury. I n the same temperature range the vapor pressure of pure ethylene glycol dinitrate is approximately one hundred times as great. Within the limits of experimental measurement, nitroglycerin and ethylene glycol dinitrate form ideal solutions. The vapor pressure of a solution is almost doubled by doubling the percentage of ethylene glycol dinitrate and is almost tripled by a 10" C. increase in temperature.
Acknowledgment (7)
partial pressure of one component of an ideal solution
logp,
Ethylene Glycol Dinitrate Vapor Pressure, Mm. Hg Mg. vapor This Naoum and per liter investigation Meyer Crater 0.165 0.0185 .... 0.272 0.0304 0.0233 0.439 0.0490 0:033 .... 0,703 0.0781 ... 0.0706 1.12 0.125 .... 1.71 0.193 o:ii5 0.2189 2.65 0.295 ... .... 3.98 0.443 ... 0.4425 5.85 0.655 ... ....
The author wishes to thank M. Brandt and W. J. Taylor of the Reynolds Experimental Laboratory, Atlas Powder Company, for suggestions and advice in the preparation of this paper, and also A. A. Young for preparing the nitric esters used in the work.
Literature Cited Chiaraviglio and Corbino, Gazz. chim. ital., 43, 390 (1913). Crater, W.de C., IND. ENQ.CHEM., 21, 674 (1929). International Critical Tables, Vol. 111, p. 212, New York, McGraw-Hill Book Go., 1928. Marshall, A., J . SOC.Chem. Ind., 23, 158 (1904). Ibid.. 49. 34T (1930). Marahali, A., 2. ges: Schiess- u. Sprengstofw., 24, 177 (1929). Ibid., 24, 422 (1929). Marshall and Peace, J . Chem. SOC.,109,298 (1916). Naoum, P.,and Meyer, K. F., J . SOC.Chem. Ind.. 49. 241T (1930). Naoum, P., and Meyer, K. F., Z . ges. Schiess- u. Sprengstofw., 24, 88 (1929). Ibid., 24, 177 (1929). Ibid., 24, 423 (1929). Rinkenbach, IND. ENG.CHEM.,18, 1196 (1926). RECEIVED February 23, 1938.
