UNINVITED CHEMICAL EXPLOSIONS W. R. TOMLINSON, JR. Chemical Research Laboratory, Picatinny Arsenal, Dover, New Jersey
L. F. AUDRIETH Noyes Laboratory of Chemistry, University of Illinois, Urbana. Illinois
ONLY
recently was it revealed by the authors ( 1 ) that many oxygenated metal ammine complexes are explosive compounds. Similar substances have been prepared, and are being prepared every day by chemists and by students of chemistry. Fortunately, there have been but few accidents thus far, but disastrous explosions have occurred in the course of work in the laboratory, pilot plant, factory, and even in the act of product delivery. All such accidents are avoidable. Ignorance and carelessness are to blame when they do happen. Knowledge of the types of materials or mixtures which may be expected to decompose with explosive violence will help to avoid many accidents. Accidents need not occur a t all if a few simple precautions and safety rules are observed. COMPOUNDS WHICH ARE EXPLOSIVES
making use of the Pauling bond energies, and the heats of formation of the simple decomposition products (t, 3). Approximate calculations can be made for all organic compounds; where data for the heat of formation are available such calculations can be carried out quite accurately for organic, inorganic, and metal-organic materials. Lothrop and Handrick (4) have shown that there is a definite relationship between heat of decomposition and oxygen balance and have indicated a simple method of calculating the former. Maximum explosive power is attained a t about "zero oxygen balance." The oxygen balance concept is an old one, but few references can be found to i t in the literature. Oxygen balance is defined as the per cent excess or deficiency of oxygen required to completely oxidize all oxidizable elements in a given compound. Values for the oxygen balance of some typical explosives are given in Table 1.
Single Compounds. Why is nitroglycerin an explosive? It releases on explosion approximately 1600 cal./ TABLE 1 Oxvaen Balance of Comnounds g. This figure is practically identical with its heat of combustion. It contains within itself all the oxygen Commund Ozvaen balance. 7" necessary for combustion. Two factors have been emAmmonium nitrate phasized in answer to the question. They are exotherAmmonium perchlorate miz decomposition and oxygen balance. These properAmmonium picrate Black powder ties characterize not only nitroglycerin, but most other Cvclonite hazardous compounds. D;lszodinitraphenol Diethyleneglycol dinitrate Many molecules contain within themselves potenDinitrotohem -.. tially dangerous energy. This energy is derived mainly Mercury fulminate -17 from the oxidation of carbon and hydrogen to carbon Nitrocellulose (14.14% '0). -24 Nitroisohutyl glycerol tnnltrate 0 monoxide, carbon dioxide, and water, and from the Nitroguanidine -31 formation of molecular nitrogen. The contribution of Nitroglycerin +3.5 Pentaervthritol tetranitrate -10 nitrogen formation to the decomposition energy of Pierie aEid chemicals cannot be stressed too strongly since the Tetryl heat of formation of nitrogen from the atoms is given as TNT 171.1kg.-cal./mol. A compound containing an appreciable amount of oxygen, along with a significant quantity Where organic explosives are concerned i t is customof nitrogen, should be suspected immediately. I t would ary in writing the probable decomposition reactions to he foolhardy to ignore the potential danger of any distribute oxygen first to carbon to form carbon monoxcompound, or mixture, whose elemental components ide, then to hydrogen to form water, and finally to carcould undergo exothermic recombination with iorma- bon monoxide to form carbon dioxide. Such hypothettion of very stable products. ical, yet logical decomposition reactions for a number of The simplest and most accurate way to evaluate the compounds are given by the following equations: potential hazard of any product to be processed or d e CHSNOS CO HIO '/nH. +'/nNz livered is to calculate its heat of decomposition to the most stable products. This can be done readily by CHSNHOH -CO + 21/nH2 + l/$Nt
-
6116
+
+
NOVEMBER, 1950
can he particularly hazardous. They are usually sensitive to impact and to friction and often quite unst.able chemically. Some idea of the exothermicity of oxidant-reductant mixtures can be gleaned from the reactions listed below: Where other atomic species must also be considered it is appropriate to assign oxygen, or any other oxidizing constituent, initially to the combination which is most stable, that is, to the product whose heat of formation is most exothermic, as illustrated by the following equat,ions:
-
N&CIO&
+ '/sN~+ '/~Clp + Ex0 + 2Nn + 50s + 01
2H10
-
SOI(NHNOa)l+ SO8(NCHsNO&
0 8
CO
+ 3HgO + CO2 + 2N1 + S
While sulfur probably is not converted to the trioxide, especially where the explosion temperature is high, the second case involves so large an excess of oxygen that this might best be assumed. The oxygen balance concept can be used, a priori, to good advantage. Since explosive power reaches a maximum when oxygen balance is near zero, it is those materials, containing just sufficient oxygen in the molecules to form HzO, COz,and Nz, that are capahle of releasing t,he largest quantities of energy per gram and will, in general, be most destructive. We would therefore urge particular care in the study of oxygenated nitrogen containing organic compounds and would suggest special caution, yes, even condemn the common practice, where picrates and perchlorates of organic bases are prepared for purposes of identification. Multicomponent Systems. The recombination of atoms from a relatively weakly bound state to much more stable states may also occur where two or more components are brought together in proper proportions-and where the energy of reaction is such that its release is effected with great rapidity--especially if large volumesof gaseous products are formed. Thus an association of oxidant and reductant can be just as dangerous as a single explosive compound. In the case of mixtures, however, it is the intinmy of contact which often det.ermines the hazard associated with such potentially reactive systems. Decrease in particle size, vith a corresponding increase in specific surface, leads (a) to a greater intimacy of contact on the part of the components of a mixture, (b) greater probability of react,ion, and (c) increase in reaction velocit,y. Black powder, consisting of an intimate mixture of potassium nitrate, sulfur, and charcoal, can be changed from the heaving or pushing type of explosive, usually referred to as a low explosive, to the energetic brisant type by decreasing particle size and increasing intimacy of contact. Such an increase in reactivity and a corresponding aspect of danger, seldom realized, can be imparted to many other commonly used systems. Thus, for example, mixtures of chlorates and of chlorites with organic matter, sulfur, and/or phosphorus, and of potassium perchlorate with reactive powdered metals, such as magnesium and aluminum,
1. 2. 3. 4.
++
- -+ +
Heot of reaction, cal./g. of mixhre
Ag,O Mg 2Ag MgO 5Ba01 2P (red) 2BaO B% (PO& Cr,Os BaSO, 2BaCrOlf S -BaO NHGlO, C COr (g) 2H20 (g) '/,N, '/sCh 5. 2KC10s 35 350% (g) 2KCI ( ) 6. GHNOs SCHIOH + 5con(g! 1 3 & 0 ( ~ ) 3N2
-+ +
+++
++
+
+
+
+
540 540 130 1020 395 1280
All other factors remaining constant, those reactions producing the greater volumes of gases are the more dangerous. On the other hand, i t is still necessary to consider the reactivity of the systems. Thus while reactions 1, 2, 3 produce little or no gas, and 4, 5, 6 considerable amounts, it is the chlorate-sulfur mixture which is the most difficult to handle safely. THE EARMARKS OF HAZARDOUS MATERIAIS Structural and Physical Peculiarities. Where single compounds are concerned i t is usually not too difficult to associate hazard with structure. One of the unusual cases which had apparently escaped popular attention for many decades comprises the group of oxygenated metal ammine complexes (see Table 2). Many of these compounds are prepared in laboratory courses. Some are powerful explosives. Too little is as yet known about sensitivity characteristics. Suffice it to point out that a compound like hexammine chromium (111) uitrate is almost as powerful an explosive as TNT. And why? An examination of its probable decomposition products makes it apparent that such a compound a p proaches oxygen balance:
-
C~(NH&(NOJ)~ 9HnO
+ Cr + 4.5Na
Certain groupings when present in a molecule can either confer upon it or enhance its explosive properties. Lothrop and Handrick (4)refer to radicals which confer explosive character directly as plosophors and those which enhance explosive power as auxoplosive groupings. The plosophors may lead either to powerful explosives or to more sensitive materials and include compounds containing the following groupings: --ONOz, -NHNOZ, =N.NOe, -NO*, -NO, -N= N-, -0-0-, -N,. Organic salts of perchloric, picric, chloric, nitric, bromic, and iodic acids would also be dangerous because of the high oxygen contents of the acids. Auxoplosive groups include nitrile, oxime, and ethel linkages. Mixtures of Organic Compounds. Some interest has been evidenced recently in the use of liquid dmitrogen tetroxide as a solvent and nitrating agent for various aliphatic and aromatic hydrocarbons. Nowhere does there appear a word of caution! We veiw this ignorance on the part of otherwise capahle chemists with
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alarm and can do no more than to condemn such foolhardy academic research. Need we do more than to and hydrocarbons point out that mixtures of liquid N204 are among the m s t powerful and sensitive high explosives. It is stated by Stettbacher (5) that. a mixture containing 70 per cent NzOl and 30 per cent nitrobensene, despite its low density, undergoes detonation a t a rate in excess of 8000 m./sec. This happens to be a completely oxygen-balanced mixture, and the components are completely miscible! TABLE 2 The Explosive Properties of S o m e Metal Ammines (I)
Ezplo-en""","
Impact Pouer," s e n ~ i t i u i t y ,g.~ sand "C. em. clvshed
tempture," Metal ammine
Hexammine chromium (111) nitrste Hexammine cobalt (111) perchloritte Hexammine cobalt (111) iodate Hexammine cobalt (111) nitrate Nitratopentsmmine cobalt (111) nitrate Chlorooentamminecobalt (111) T6ioc3ranatopentammineoobalt (111) perchlorate Dichloradiethylenediamine cobalt (111) ~erchlorate ~mmo&u& hexanitrocobaltate (111) Tetrammine copper (11) nitrate Mercuric fulminate Lead aside TNT (sym-trinitrotoluene)
265
32
40.7
360 355 295
100" 50
39.2 14.7 35.9
310
48
35.5
325
55
20.1
300
85
9.9
230 330
33 19 5 10 95"
19.0 17.2 21.0 18.0 42.0
...
335 470
a For details of tests see Picatinny Arsenal Technical Report No. 1401, March 18, 1944 and Bureau of Mines Technical Bulletin No. 346. "Phvsical Tes'tine. of ~xdosives."
In this same connection we mould also wish to point out the inherent danger in the use of red fuming nitric acid for laboratory nitrations! Aniline and fuming nitric acid constitute the components which are used in the operation of some jet assisted take-off (JATO) units (6). This same combination constitutes one of the liquid "Sprengel" explosives (7). The alkali sensitivity of nitroparaffins is perhaps not too well recognized (8). I t appears that many sensitive materials can be formed as intermediates depending on the base involved, leading eventually to salts of fulminic acid. All of the intermediates as well as the end uroduct are more sensitive than the original nitromethane. In connection with peroxides it must be emphasized that inst because a ~eroxideis sold commerciallv is no marantee that i t is nonhazardous: manv industrial peroxides are far from inert (9): D A ~to the extremely dangerous nature of peroxides, in general, they must all be regarded as dangerous, unless the contrary is definitely known t o be the case. Hydrogen peroxide is normally considered quite innocuous, perhaps because a dilute solution finds common household and industrial
use. Concentrated hydrogen peroxide, on the other hand, is exceedingly dangerous. When mixed with organic material such as glycerin, explosive mixtures are obtained. The ultimate in cleanliness is required to prevent dangerous contamination of concentrated hydrogen peroxide. The presence of peroxides makes the distillation of ether dangerous and this operation should always be preceded by washing the ether with a concentrated solution of ferrous sulfate or sodium hydrogen sulfite. This procedure reduces considerably the concentration of peroxides remaining in the still residues. Mixtures of Inorganic Materials. Many inorganic mixtures, typified by the pyrotechnic type composition, contain inorganic oxidants and reductants, the general properties o f which should be carefully considered before they are mixed. While the following list is not allinclusive, it contains some of the more hazardous mixtures. (1) Chlorates mixed with red phosphorus, sulfur, sulfides, sulfites, charcoal and/or powdered metals are sensitive to impact and friction. I n fact, spontaneous explosions of potassium chlorate both alone and in mixtures have been observed repeatedly. Therefore, the more stable potassium perchlorate is to be preferred wherever possible. (2) Chlorates mixed with acidic snbstances are unstable and may explode spontaneously. Where such mixtures must be used calcium (magnesium) oxide or carbonate is added to neutralize any acid which may be liberated during storage. (3) Peroxides mixedwith red phosphorus, sulfur, sulfides, and charcoal are likely to be very sensitive to impact and friction. (4) The addition of red phosphorus, sulfur, sulfides, thiocyanates, and other sulfur containing compounds to inorganic oxidant-reductant mixtures generally lowers the ignition temperature and may increase the sensitivity to impact and friction if very strong oxidants such as chlorates, perchlorates, and peroxides are employed. (5) Ammonium perchlorate, ammonium nitrate, and ammonium and inorganic picrates are explosives in themselves, and, if used in inorganic mixtures, render them more hazardous. (6) Silver salts decompose a t relatively low temperatures and therefore would tend to lower the ignition temperatures of the mixtures. The same is true of red lead, lead dioxide, and lead nitrate. Other heavymetal compounds containing oxidizing radicals may be have in a similar fashion. (7) Certain pyrophoric metals suchas zirconium and titanium, when dry, are likely to catch fire during handling. It is for this reason that they are shipped and stored under water or water-alcohol mixtures. Wetmixing is to be preferred for mixtures containing these metals. It is interesting to point out, however, that wet zirconium powder containing up to 15 per cent water is even more dangerous than the dry powder, since upon ignition the dry powder will burn whereas the moistened powder will e~plode. This metal must there-
NOVEMBER, 1954
fore be stored with a t least 25 per cent water and/or alcohol. (8) Permanganates are very powerful oxidizing agents. Mixtures containing permanganates and powdered metals are likely to be very sensitive to impact and friction. This sensitivity will be increased if sulfur, sulfides, red phosphorus, or charcoal are also present. (9) Mixtures of finely powdered metals, such as aluminum and magnesium, and salts containing oxidizing anions such as nitrates, perchlorates, and chlorates, constitute hazards. One of the most widely used photoflash compositions consists of a finely divided, intimate mixture of barium nitrate and aluminnm-magnesium alloy. This composition is not only easily ignitible but extremely sensitive to friction and impact. We would particularly call the attention of such inorganic mixtures as represented by the last category to teachers of yualitatiue analysis. Our own examination of "unknowns" reveals that powdered aluminum and magnesium are often used in such combinations, since these metals are commercially available. Mixtures of these metals with nitrates, chromates, perchlorates, and other common oxidizing radicals represent hazardous combinations. Very often students are urged to "grind" an unknown so that at,tack by agents can be effected more efficiently! Such compositions are notoriously sensitive to friction. May we urge instructors to check such stwEent unknowns and to eliminate any mixtures which could possibly be dangerous. HOW TO AVOID ACCIDENTS
Dangerous reactions generally require energy for their initiation. This energy may be supplied in the form of heat, light, electrical, or mechanical energy, and hy impact or friction. I t is extremely important in handling all dangerous or suspect materials to avoid application of abnormal amounts of energy from any source whatsoever. Where dangerous and unstable materials are concerned, the following precautions should he observed: (1) Avoid theuse of open flames. (2) Avoid direct application of electrical energy, either electrostatic or that due to live circuits. (3) Avoid application of energy deriving from the impact of two hard surfaces, especially where the contact surface is very small. The force involved, when considered as localized, is very large. Glass-stoppered bottles and frit,t,edglass funnels are taboo for dangerous materials. (4) Conduct any operation involving ( a ) solvent vapors, (b) acid vapors, (c) corrosive liquids, and/or (d) gas evolution in a hood with the exhau~tfans running. This is necessary in order to (a) avoid vapor accumulation and possible subsequent explosion, (b) re-
duce toxicity effects, and (c) prevent corrosion of laboratory equipment. (5) Take precautions against the possibility of explosions in vacuum operations. Flat-bottomed flasks should not be used in vacunm distillation systems. Defective equipment should not be used for vacuum work. These precautions are particularly necessary if sensitive materials are to be distilled. Barricading is, of course, essential. (6) Bottles of acids, bases, andsolvents should not be stored adjacent to each other. There should be some segregation to decrease the possibility of forming dangerous mixtures in the case of breakage. (7) Unnecessary chemicals should not be allowed to accumulate in the laboratory. (8) Each laboratory must be kept clean and free from explosive dust. Common-sense rules of "good housekeeping" should be applied. (9) The quantity of explosive material stored in a laboratory should be kept a t a minimum. (10) No "horse-play'' can be tolerated. ( 11) Any operation requiring the grinding of reactive compounds and mixtures must be conducted using all safety precautions. Grinding should be carried out only after some indication of the probable sensitivity of the material is known. Grinding must be done (a) behind a barricade, (b) on paper, or (c) using a wooden pestle. The hands of the operator should be gloved. (12) Protective equipment must be utilized wherever applicable. Barricades, goggles, face masks, asbestos gloves, and gas masks should be available. (13) Dangers of static electricity should be reduced (a) by minimum use of outer woolen clothing, (b) by wearing safety shoes, (c) by conductive flooring, and (d) by adequate grounding of containers and receptacles used to store or transfer ignitible solids or solvents. (14) Adequate fire prevention measures augmented by drills and inspections must be placed into effect. LITERATURE CITED (1) TOMLINSON, W. R.,K. G. O ~ S O NAND , L. F. AUDRIETH, J. Am. Chon. Soc., 71, 375 (1949). (2) BRANCH,G. E. K., AND M. CALVIN,"Theory of Organic Chemistry," Prentice-Hall, Inc., New York, 1941. F. R., AND F. D. ROSSINI,"Thermochemistry of (3) BICHOWSKY, Chemiod Substances," Reinhold Publishing Corp., New York, 1936. (4) LOTHROP, W. C., AND G . R. HANDRICK, Chem. Reus., 44, 419 (1949). (5) STETTBACHER, A,, "Spreng-und Sohie~~toffe, Atomzerfallselemente und ihre Entladungserscheinmgen))' Zurich, 1948. AWD H. H. M. PIKE,J. (6) WAEELER, W. H., H. W~TTAKER, Inst. Fuels (London), 20, 137 (1947). (7) DAVIS,T. L., "Chemistry of Powder and Explosives," John Wiley and S n s , Ino., New York, 1943, p. 353. (8) The Commercial Solvents Corporation, "The Nitroparaffins," a descriptive pamphlet. (9) WEISS,M. W., Znterehem. Rev., 8,63 (1949).
