mfety in the chemkol lcrborcrtory
edited by MALCOLM M. RENFREW University ol Idaho MOSCOW, ldaho 83843
Nitric Acid, Nitrates, and Nitro Compounds Energetic Servants, but Overwhelming Masters Leslie Eretherick Chemical Safety Matters, "Woodhayes", West Road, Bridport. Dorset DT6 6AE, England
Nitrates andnitro compounds are derived from nitric acid, which has a long history extending back to around 1100 A.D. This was when moderately concentrated nitric acid was f i s t made by heating a mixture of saltpeter and capperas, or potassium nitrate and hydrated iron(E) sulfate, as we say nowadays. Preparation of the more concentrated acid (fuming, above 70% nitric acid) became poasible in 1658 with the availability of concentrated sulfuric acid. Although nitric acid has been aroundliterally for hundredsof years, and some naturally occurring inorganic nitrate salts for even longer, the organic derivatives of nitric acid have been known in many cases for less than 150years. The earliest ammatic compounds to be isolated in reasonable purity from coal tar were naphthalene (1819) and phenol (1841), and their nitration with fuming and coneentrated nitric acid, respectively, were being effected commercially soon after. The separation of benzene and toluene fractions from coal tar, together with processes for their nitration with nitric acid, nitriosulfuric acid mixtures, or sulfuric acid and potassium nitrate were patented in 1847 (1). This article is concerned with the potential, and often all too real, hazards associated with these long-established materials and processes.
is not observed, and many such incidents have occurred in the past decade. As a typical example, when 1-g samples of powdered milk were digested at 80 "C with 5 mL of 70% nitric acid in 25-mL pressure vessels (minimum outer wall thickness 10.4 mm) rated at 125-bar maximum working pressure, all three vessels ruptured soon after reachine 80 OC. The direct cause of the accident was a runaway exothermic oxidation reaction of the 1-g samples used, whereas the vessel manufacturer's instruction sheet had suggested a maximum sample size of 1GQ mg. It was calculated that an energy release of 23.3 kJ had occurred during the runaway oxidation (3), and in combination with abundant gas production the pressure surge exceeded the yield strength. The need for effective pressure relief in sealed digestion vessels has been stressed, especially if microwave heating is used, because it is then not possible to use metal outers to surround the sealed Teflon PFA vessel (4). The oxidizing effect of nitric acid is, of course, somewhat enhanced if the substrate is itself a reducing agent, and an extreme example of this may be found in the use of nitricacid andbydrazine as arocket-propellant oair. Other less extreme reactions with form'aldehyde and sulfur dioxide have also been recorded (5).
Nltrlc Acld It is a fact that most chemical reaction hazards involve oxidation reactions, and the oxidant that most frequently features in reported accidents is nitric acid. Thus, in the next edition of a weU-known compendium of hazardous reactions (2), the section devoted to such reactions of nitric acid will extend over 32 pages of text. The two most significant characteristics of nitric acid in the present context are that it is a powerful oxidant even when cold or somewhat diluted and that gases are almost invariably evolved when it functions as such. If the material heing oxidized is organic, considerable volumes of carbon dioxide as well as oxides of nitrogen may be produced. This can cause severe problems when samples of various organic materials are being prepared for subsequent analysis by digestion withnitric acid in Teflon-lined pressure vessels, if scrupulous attention to detail
Inorganic NRrate Salts Most metal nitrate salts will fundion as oxidants in the same general way as nitric acid, but the facts that they are solid and must be dissolved (usually in water) to ensure intimate liauid contad for reactions and that the metallic ration represen- (except for lithium) a considershle proportion of the salt molecule combine effectively M reduce the activity. Molten mixed nitrate salt baths, however, used for metal finishine operations, may approach or exceed the ox; dizinenower of nitricacid. deoendineon the salts used and the temoekt&e. . ~.a n i emlosive reactions in these baths were formerly commonplace (6).Silver nitrate is unique in that it may produce exceptionally hazardous explosive products under two particular and distinct circumstances. The first is when the aqueous solution is mixed with ammonia and sodium or potassium hydroxide solutions and allowed to stand. A black
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deposit ("fulminating silver", mainly silver nitride, (Ag3N))swn forms, and this may ea~iode violently on slight disturbance.The seeond circumstance iswhen solid silver nitrate comes into contact with ethanol. This leads to the formation of the chemically different but equally explosive silver fulmi0 (7). nate, AgCNOf far greater hazard potential are the nitrate salts of the nitrogenous bases, ammonia, hydroxylamine, and hydrazine. Ammonium nitrate, on heating in aqueous solution under carefully controlled conditions, decomposessmoothly in an endothermic reaction to give water and nitrous oxide, according to equation 1.
Note that endothermic reactions cannot nn-.. dergo a thermal runaway, since heat is absorbed during the reaction. Here, the salt is oxygen balamed, with no surplus over that required for the products of decomposition. ~
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However, when treated as the solid, it decompnnes exothermically according M eq 2, giving steam, nitrogen, and oxygen, now showing an oxygen balance of +5m. If decomposition of the solid is effected either by h e a t i i under confined conditions or by the explosive shoek from a detonator, the decomposition of ammonium nitrate is explosive, with a detonation velocity of around 1 kmls. The explosive energy released from ammonium nitrate can be increased considerably by admixture of a mall proportion of oil (to give ANFO), or aluminum powder (giving Ammonal) as fuel for the excess oxygen, to restore the oxygen balance to zero. Energy release ia always maximal at zero balance (stoichiometric composition). Ammonium nitrate, originally developed as a fertilizer, now fmds wide application as a cheap explosive,and in bulk it is now classed as a major industrial hazard. Presence of various impurities markedly reduces its stability. Many large accidental explosions involving ammonium nitrate have been recorded, the first, and in some ways the most surprising, heing at Oppau, Germany, in 1921, where the double salt of ammonium nitrate
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with ammonium sulfate (2:l) formed as a byproduct was stored for use as a fertilizer in an open heap of several thousand tons. As the nitrate contained some 45 wt% of sulfate as a diluent, and as it was stored in the open, no possibility of hazard appeared to exist. This belief was reinforced by the Long-established practice of breaking up the caked mound of salts with small charges of blasting explosives prior to loading operations, and this had been done for years on some hundreds of occasions. The last occasion was on 21 September 1921, when the whole mound of 5.4 kt of the double salt, equivalent to 3 kt of ammonium nitrate, detonated. The devastation was enormous, with a huge crater, nearly 600 fatalities, 1,500 injured and 7,000 homeless, with severe shock effects over a 10-km radius (Fig. 1). Thereason for the unexpected detonation could not be established a t the time because the explosive tests then available could not demonstrate that the double salt with 55% nitrate content would undergo detonation. However, it has been found recently that a modern detonation test does give positive results (8). I t has occurred to me that adventitious contamination of the blasting area with spilled fuel or lubricants from transport vehicles might perhaps have played a part in this disaster. Since Oppau, there have been a number of large-scale detonations of ammonium nitrate, often involving the bagged or hulk fertilizer exposed to fire conditions, the most recent incident in November 1988 a t Kansas City involving two road trailers laden with a total of 21 t (9). Hydroxylaminium nitrate and hydrazinium mono- and di-nitrates are all examples of redox salts (reducing cations, oxidizing anions) with positive oxygen balance (100, 20, and 10090, respectively), of lower stability than the ammonium salt, and capable of energetic or explosive decomposition with little provocation (10). Organic
Nitrate Salts
As might perhaps be expected from the
previous section, the nitrate salts of organic bases, most of which are camhustihle, and particularly those salts of relatively low mo-
lecular weight bases, where the "diluting effect" of the rest of the structure upon the nitrate content is not great, exhibit evidence of instability under various circumstances. Thus. rail tanks of 80%mixtures of methvlaminikn nitrate with 200: watpr eaplo&d during pumping oprrations, the dinitrate of 1,2-dlnminocthanr was formerly used ax a military eaplosi\,e, and pyridmium nitrate erphdes on heating, hut not with shurk or frrctiun ( 1 1 ) . I'rea nitrate, which is nearly oxygen balanced, explodes on heating and has been used as a component in mixed explosives (12). Nitrate salts of organic bases are best avoided in laboratory work. Alkyl
Nltrates
These compounds are esters formed from nitric acid and alcohols under closely controlled dehydrating conditions. Their limited thermal stability is due in part to the presence of a C-O-N linkage in the structure, and in part to the high concentration of hound oxygen round the nitrogen atom. Methyl nitrate is nearly oxygen balanced, with high shockand thermal sensitivity, exploding at 65 OC. The energy of decomposition, 153.8 kJ/mol, 4.42 kJ/g, is rather high, but methyl and ethyl nitrates are too sensitive and volatile for practical use. 2-Propyl nitrate, -68% oxygen balance, shows selfaustainine exothermic decomoosition/com-~ bustion hehnvior suitabk lor rorkrt monopropellant rystems. I t has also brm evnluated as n fuel fur internal combuatim engines to run underwater without an air supply. Better known are the nitrates derived from polyhydrie alcohols, which find wide application as commercial explosives. The oldest is 1,2,3-propanetriyl nitrate (glyeeryl trinitrate, "nitroglycerine", or NG), first made in 1846, with an oxygen balance of +5.9%, a very high sensitivity to mechanical shock and a very great explosive power, deriving from high liquid density, rapid detonation with near maximal energy release, and evolution of 7.25 mol of gas per mole of nitrate. Although its extreme sensitivity could be somewhat reduced by transporting it in the frozen state, many serious accidental explo(Continued on page A222) ~
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Figure 1. BASF site at Oppau after ammonium nitrate explosion in 1921. Volume 66
Number 9
September 1989
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sions occurred in' its use. Nobel's discovery of its desensitization by absorption onto kieselguhr in 1866 to give the easily handled dynamite revolutionized the use of commercial explosives. More recently, nitrate esters of other polybydric compounds, such as ethane-l,2-diol, tetrakis(hydroxymethy1)methane (pentaerythritol) and mannitol have been used for the same purpose (13).
Acyl Nllrates This class of compounds, formally mixed anhydrides derived from a carboaylir acid and nitricarid, ischaracterired by low thermal stability and a tendency toward explosive decomposition on heating or other stimulus (14). Acetyl nitrate, which is readily formed from cold acetic anhydride and nitric acid and which has been used as a powerful nitratiue reaeent. will exolode violentlvon raoid heitin'to 1'00 'C. A d samnlea of tyryl nitrate and benzoyl nitrate detonate on heating, and the latter sometimes on exposure to light. Formyl nitrate, as a redox compound of +60% oxygen balance, is prohably incapable of isolation.
Allphatlc Nitro Compounds The lower nitroalkanes, formerly produced by indirect reactions but now directly by vapor-phase nitration, have been widely used as commercial solvents aa well as reactive intermediates and high-energy fuels. This latter application arises from the inherent flammability and an intrinsic oxygen sunnlv. .. amountine to a -42% oxveen halancr in the cave of nitromethane. The detonation pofcntial of nitromethane initiated hy shock or strong heating was known some 50 years ago, and precautions were formulated, but it was not until two detonations in rail tankers had occurred 15 years later that it was realized that the shock produced by sudden application of gas pressure or from forced flow throueb restricted onenines .. during shunting uperations was muugh todetonace the liquid (151 Nitroethane and the nitropropanes are less srnsitiw in this respect. In the presence of bases and absence of water, nitroalkanes will react to formsalts of the isomeric nei-nitroalkanes as shown in eq 3.
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+ NaOEt
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RCH,NO,
RCH=N(O)ONa
+ EtOH
(3)
These aci-salts are easily detonated by impact or friction and are powerful explosives. Nitramethane forms sodium aci-nitromethsnide in this way, and when moistened with water, it exploded violently, apparently owing to formation of sodium fulminate (16): the same fact has been rediscovered 93 years later (17). The sodium ions in molecular sieves are sufficiently basic to form oeisalts,and an explosion when nitromethane was being dried with a large-pore (13A) sieve was attributed to this cause (18). Amines and metal oxides act on nitroal-
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kanes in the same way. Nitromethane has alsobeenshownto act as amildoxidant, and is capable of causing explosions if heated under confinement to high tempeatures with hydrocarbons or other oxidizable materials (19). Polynitroalkanes show the same general properties as the mononitro compounds, hut to a greater extent. In particular, the more nitro groups that are present, the closer will be the oxygen balance to zero, or even to positive values. As an extreme case, tetranitromethane, with +300% balance, is not only explosive in its own right, but is also an extremely powerful oxidant. A 10-gsample of a stoiehiometrie mixture with toluene prepared during a lecture demonstration detonated spontaneously, killing 10 of the audience (20).
Aromatic Nibo Compounds As noted in the introduction, aromatic nitro rompound~have heen of great technical interest and significance, mainly as amine precursors, since the begimings of industrial organic chemistry in the early d a y of the manufacture of artificial dyes. The fact that highly nitrated aromatic compounds possessed explosive properties must have been noticed soon after the preparation of picric acid (as a silk dye) in 1841,and the development of the explosives industry, with few exceptions hased upon polynitro aromatic compounds, followed thereafter. However, the deliberate violent decomposition of compounds specifically designed to explode does not directly concern us here, unexpededly violent reactions being of more concern. Nitrobenzene, long used as a high-boiling solvent (210' C ) in organic chemistry, is not
an inert solvent, and will undergo vigorous or violent reactions withLewis acids (alumnum chloride, tln(lV1 chlorrde, aulfur~c aerd) at elevated temperatures. The violent or eaplo~rvereartlon of mononrtro d~rrvntives of benzene, toluene, or naphthalene when heated to high temperatures with solid o~concentrated aqueous solutions of sodium or potassium hydroxide was first describedin 1871.The same behavior of uitrobenzene under a variety of conditions was again describedin 1899,but then lapsed into obscurity. I t was rediscovered the hard way, in a series of accidents in 1953-1976. The reaction appears to involve the formation of azobenzene and its N-oxide, though in the earlier work the formation of o-nitrophenol had been noted. The heat of decomposition of nitrobenzene is relativelv.hieh, - . a t 3.0kJ/e (0.72 kCal/g). Nitrobenzene will also func; tion as an oxidant when hot, for example in the notoriously vigorous Skraup reaction. Presence of traces of potassium, or of its alloy with sodium, in nitrobenzene renders it extremely sensitive to shock-initiated explosion (21). Alkalimetalsalts of 2- or 4-niixophenol or 2- or Cnitrobenzenethial (or a mixture of 2nitrophenol and solid potassium hydroxide) are all thermally unstable, some at modest temperatures (22). As might be expeded, the same behavior is apparent to a greater degree in polynitroaromatics, and the tendency of di- and tri-nitroaromatics to lead to violent or explosive reactions when heated with alkalies or ammonia was sufficiently well-estahlished for a general warning of the hazard to have been issued to the German chemical industry in 1914 (23). Since then, there have been a consider-
able number of incidents involvine - violent derornnositnn or exnlosion of assorted ni.... troaromatic derivatives in contact with al. kali under various hor conditions. These incidents were analyzed some time ago (241, and the general pattern was reminiscent of the behavior associated with formation of oci-nitro salts in the nitroalkanes. However, there is a fundamental difference between the two groups of compounds, in that the hydrogen formally needed to produce the parent oei-nitro structure C==N(O)OH is available on the nitro-bearing carbon atom in the nitroalkane, but this cannot be the case in aromatic nitro compounds. To effect this change in aromatic systems one needs transfer of hydrogen from another ring C atom, with bond rearrangement. This can only happen either from an unsuhstituted oor p-position, or from an H-bearing suhstituent atom there that can isomerize to a doubly bound form, to give in either case an oor p-quinonoid ring structure doubly bound to the oei-nitro group, which is then capable of salt formation with the hase. Examples of suhstituents that can isomerize in this way are amino, alkylamino, hydroxy, or thiol groups, giving the douhly bound imino, alkylimino, 0x0, or thiono suhstituents, respectively. This isomerization of a nitroaromatic compound to its aci- form is shown in the generalized structures in Figures 2 and 3, and all of the* have been involved in unexpectedly violent decomposition on heating in presence of various bases, or as the isolated salts. Remember that meto-substituted aromatics cannot give quinanoid forms. ~
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(Continued on page ,4224)
