eGERWARD A. COOK, EDWARD SPADINGER, ALFRED D. KIFFER, AND CHARLES V. KLUMPP Tonawanda Research laborafory, Linde Air Products Co., A Division of Union Carbide and Carbon Corp., Tonawanda,
N. Y.
Explosion limits o f ozone-containing mixtures are o f importance in connection with the safe production, handling, and trensportation o f ozone. These explosion limits vary with the strength of the stimulus and with other conditions. However, figures can b e given for ozone concentrations a t and below which explosions would not b e expected in single-phase gaseous or liquid mixtures o f ozone and oxygen free o f any contaminating organic material or other type o f ozone-decomposition catalyst. Solutions containing appreciably more ozone than the limits may detonate with great violence when stimulated b y a sufficiently strong spark or heavy impact, and sometimes even when no obvious strong stimulus i s observed.
ECENT increasing use of ozone as an industrial chemical has made i t desirable to find out under what conditions gaseous and liquid mixtures of ozone and oxygen mag be handled safely. I n considering possible Txaps to store and ship ozone, one might think first of handling it as a gas. Experience has shown, hoRrever, that at atmospheric pressure and room temperature ozone gas, either pure or diluted, decomposes spontaneously in all kinds of vessels, even those of apparently clean glass, at a rate too high to permit storage for more than a few hours or days without appreciable loss. The possibility of storing ozone in solution in some liquid solvent at room temperature might be considered; however, with all solvents tried so far, solutions at room temperature lose their ozone content by spontaneous decomposition of the ozone. A more promising possibility is the handling of liquid ozone, properly diluted, at low temperatures. Therefore a preliminary investigation of the safety of liquid solutions of ozone in oxygen is reported. Since it is necessary to handle ozone in gaseous form before it can be condensed to a liquid, experiments on the explosibility of gaseous mixtures of ozone and oxygen m-ere carried out first. Under certain conditions, mixtures of ozone and oxygen explode when ignited by a spark or other stimulus. The conditions under which explosions will take place may be specified in a manner analogous to the explosion limits of mixtures of oxygen (or air) with combustible gases. I n the case of ozone, no combustion is involved, and there is only one limit instead of the tmo (‘(upper’’ and “lower”) which are familiar in the case of combustibles. In the absence of gaseous impurities, this aingle ozone limit varies with pressure, temperature, shape and dimensions of the container, nature and area of surfaces to which the ozone-oxygen mixture is exposed, and .ryith the energy of excitation. At ozone concentrations above the explosion limit, the violence of the explosion increases with increasing ozone concentration and is believed to be greatest for 100% ozone. The only pertinent reference found in the literature before this work was undertaken x a s an article by Riesenfeld (16), a pioneer in research on the properties of ozone. Rieeenfeld stated that the explosion limit for gaseous mixtures of ozone and 736
oxygen in a glass gas buret is about 12 mole % ozone. Pressure and temperature were not specified, but are presumed to be 1 atm. and about 20” C., respectively.
Gas-Phase Explosion Limits Description of Method and Results. Preliminary tests were made by sparking gaseous ozone-oxygen mixtures at atmospheric pressure and about 2.5’ C. in a 2.5-cni. i.d. glass bulb. Stopcocks used for filling the glass bulb were greased with hpiezon hI grease. Instead of a sharp explosion limit, partial decomposition of the ozone took place as a result of the sparking. When the starting mixture contained 9.5 mole % ozone, 20% of the ozone was decomposed by the spark; at each higher ozone concentration tested, the extent of the decomposition increased until, viith 12.9% ozone, 95% of the ozone was decomposed by the spark. After these tests the apparatus was redesigned as shown in Figure 1. The stopcocks were lubricated with an entirely inorganic grease ( 1 4 ) made by stirring colloidal silica into concentrated sulfuric acid, and the size of the explosion bulb was increased to 9.9 cm. i.d. The bulb had tungsten electrodes mounted so that the spark gap was in the center of the sphere. The sphere mounted next to it mas of the same size. By the following procedure both spheres were filled with ozone of the same concentration. The entire system was evacuated through stopcock E. Then the desired amount of ozone mas fed into both vessels simultaneously by opening stopcocks A , B, C, and D, thus flashvaporizing a little ozone-oxygen mixture (containing about 17 mole yo ozonc) nithdramm from the bottom of the liquid storage reseivoir, until the pressure in the bulbs was at a precalculated value. Stopcock A was then closed and E opened to add oxyeen gas simultaneously to both vessels until the pressure was 760 mm. of mercury The stopcocks on both vessels Twre then immediateiy closed and a piece of solid carbon dioxide was applied for a few minutes to the side of each to help in mixing the gases. The cooled spot was allowed to warm to room temperature, and a spark was then passed betveen the electrodes in the explosion vessel.
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~
~
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~
At no time was any flash observed that could be ascribed to an explosion, even when the test was viewed in the dark. Flashes were probably not intense enough to be seen against the brilliant light emitted by the spark. No noise was heard, nor was the vessel shattered, in tests with up to 17 mole % ozone. Whether or not an explosion took place was decided by observing if the pressure (indicated by the manometer) increased after sparking in accordance with the reaction, 203 302. In any event, the gases in both vessels were analyzed by purging them through separate potassium iodide solutions with a stream of oxygen ( 2 ) . When no explosion took place, the composition of the gases in the two bulbs as determined by analysis for ozone agreed very closely. When an explosion took place, there was very little or no ozone left in the sample from the explosion vessel. The starting composition was taken as that determined by analysis in the reference bulb. The stimulus applied was a d.c. spark. It was made by slowly charging a condenser, connected across the gap, through a bank of resistors. When the voltage was sufficient to cause a spark discharge, the condenser discharged itself through the gas. The resistance of the bank was high enough so that no appreciable current entered the spark from the d.c. source during the duration of the spark. The energy of the spark was calculated from values for the voltage at the time the discharge took place and the capacitance of the condenser (6, 8). By adjusting the distance of the gap between the tungsten electrodes and varying the capacitance of the condenser the spark discharge voltage was varied. I n the tests made with the apparatus shown in Figure 1, there was a clear distinction between explosive and nonexplosive mixtures. No composition was encountered that gave appreciable partial decomposition when sparked. This result may have been due to either the larger diameter of the explosion vessel (less surface to stop chain reactions) or to discontinuing the use of the organic stopcock grease, or to a combination of these. Altogether, 32 tests were made. All were consistent with the results given in Table I. The spark energies given should be considered semiquantitative only, since the quenching effect of the spark gap (IS)mas not taken into consideration. .-)
Table 1.
Spark Explosion Tests on Gaseous Mixtures of Ozone and Oxygen
(Made at roam temperature and 760 mm. mercury pressure in 9.9-cm. i.d. glass bulb) Energy of Spark Concn. of Ozone, Mole yo Spark, Gap, NO Joule Mm. explosion Explosion 15.5 0.004 15.0 10.4 0.05 11.1 10.36 11.45b 0.17 9.1 9.6 0.47 4 Not recorded. b Ozone prepared from oxygen containing less t h a n 2 p.p.m. total impurities.
Because a 0.47-joule spark is much more violent than static sparks would normally be, it i s concluded that in the absence of impurities, gaseous mixtures of ozone and oxygen a t atmospheric pressure and room temperature are reasonably safe from explosion if the ozone concentration is not over about 9 mole % (13 wt. %). Discussion. After most of this work had been completed, an article by Schumacher (18) appeared. His experiments were similar to those described above, except that he used a heated platinum wire as a stimulus instead of a spark. I n an 11-em. diameter glass bulb (18, p. 238) he obtained partial decompositions ranging from 5.3% decomposition for a gas mixture containing 6.7 mole % ozone to 90% decomposition when April 1956
there was 11.2% ozone in the mixture. Schumacher's results are similar to the preliminary ones obtained by these authors. The partial decomposition might have resulted either from the rather prolonged effect of the hot platinum wire or from the catalytic effect of stopcock grease. No mention of the kind of lubricant used on his glass stopcocks is made in this particular article, but in another article (17, p. 199) in the same series, Schumacher mentions the use of a specially prepared stopcock grease having very low vapor pressure. This grease probably is similar to the one with which these authors obtained partial decomposition upon sparking. Traces of organic matter are known to catalyze ozone decomposition. GLASS CAPILLARY
A
,cL
rr
TO MANOMETER
u
Q
LIQUID 03'02
Figure 1.
Gaseous ozone explosion limit apparatus
In later experiments with a hot platinum nire as stimulus, Schumacher obtained an explosion limit of 11 mole % in a n 11em. diameter glass bulb a t 20' C. and 1 atm. pressure; 90% of the ozone was decomposed. This result corresponds to the value for a stimulus of 0.05 joule. At 10.3 mole % ' ozone Schumacher decomposed 49% of the ozone present (18, p. 238). He ascribes this to decomposition on the hot wire. Although the gas-phase explosions which these authors obtained in the range below 17 mole % were so mild that they did not shatter the glass bulb, experiments a t the University of Missouri (6) showed that even a t concentrations of ozone as low as 9.2 mole % it is possible with very strong stimulus t o obtain detonations in the gas phase if the containing vessel is cylindrical and has sufficient length. Mechanism of Gaseous Ozone Explosion. An ozone explosion starts with a stimulus, which is usually some form of energy such as an electrical spark or heat generated by impact or from a chemical reaction. The most likely initial reaction seems to be:
X
+ + 24.1 kcal. -, + 0 + X 0 8
0 2
(1)
in which X is the atom, molecule, or electron which conveys the energy to the ozone. This reaction is endothermic by about 24.1 kcal. a t 1250" K. ( 7 ) and probably about the same a t room temperature. The value of 24.1 kcal. also represents the probable activation energy for the decomposition of gaseous ozone. The next step is a very exothermic reaction.
0 $. 0
3
+.202 + 93.2 kcal.
(21
When the ozone concentration is sufficiently high, the heat of Reaction 2 results in the dissociation of more ozone by Reaction 1, thus setting up a chain reaction. Near the flame front, much of the excess thermal energy is used up by the dissociation of oxygen :
X
+ 02 + 117.4 kcal.
-.+
20
+X
(3)
A part of the energy is probably radiated away: 2 0 -C
0 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
+ hv
(4)
737
For further details the reader is referred to the interesting quantitative description ( 7 ) of an ozone flame front published by Hirschfelder and associates. Comparison with Acetylene. The statement has been made that if ozone were sufficiently pure it, like acetylene, should be stable enough t o permit storage and transport at rooni temperature, provided suitable precautions were taken against explosion. (In the case of acetylene, these precautions include dissolving the acetylene in acetone and filling the acetvlene container with a porous mass which will effectively prevent the propagation of any explosion.) The fact is, however, that no sample of ozone has yet been prepared, so far as the authors know, which does not decompose spontaneously a t room temperature a t an appreciable rate. The following comparison is of interest in this connection.
available for the energy of activation of ozone is 24 kcal. ( 7 , 1 9 ) . For the thermal decomposition of acetylene a t temperatures in the range 450" to 660" C. the energy of activation has been estimated a t 29.9 (3) and 40.5 ( $ 1 ) kcal. per mole. Assuming that these figures would not be much different a t room temperature, the best values available f3r the energies of activation of ozone and acetylene are consistent with the experimental fact that ozone gas does, but acetylene gas does not, decompose spontaneously at, room temperature. Therefore the spontaneous decomposition of ozone probably is due primarily to the instability of the ozone molecule rather than to catalysis, but the rate of this decomposition may be hastened greatly by catalysis.
liquid-Phase Explosion Limits Sothiny has been found in the published literature on the explosion limits of liquid mixtures of ozone and oxygen. Spark Tests on Small Samples. Spark explosion tests were first made on small quantities (0.5 to 1.0 ml.) of liquid ozone-oxygen mixtures, using glass explosion vessels like the one shown in Figure 2 and apparatus as shown in Figure 3. To prevent contact of ozone with the mercury a little concentrated sulfuric acid was kept on the mercury in the manometer.
GLASS
WIRE
i
Figure 2.
Liquid ozone explosion vessel
Both ozone and acetylene are thermodynamically unstable a t room temperature, as shown by the following values ( 1 6 ) for 25' C.: H e a t of formation (endothermic). koal./mole Free energy required for formation, koal./mole
Acetylene
Ozone
54.2 50.0
34 0 39 1
From these figures it is seen that acetylene is thermodynaniically more unstable with respect to carbon and hydrogen than ozone is with respect to oxygen. Experimentally, it is known that undiluted ozone and acetylene are both highly explosive in their gaseous and liquid states. The same is true of solid acetylene. Experimental data are not available on the explosibility of solid ozone, but there is good reason to think that it tvould explode with great violence if sufficiently stimulated. Pure acetylene gas does not decompose spontaneously a t room temperature, n.hereas even the purest ozone made so far does. The most stable ozone described in the literature ( 2 3 ) was reported to decompose a t a slow but measurable rate, even a t 16" C. Samples of ozone r e r e prepared in this laboratory from oxygen by heating potassium permanganate and purifying until the total concentration of impurities mas less than 2 p.p.m. as analyzed by the very sensitive method of Shepherd (20); all showed measurable, spontaneous decomposition a t room temperature in clean glass containers. The question arises: I s this slow decomposition at room temperature of even the purest gaseous ozone due to catalysis by the glass or by something on its surface, or is it due to the instability of the ozone molecule? A reaction which has a measurable rate a t room temperature and a t ordinary pressures usually does not have a n energy of activation much over 24 kcal. per mole ( 1 2 ) . The best figure
538
Figure 3.
Part of apparatus used to determine liquid ozone explosion limits OS-mi. samples
Vessels B and C (Figure 3 ) were evacuated. Vessel C was then filled with a gaseous mixture of ozone and oxygen of known composition. A part of this mixture was condensed into explosion bulb B by cooling 13 with liquid nitrogen. Since the gas mixture had to pass through the capillary manifold, and since the stopcock above vessel C was closed as soon as the flow of gas out of vessel C stopped, there was no preferential f l o w of ozone into vessel B. From pressure readings made before and after the condensation, and from the known volume of vessel C, the weight of condensed liquid plus gas in vessel B was calculated. Vessel C was then again evacuated and oxygen allowed to vaporize from B t o C. (The vapor pressure of ozone is negligible a t -183' C. and below.) By reading the pressure and determining the ozone concentration in the vapor collected in C, the quantity and composition of the solution left in B was calculated. This solution was stirred by tapping the tube leading from the manifold t o B. A high voltage a.c. spark was then passed through the liquid to determine whether or not it was explosive. Just before sparking the solution, the operator (Tvearing gloves)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ROCKET PROPELLANTS reached behind the shield and replaced the Dewar vessel with a beaker full of liquid oxygen. (The explosion usually shattered the beaker.) This procedure saved breaking a Dewar every time a n explosion took place. I n some of the tests, especially with higher concentrations of ozone, the explosion vessel was separated from the manifold below stopcock D by means of a torch. This separation inade it impossible for the force of the explosion t o whip vessel B around in such a way as t o injure the manifold.
PISTON
1
GL
Figure 4.
TOP VIEW O f STEEL CYLINDER WITH EXPLOSION BULB IN PLACE
Part o f apparatus used for impact tests
bulb into the capillary.) Next the piston was inserted through the slot so t h a t i t rested lightly on the thin glass bulb. The 500-gram weight was then released. When the weight struck the piston, the piston sheared o f f and crushed the thin glass bulb, and compressed the liquid into the bottom part of the hole, t o which the slot did not extend. The operators were, of course, protected by gloves, goggles, and shield. Whether or not a n explosion took place was determined by sound. I n each test the weight was immediately dropped a second time. If no explosion resulted from the second impact, it was assumed t h a t the decomposition was either complete the first time or t h a t the mixture was safe. Nineteen tests were made. Although consistent explosions were not obtained until the apparent ozone concentration reached a value of over 23.0 mole %, there were indications of partial decomposition a t 23.0 mole yo and a little below. The results are in reasonably good agreement with the spark explosion tests. Spark Tests on Larger Samples. I n order to confirm the spark and impact test results obtained with 0.5 to 1.0 ml. of liquid, experiments were carried out on larger volumes (10 to 21 ml.). Because of the great violence of explosions of liquid ozone, these exDeriments were not conducted in the Research Laboratorv building, but inside an auxiliary building in a cubicle surrounded on four sides by steel plates. The explosion was observed with the help of a mirror through a slot in one of the plates. The oxygen used to make and dilute the ozone was regular Linde cylinder oxygen purified only by passing it through two traps cooled by a dry ice-acetone mixture. A part of the apparatus is shown in Figure 5. The manifold, which was outside the cubicle, was made of 2mm. i.d. capillary tubing. The manometer contained mercury which, on the ozone side, was covered with concentrated sulfuric acid. The ozone-oxygen mixture was led inside the cubicle t o the combined liquefaction, storage, and explosion vessel cooled with boiling liquid oxygen. When the total gas flow was 1
Attempts t o obtain sparks underneath the liquid by applying 15,000 volts d.c. failed. The actual a.c. voltage successfully used was not measured, but was probably about 20,000 volts. All stopcocks were greased with a mixture of colloidal silica and concentrated sulfuric acid ( 1 4 )to avoid the catalytic effects of traces of organic matter. Spark gaps were varied from 0.6 t o 1.05 mm. Altogether, 12 tests were made a t various concentrations of ozone. The lowest apparent concentration a t which a n explosion took place was 18.6 mole %; this was a mild explosion. Consistent, violent explosions were not obtained until the apparent concentration was 21 mole % ' or higher. This work was done before the exact solubility 1/2 INCLl of ozone in liquid oxygen a t -183" C. had been STEEL PLATE DRY ICE-ACETONE determined. This figure has now been estabCOOLED TRAP lished ( 1 1 ) as 17.6 mole % ozone. It follows, therefore, that in the explosion tests a small quantity of the heavy, ozone-rich phase must have been present whenever the apparent ozone concentration was above 17.6 mole %. This second phase could not be detected visually. Impact Tests. An elongated glass bulb (Figure 4) was sealed t o a glass capillary tube, which in turn was sealed to the ozone manifold SAMPLING BULB (shown in Figure 3) instead of vessel B. By the FOR ANALYSIS same general technique as described above, about 0.5 ml. of a liquid mixture of ozone and oxygen TEST VESSEL was condensed into the small bulb on the capillary. The explosion vessel was removed from the manifold with a torch and then kept in a Dewar frlERCURY SAFETY BUBBLER vessel filled with liquid oxygen until the impact tester was ready. Figure 5. Apparatus for liquid ozone explosion tests Figure 4 also shows a steel cylinder with a slot 5- to 25-mi. samples cut in one side. This cylinder was equipped with a removable close-fitting piston. Above the piston was mounted a vertical 1-inch iron pipe, 10 feet high, which acted as a guide for a 500-gram weight. cu. foot per hour or less and the pressure was 10 em. of mercury At the start of a test, the 500-gram weight was suspended a t above atmospheric, all the ozone and most of the oxygen conthe top of the guide pipe. The bottom of the slotted steel densed in the liquefaction vessel. cylinder was placed in a copper vessel containing a little liquid When sufficient liquid oxygen and ozone had collected, the oxygen, but no liquid oxygen was allowed t o enter the slot. stopcock was closed and p a r t of the oxygen was boiled off the Then the bulb containing the ozone sample was quickly taken solution by applying a vacuum to the space above the main out of the Dewar and laid in the cylinder as shown in Figure 4. portion of the liquid. After the volume of solution had been (The capillary was kept a t a slight angle with the floor so that reduced sufficiently by this step, the evaporation was halted and none of the liquid ozone-oxygen mixture could run out of the the solution was allowed t o stand for at least 15 minutes so t h a t
April 1956
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739
it warmed up to the temperature of boiling liquid oxygen. The solution was then mixed by bubbling oxygen gas through it a t about 0.7 cu. foot per hour for 30 seconds. This warm-up and mixing procedure was followed every time the solution was concentrated. The liquid in the explosion vessel was sampled for analysis in the folloning manner The analysis bulb (volume = 505 ml.) was connected to the manometer and evacuated, all the other stopcocks being closed. By passing all gases through a heated tube filled with manganese dioxide supported on porous aluminum oxide pellets before allowing them to enter the pump, the vacuum pump was protected from ozone. The stopcock leading to the vacuum pump was closed when the desired vacuum waa obtained, and the stopcock between the analysis bulb and the liquid sample was carefully opened so that the pressure in the bulb built up to 200 to 300 mm. of mercury. During the sampling period the ozone and oxygen in the capillary emerged from the liquid oxygen bath, vaporized in the warm capillary section, and flowed into the sample bulb. Because of the rapid heating of the liquid in the small capillary, the chance of the oxygen distilling into the bulb preferentially to the ozone was greatly reduced, and when the sample was taken rapidly-i.e., within about 30 seconds-the error due to the greater volatility of the oxygen was very small. The temperature and pressure of the gas sample were recorded and the sample was purged with oxygen through a bubbler filled nith aqueous potassium iodide. (For the tests described here, the sample was purged from the bulb with oxygen for 5 minutes a t a flow of about 2.0 cu. feet per hour.) The iodine liberated in the bubbler was estimated in the usual way by acidifying the solution with sulfuric acid and titrating n i t h 0 . l N sodium thiosulfate. From the temperature and pressure of the gas sample, the known volume of the bulb, and the amount of ozone determined by the titration, the sample composition was calculated. The error due t o the volume of sample in the capillary connections was negligible. A spark was passed through the liquid ozone solution by momentarily closing the 110-volt primary circuit of a "sign" transformer, the high voltage secondary of which was attached to the tungsten electrodes sealed in the explosion test vessel. All explosion tests mere made after the Dewar flask had been lowered away from the explosion vessel and shielded from the explosion, leaving only a glass tube filled with liquid oxygen to refrigerate the ozone. Through this tube the liquid ozone v a s observed visually from outside the explosion cubicle until the outside of the tube frosted over. All solutions were sparked repeatedly if an explosion did not take place immediately. Twelve tests were made. KOexplosion took place until the apparent ozone concentration was over 20 mole %. Tests On High Concentration Ozone. I n order to see what would happen a t high concentrations of ozone, tests were made on 0.5-ml. liquid samples containing 66 mole 7 0 ozone. Both the spark and impact (500-gram weight from 10 feet) tests were carried out, and in both cases violent detonations were obtained. llilder impact teste, however, sometimes failed to explode high concentration liquid ozone. Discussion. With the now available data for the exact limiting solubilities of ozone in liquid oxygen at various temperatures ( 1 1 ) and the experimental work described here, it is possible to give values of what might be called "safety" limits for liquid ozone mixtures a t various temperatures (Table 11). I n the temperature range -183" t o -195.8' C . the ozone concentration in the dense phase varies from 67.2 to 84.3 mole %. If any of this dense phase is present, stored ozone cannot be considered safe, since the dense phase is highly explosive when sufficiently stimulated. These authors have no laboratory data on explosion limits a t temperatures above -183' C. Above -179.9' C., where liquid ozone and oxygen are miscible in all proportions, the explo-
740
sion limit m-ould probably vary with the strength of the stimulus, a8 in the case of gas-phase mixtures. I n view of the authors' results a t - 183" C., it would probably be best to keep the ozone concentration below 20 mole % (27 wt. yo)when the temperature is -180' C. or higher, unless protection against unexpected explosions is provided. It is possible, of course, that future research might indicate a method of making higher concentrations safe.
Table II.
Safety Limits for Solutions of Ozone in Liquid
Oxygen Temperature, 0
c.
-183.0 -184.0 - 18G.0 -188.0 -190.0 -192.0 -194.0 -195.8
Safety Limit, % Ozone Mole % wt. % 24.3 17.6 l5,O 20.9 10.8 15.4 8.8 12.6 11.1 7.7 10.1 7.0 9.9 6.8 10.0 6.9
Comparison with 90% Hydrogen Peroxide. Flfteen years ago, hydrogen peroxide a t concentrations of 90% (balance is water) was considered to be very dangerous. Kow 90% hydrogen peroxide is an article of commerce and safe when properly handled. It is safe and stable largely because of the absence of impurities n-hich catalyze its decomposition. A similar situation has not been found t o be true with ozone. In this laboratory, ozone, even when made from highly purified oxygen, has been found to explode if sufficiently stimulated except when greatly diluted by liquid oxygen. To give a direct comparison betu;een liquid hydrogen peroxide and liquid ozone, a sample of 90% hydrogen peroxide Eupplied by the Becco Chemical Division of Food Machinery and Chemical Corp. n-as subjected to tests similar to those used for the liquid ozone mixtures, except that the tests on hydrogen peroxide were carried out a t room temperature. \Then both electrodes were immersed, no spark could be made to pass between the electrodes, apparently because the peroxide was too good a conductor; when the high voltage was applied, the solution simply became warm and gases were evolved. The electrodes were then arranged as in Figure 2 so that one r a s in the gas phase and the other was underneath the surface of the liquid. A spark was readily obtained then, but no explosion took place. Repeated impact (hammer dropping on piston from height of 10 feet) also gave no explosion. These and man)tests reported in the literature have shown that 90% hydrogen peroxide, while dangerous when contaminated with certain catalysts and organic matter, is stable when pule and is thus vastly different from high concentration liquid ozone. When 90% hydrogen peroxide is used in the field-e.g., as a rocket fuel oxidant-it occasionally becomes contaminated. When this happens, the temperature of the peroxide starts t o rise, thus giving a warning before the peroxide explodes. The warning time umally is sufficiently long to make it possible to prevent explosion by dilution with water or cooling. This sort of thing is probably not true of high concentration liquid ozone. The authors' experience would indicate that if a small amount of impurity were accidentally introduced, or if theie were a sufficient stimulus, the whole mass would probably detonate without warning.
Hazard 5 From the experiments described in this article it must not be concluded that it is necessary only to keep the ozone concentration below a certain value to make solutions of ozone safe
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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ROCKET PROPELLANTS Actually, there are many hazards. Contamination with impurities which catalyze ozone decomposition could lead t o rapid liberation of oxygen which might build up pressure in a container and rupture it. Addition of sufficient organic matter might make the ozone solution explode. Leakage of the container or loss of refrigeration would permit both diluent and ozone to vaporize; if the diluent is more volatile than ozone, as in the case of oxygen, the concentration of ozone could build up and rapidly reach explosive levels in the liquid phase; further evaporation would lead to an explosive vapor phase (10). Too deep refrigeration, as with liquid nitrogen (boiling point = -195.8’ C.) instead of with liquid oxygen (boiling point = -183.0” C.), may lead to separation of the dense, highly explosive ozone-rich phase. Besides the hazards mentioned, there are many other problems such as corrosion, pumping, metering, valving, etc. If ozone-oxygen mixtures are condensed a t temperatures below - 179.9’ C. it is necessary t o guard continually against condensing out two phases instead of just the “safe” (oxygen-rich) phase. As the tests described were all on a relatively small scale, similar experiments would have t o be carried out on a larger scale before equipment could be properly designed for storing and handling large quantities of ozone. Recently a patent ($9)has been issued, claiming the stabilization of liquid ozone by the process of making it from oxygen freed from most of its organic impurities. Independent qualitative evidence found in this laboratory indicates that making ozone from specially purified oxygen does help to increase the stability of the ozone. During the past few years rather extensive work also lias been done in this laboratory on the physical properties of ozone (1, 9-11). At first there were many unexplained explosions, but after making ozone from oxygen freed of the traces of organic impurities it normally contains, work was continued with liquid ozone of 100% concentration for a period of 2 years without any unexpected explosion. At the end of this time a bad detonation occurred while a sample of liquid ozone was flowing through a capillary tube. This explosion might have been due t o the trapping and compression of a bubble of oxygen (from slight decomposition of ozone) in the liquid. Adiabatic compression of a bubble is said to be one cause of unexpected explosions in liquids, so that occasionally a liquid may be detonated by the gentlest of blows ( 4 ) . There seems to be no question that high purity does decrease the probability of explosions taking place as a result of mild, random stimuli. However, since there is always the possibility of static sparks and adiabatic compression of bubbles, high purity does not appear to be a sufficient safeguard against detonations of concentrated liquid ozone. Handling of the ozone behind suitable barriers by remote control would seem to present the best method of carrying on larger-scale research on this interesting material.
(5) Boyle, A. R., Llewellyn, F. J., J. SOC.Chem. I n d . 66, 99-102 (1947). (6) Gordon, W. E., private communication (see Hoshowsky, S. A,, Ph.D. dissertation, “The Detonation of Ozone,” University of Missouri, 1950). (7) Hirschfelder, J. 0.. Curtiss, C. F., Campbell, D. E., J . Phys Chem. 57, 409 (1953). (8) Huff, W. J., U. S. Bur. Mines, Rept. Invest. 4031, p. 28, October 1946. (9) Jenkins, A. C., Birdsall, C. M., J. Chem. Phys. 20, 1158 (1952). (10) Ibid., 22, 1779 (1954). (11) Jenkins, A. C., DiPaolo, F. S., Birdsall, C. M., “The System Ozone-Oxygen,” J. Chem. Phys. 23, 2049 (1955). (12) LaMer, V. X., Science 86, 614 (1937). (13) Lewis, B., Elbe, G. yon, “Combustion, Flames, and Explosions of Gases,” p. 394, Academic Press, New York, 1951. (14) Love, C. H., McBerty, F. H., Fiat Final Report 743 t o Office of Military Govt. (U. S.),p. 3, April 24, 1946. (15) Riesenfeld, E. H., 2. Elektrochem. 29, 121 (1923). (16) Rossini, F. D., Wagman, D. D., Evans, W. H., Levine, S., Jaffe, I., Natl. B u r . Standards (U.S.) Circ. 500 (1952). (17) Schumacher, H. J., Anales asoc. qulm. argentina 41, 197-202 (December 1953). (18) Ibid., pp. 230-48. (19) Schumacher, H. J., “Chemische Gasreaktionen,” p. 437, Theodor Steinkopf, Dresden, 1938. (20) Shepherd, M., J. Research Xatl. Bur. Standards 12, 185 (1934); Ibid., 21, 831 (1938). (21) Taylor, H. A,, Van Hook, A., J . Phys. Chem. 39, 815 (1935). (22) Thorp, C. E., Kinney, L. C., Remaly, R.F. (to Air Reduction Co.), U. S. Patent 2,700,648 (Jan. 25, 1955). (23) Warburg, E., Ann. Physik IT.’ 9, 1286 (1902).
RECEIVED f o r review May 12, 1955.
ACCEPTED February 1, 1956.
Literature Cited (1) Birdsall, C. M., Jenkins, A. C., DiPaolo, F. S., Beattie, J. A., Apt, C. M., J. Chem. Phys. 23, 441 (1955). (2) Birdsall, C. AI., Jenkins, A. C., Spadinger, E., Anal. Chem. 24, 662 (1952). (3) Blyumberg, E. A., Frank-Kamenetskii, D. A., J . Phus. Chen. (U.S.S.R.)20, 1301 (1046). (4) Bowden, F. P., Yoffe, A. D., “Initiation and Growth of Explosion in Liquids and Solids,” p. 29, Univ. Press, Cambridge, 1952.
April 1956
U. 8 . NAVY PHOTO
Largest single-stage American built rocket, designed specifically for upper atmosphere research, i s the Viking 12 shown here in take-off a t White Sands, N. Mex.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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