Metal Azides

CRCR33METAL AZIDES33.1 GENERAL DISCUSSIONAzides form highly explosive shock- andheat-sensitive salts with many metals. Theyare derivatives of hydrazoic acid, HN3, con-taining a NULN moiety attached to the metal,by ionic or covalent bond. The azide grouphas a linear structure that is stabilized by res-onance as follows:RN N N RN N N+ Azide anion is a good nucleophile. Manyazide reactions proceed via formation ofan active intermediate, nitrene, analogous tocarbene. This results from the photolysis ofhydrazoic acid:HNN N HN + N2+h(hydrazoic acid) (nitrene)Alkali-metal azides are prepared by passingnitrous oxide into the molten metal amide.Azides of other metals are made by reactingsodium azide with the corresponding metalor alkyl salts. Whereas alkali-metal azidesare inert to shock, the salts of copper, silver,lead, mercury, and most heavy metals aredangerously shock sensitive.Azides react explosively or form otherexplosive azides when they come in contactwith a number of substances. With acids,almost all metal azides react to form hydrazoicacid, which is dangerously sensitive to heat,friction or impact. Azides can react with saltsolutions of many metals forming azides ofthose metals, some of which especially, theheavy metal azides are highly sensitive tofriction and impact. The rates and yields ofsuch reaction products would depend on theequilibrium constants and solubility products.For example, soluble alkali-metal azides canreadily form lead or cadmium azide whenmixed with a salt solution of lead or cadmium.The hazardous properties of some of thecompounds of this class are discussed below.The metal derivatives of all azides (espe-cially the heavy metal derivatives) are ther-modynamically unstable. These endothermiccompounds have high positive Gibbs freeenergy and can explode on impact or heat-ing. The explosivity of these compounds isa function of their thermodynamic unstabilityand decreases in the following order for someA Comprehensive Guide to the Hazardous Properties of Chemical Substances, by Pradyot PatnaikCopyright 2007 John Wiley & Sons, Inc.613CRCRCRCR614 METAL AZIDESTABLE 33.1 Heats of Formation of Selected AzidesHeat of Formation HftrianglerightstriangleleftAzideaFormula (kcal/mol)Mercuric azide Hg(N3)2C133.0Cadmium azide Cd(N3)2C107.8Lead azide Pb(N3)2C104.3Hydrazoic acid HN3C70.3Silver azide AgN3C66.8Cuprous azide Cu2(N3)2C60.5Strontium azide Sr(N3)2C48.9Sodium azide NaN3C16.8Calcium azide Ca(N3)2C11.0aPure solid in crystalline form.of these azides: mercuric azide cadmiumazide lead azide hydrazoic acid silverazide zinc azide cupric azide cuprousazide strontium azide calcium azide.The decreasing pattern above is of littlepractical interest, however, as all the heavymetal azides detonate violently upon heatingand mechanical impact. Table 33.1 lists theheat of formation Hftrianglerightstriangleleft for some azides.It may be seen that explosivity decreaseswith a decrease of Hfand at a lowvalue of C16.8 kcal/mol, sodium azide isnonexplosive. Discussed below are individ-ual compounds of commercial interest orthose presenting severe explosion hazard.The explosive properties of additional com-pounds are highlighted in Table HYDRAZOIC ACIDFormula HN3; MW 43.04; CAS 7782-79-8Structure:HNN N+Synonyms: hydrogen azide; azoimide; tria-zoic acid; hydronitric acidUses and Exposure RiskHydrazoic acid is used in making heavymetal azides for detonators. It forms readilywhen sodium azide reacts with acid orhydrazine is mixed with nitrous acid.Physical PropertiesColorless mobile liquid with a strong pun-gent odor; bp 37C(98F); mp NUL80C(NUL112F); soluble in water and organicsolvents.Health HazardThe acute toxicity of hydrazoic acidthrough inhalation and other routes of expo-sure has been found to be high to very high.The symptoms and the intensity of poison-ing are similar to sodium azide. It is, how-ever, less toxic than hydrogen cyanide. Inhumans, inhalation of its vapors can produceirritation of eyes and respiratory tract, bron-chitis, headache, dizziness, weakness, anddecreased blood pressure (Matheson 1983).Prolonged exposure to high concentrationscan result in collapse, convulsion, and death.An exposure to 1100 ppm for 1 hour waslethal to rats. Chronic exposure to a lowlevel of this compound in air may producehypotension.Animals given intraperitoneal dosages ofhydrazoic acid showed the symptoms ofheavy breathing, convulsions, depression,and fall in blood pressure. It affected thecentral nervous system, but no damage wasobserved in the liver or kidney.LD50value, intraperitoneal (mice): 22 mg/kgCRCRCRCRHYDRAZOIC ACID 615TABLE 33.2 Miscellaneous Metal AzidesCompound/CAS No. Formula/MW Explosive PropertiesLithium azide19597-69-4Li(N3)48.96Colorless crystalline solid; inert to shock and stable atelevated temperature; reactions with lead, copper, silver,cadmium zinc, and mercury salts can form explosiveazides of these metals; reactions with chlorine, brominevapors and carbon disulfide can be violentMagnesium azide39108-12-8Mg(N3)2108.35Crystalline solid; insensitive to shock; explodes on heating;reacts with acids forming hydrazoic acid; reacts with anumber of heavy metal salts in solutions forming theirexplosive azides; susceptible to form unstable andexplosive azidodithioformate when its solution is mixedwith carbon disulfide; forms explosive bromine azidewith bromine vaporsPotassium azide20762-60-1K(N3)81.118Insensitive to shock or impact; rapid heating to hightemperature can cause explosion; it melts first and thendecomposes above melting point; evolves nitrogen ondecomposition; reacts with acids forming explosivehydrazoic acid; forms explosive potassiumazidodithioformate when its aqueous solution is mixedwith carbon disulfide; reacts with bromine vapor, formingexplosive bromine azide; reaction with methylenechloride, chloroform, and other halogenated hydrocarbonsolvents can produce diazidoalkanes; upon distillation orconcentration, may explode; reactions with a number ofsalts of lead, copper, silver, mercury, cadmium, and otherheavy metals can produce explosive heavy metal azidesAmmonium azide12164-94-2NH4(N3)60.059Explodes on rapid heating, friction, and impact; reacts withacids and metals to form hydrazoic acid and metalazides, which explode readilyCalcium azide19465-88-8Ca(N3)2124.12Colorless crystalline solid; mildly endothermic,Hftrianglerightstriangleleft C 46 kJ/mol; not sensitive to shock; explodes onheating at 150C (302F); reactions with acids can formhydrazoic acid (explosive); susceptible to the formationof many heavy metal azides when mixed with their saltssolutionsStrontium azide19465-89-5Sr(N3)2171.66More endothermic than calcium azide;Hftrianglerightstriangleleft C 205 kJ/mol; moderately sensitive to impact;explodes on heating to 170C (338F); susceptible toform heavy metal azides; and hydrazoic acid ontreatment with acids and heavy metal salt solutionsBarium azide18810-58-7Ba(N3)2221.37Crystalline solid; one of the metal azides that is not anendothermic compound, Hftrianglerightstriangleleft NUL 22.2 kJ/mol; however,highly sensitive to shock or impact when dry; whendamp or moistened with a solvent or in solution, it isrelatively insensitive to impact and safe to handle;presence of metal ions as impurities can enhance thedanger of explosion; explodes when the anhydrous salt orthe nonhydrate is heated to 200C (392F), formingnitride; forms explosive products, hydrazoic acid andazides of lead,(continued)CRCRCRCR616 METAL AZIDESTABLE 33.2 (Continued)Compound/CAS No. Formula/MW Explosive Propertiessilver, and copper in contact with acids or thecorresponding salts of these metals, respectively;DOT restricted material, Label: Class A Explosiveand Poison; UN 0224 (when containing less than50% water); Flammable Solid and Poison, UN1571 (when containing 50% or more water)Zinc azide14215-28-2Zn(N3)2149.42Colorless crystalline solid; explodes easily; highlysensitive to heat, shock, and static charge; formsexplosive azides with heavy metal salts; formsexplosive hydrazoic acid an reactions with acidsMercury (II) azide14215-33-9Hg(N3)2284.63Dangerously explosive; highly endothermic salt,HfC 556.5 kJ/mol (Bretherick 1995); explodeswhen subjected to heat or shock; can explode evenon mild scratching of large crystalsMercury (I) azide38232-63-2Hg2(N3)2485.06Less sensitive than the Hg(II) salt, and other heavymetal azides; explodes on heating to above 270C(518F); forms explosive azides of heavy metalswhen mixed with their salt solutions; formsexplosive hydrazoic acid with acidsCadmium azide14215-28-3Cd(N3)2196.45Dangerously explosive similar to the azides ofHg(II), Pb and Ag; highly endothermic, Hftrianglerightstriangleleft451 kJ/mol (Bretherick 1995); sensitive to heat,shock, and friction; explodes on heating; slightfriction or shock can cause explosion when the saltis in dry state; its solutions susceptible to explodespontaneously on long standing, especially whensaturated, or upon crystallization or in presence ofimpurities; reacts with heavy metal salts solutionsor acids, proceeding to explosive violenceSilicon tetrazide27890-58-0Si(N3)4196.17Dangerously explosive; explodes spontaneously whenpure (Mellor 1967); highly sensitive to heat andshock; crystallization and purification should behandled very carefully; explodes on reactions withacids; susceptible to react with heavy metal saltsforming their azidesCobalt azide14215-31-7Co(N3)2142.97Explodes on heating at 200C (392F); susceptible toform hydrazoic acid with acids and heavy metalazides in contact with heavy metals or their saltsNickel azide59865-91-7Ni(N3)2142.70Explodes at 200C (392F); no data available onshock or impact sensitivity; forms hydrazoic acidwith acidsPalladium azide13718-25-7Pd(N3)2190.44Crystalline solid; explodes on heating; sensitive toimpact when dry; forms hydrazoic acid with acidsCopper amine azide Cu(NH3)2(N3)2181.65Dark green crystals; explodes when heated to 105C(221F); reacts with acids forming hydrazoic acid;DOT-classified forbidden substance; Hazard Label:ForbiddenCRCRCRCRSODIUM AZIDE 617Exposure LimitCeiling 0.1 ppm vapor (ACGIH).Fire and Explosion HazardIn pure form or highly concentrated solution,hydrazoic acid is a dangerous explosive com-pound. It is unstable and sensitive to heatand shock. The explosion hazard decreasessignificantly with more dilute solutions.It forms shock-sensitive metal azideswhen react with metal salts, and fluorineazide with fluorine (Lawless and Smith 1968)and susceptible to form chlorine azide andbromine azide with chlorine gas and brominevapor. All these products can explode vio-lently on impact. With carbon disulfide itforms a violently explosive salt (Mellor1946; NFPA 1997).Disposal/DestructionHydrazoic acid may be destroyed by con-verting it to sodium azide. The latter isdecomposed with nitrous acid in a hood(National Research Council 1995). The fol-lowing method is used. It is diluted in waterto a strength below 5%; or its solution inorganic solvents that is immiscible in wateris shaken vigorously with water in a sepa-ratory funnel. The aqueous solution contain-ing hydrazoic acid is neutralized with sodiumhydroxide and separated from any organiclayer. Sodium azide, so formed, is destroyedby reacting the aqueous solution with anexcess of sodium nitrite followed by 20%sulfuric acid until the solution is acidic. Thereaction is carried out in a three-necked flaskequipped with a stirrer, a dropping funnel,and a gas outlet line to vent out nitric oxide.The reaction mixture is flushed down thedrain.33.3 SODIUM AZIDEEPA Classified Acute Hazardous Waste,RCRA Waste Number P105DOT Label: Poison B, UN 1687Formula NaN3; MW 65.02; CAS 26628-22-8Structure:NNN+Na+ionic bond between sodium ion and azideanion, hexagonal crystals.Synonyms: azium; smiteUses and Exposure RiskSodium azide is used in making othermetal azides, therapeutically to control bloodpressure, as a propellant for automotivesafety bags, as a preservative for labora-tory reagents, as an analytical reagent, andin organic synthesis. It is also used as anantifading reagent for immunofluorescence(Boeck et al. 1985).Physical PropertiesColorless crystalline solid; decomposes at300C (572F); density 1.846; readily solublein water 41.7 g/100 mL at 17C(62F),soluble in alcohol 0.314 g/100 mL at 16C(60F) insoluble in ether.Health HazardSodium azide is a highly toxic compound;the order of toxicity is the same as that ofhydrazoic acid. It is converted to hydrazoicacid in water. The aqueous solutions ofsodium azide contains hydrazoic acid, whichescapes at 37C(98F), presenting a dangerof inhalation toxicity.Sodium azide, by itself, is a severe acutetoxicant causing hypotension, headache,tachpnea, hypothermia, and convulsion.The toxic symptoms from ingestion of100200 mg in humans may result inheadache, respiratory distress, hypermotility,and diarrhea. An oral intake of 1020 gCRCRCRCR618 METAL AZIDESmay be fatal to humans. The target organsare primarily the central nervous system andbrain. There are reports on azide poisoningof brain and nerve tissue (ACGIH 1986;Mettler 1972). Owing to the availability ofelectron pairs in azide ion for coordinatebonding, azide forms strong complexes withhemoglobin, which blocks oxygen transportto the blood (Alben and Fager 1972).LD50value, oral (rats): 27 mg/kgSodium azide is strongly mutagenic in theAmes test. However, it shows a very weakmutagenic effect on Saccharomyces cere-visiae C658-K42 (Morita et al. 1989). Macoret al. (1985) found that light decreased themutagenic effect of sodium azide to inducehereditary bleaching of Euglena gracilis.Carcinogenicity of this compound on animalsor humans has not yet been fully established,although skin and endocrine tumors in ratshave been observer (NIOSH 1986).Exposure LimitCeiling 0.3 mg/m3in air (ACGIH).Fire and Explosion HazardAlthough metal azides are known explosives,sodium azide is inert to shock. However, vio-lent decomposition may occur when heatedto 275C (527F). It forms lead, copper, andother metal azides, which are highly deto-nating when it comes in contact with thecorresponding metal salts. Therefore, pour-ing sodium azide solution into lead or copperdrain must be avoided.Reactions with chlorine or bromine vaporproduce chloroazide or bromoazide, whichcan explode spontaneously. Explosions canoccur when sodium azide reacts with carbondisulfide, chromyl chloride (Mellor 1946,Suppl. 1967), and dibromomalononitrile(MCA 1962). Such violent explosions maybe attributed to the formation of highlyunstable reaction products that are extremelysensitive to shock and/or light.Disposal/DestructionSodium azide may be destroyed by nitrousacid treatment (National Research Council1995):2NaN3C 2HNO2NULNULNUL! 3N2C 2NO C NaOHThe reaction is carried out in a three-necked flask equipped with a stirrer, a drop-ping funnel, and an outlet for toxic nitricoxide to carry the gas to an exhaust system.A 20% aqueous solution of sodium nitritecontaining about 1.5 g of the salt per 1 g ofsodium azide is added onto a 5% aqueoussolution of sodium azide. This is followed byslow addition of 20% sulfuric acid, until themixture is acidic. The decomposition is com-plete when starchiodide paper turns blue,indicating the presence of excess nitrite. Thereaction mixture is washed down the sinkwith a large volume of water.AnalysisSodium azide may be analyzed by react-ing the aqueous solution with iodine. Thereaction is induced by 2-mercaptopyrimidine(Kurzawa 1987). The excess of iodine isback-titrated using potassium iodide, sodiumthiosulfate, and starch indicator. The azidefunctional group may be identified by FTIR.33.4 LEAD (II) AZIDEDOT Label: Class A Explosive. Forbidden(dry); when wetted with not less than 20%by weight water or mixed with mixture ofwater and alcohol, it is labeled Explosive1.1A. UN 0129Formula: PbN6; MW 291.25; CAS 13424-46-9CRCRCRCRSILVER AZIDE 619Structure: Pb(N3)2Uses and Exposure RiskLead azide is used as a primary explosive indetonators and fuses to initiate the boosteror bursting charge. Generally, it is used indextrinated form. Lead azide is also used inshells, cartridges, and percussion caps.Physical PropertiesColorless needles or white powder; bp 350C(662F) (explodes); density about 4.0 g/cm3;slightly soluble in water 0.023% at 18C(64F) and 0.09% at 70C (158F), solublein acetic acid.Health HazardToxicity data for lead azide are not avail-able. Its aqueous solution is toxic, exhibitingpoisoning effect of lead.Fire and Explosion HazardLead azide is a primary explosive. Its det-onation temperature is 350C (662F), andthe detonation velocity is 5.1 km/s (Meyer1989). Its heat of combustion and heat of det-onation are 631 and 368 cal/g, respectively(or 184 and 107 kcal/mol). The released gasvolume is 308 cm3/g at STP. It forms highlyshock-sensitive copper and zinc azides whenmixed with the solutions of copper and zincsalts. Its contact with these metals or theiralloys over a period of time results in theformation of their azides, too. Reaction withcarbon disulfide is violently explosive. Thereis a report of an explosion resulting from theaddition of calcium stearate in a lead azidepreparation (MCA 1962).Disposal/DestructionLead azide is decomposed by treatment withnitrous acid or ceric ammonium nitrate (Wear1981).33.5 LEAD (IV) AZIDEFormula PbN12; MW 375.28; CAS 73513-16-3Structure: Pb(N3)4Synonym: lead tetrazideUses and Exposure RiskThe tetrazide salt, unlike the diazide has nocommercial use because it is too unstable.Health HazardNo data are available on the toxicity of thisunstable substance. It is expected to be highlytoxic. The toxic effects should be those oflead.Fire and Explosion HazardThe tetrazide salt is unstable, decomposingspontaneously. It is highly sensitive to heat,shock and static charge. The decompositioncan occur explosivel