Heavy-Metal Azides

Lead azide is only very slightly soluble in water, but the solubility is greatly increased by the addition of acetates or nitrates [3,17]. This increased solubility results from the complexation of the lead ion with the added anions. Because the solubility is temperature dependent, slow cooling can be used as a growth technique. The resulting crystals are of the orthorhombic modification (a-lead azide) [18]. 

The monoclinic form ( 3-lead azide) was first grown by Miles by a diffusion technique [18]. Dilute solutions of a lead salt and sodium azide were allowed to diffuse together in a water medium to cause crystallization. Numerous spontaneous explosions occurred using this method however, the crystals, once 

A second monoclinic modification (7-lead azide) and a triclinic modification (5-lead azide) have been identified (Chapter 3). These modifications form by changing the pH of the growing solution and by doping. It is found that polyvinyl alcohol, added to 5% solutions of ammonium acetate, favor the gamma modification when slow cooled below 40°C [21]. Other additives favor the formation of other polymorphs for example, dextrine favors the a-polymorph and eosin, the j3-polymorph [21]. These latter polymorphs are reported to be more light sensitive than either the alpha or beta polymorphs. 

The gel growth technique, used infrequently because of difficulties in controlling the purity and growth parameters, has nevertheless been used successfully for some materials having low solubility in the gel medium, normally water. 

The gel growth of lead azide occurs due to the reaction Pb(X) + 2NaNa--------  

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The major use of inorganic azides exploits the explosive nature of heavy metal azides. Pb(N3)2 in particular is extensively used in detonators because of its reliability, especially in damp conditions it is prepared by metathesis between Pb(N03)2 and NaN3 in aqueous solution. 

As a heavy metal azide, it is considerably endothermic (A// +279.5 kJ/mol, 1.86 kJ/g). While pine silver azide explodes at 340°C [1], the presence of impurities may cause explosion at 270° C. It is also impact-sensitive and explosions are usually violent [2], Its use as a detonator has been proposed. Application of an electric field to crystals of the azide will detonate them, at down to — 100°C [3], and it may be initiated by irradiation with electron pulses of nanosecond duration [4], See other catalytic impurity incidents, irradiation decomposition... 

Insensitive to impact, it decomposes, sometimes explosively, above its m.p. [1], particularly if heated rapidly [2], Although used in aqueous solutions as a preservative in pharmaceutical preparations, application of freeze-drying techniques to such solutions has led to problems arising from volatilisation of traces of hydrazoic acid from non-neutral solutions, condensation in metal lines, traps or filters, and formation of heavy metal azides in contact with lead, copper or zinc components in the drying plant [3,4],... 

The majority of the metal azides are sensitive explosives and exposure to heat, friction or impact is usually undesirable. Contact of most azides, particularly readily soluble ones, with acids will produce hydrogen azide, itself an explosive and highly toxic low-boiling liquid. In presence of heavy metals, it may give other equally hazardous heavy metal azides. These may also be formed from contact of soluble azides with heavy metals. 

Lead azide is an explosive solid that can be detonated by shock (or by heating to 350 °C), while sodium azide exists as stable white crystals that decompose smoothly on heating (unless allowed to react with extraneous material). Why are heavy metal azides so explosive Why are lead azide detonators not sheathed in copper or brass ... 

Mercury fulminate is easy to produce, has been known since earliest times and is still widely used. The scarcity of mercury has however led to many attempts to replace this substance by something else, in particular by substances containing a different metal. Some success was achieved as a result of work of Will and Lenze [5] in 1892 on the application of heavy-metal azides as initiating agents. 

The infra-red spectra of the heavy metal azides, which are the most interesting because of their explosive properties, were investigated by Gamer and Gomm [37], Lecomte et al. [38], and the Raman spectra have been studied by Kohlrausch and Wagner [36], and by Deb and Yoffe [26]. The results are given in Table 29. 

Fig. 50. Column for continuous manufacture of heavy metal azides and lead styphnate, according to Meissner [110] /,2—inflow of reacting solutions, 3,4—reaction column, 5—air nozzle with exit openings, 6 and 7 directed up- and downwards, respectively,...

According to Wohler Martin (Ref 6), Zn (N3)a is detonated under impact of a 2 kg wt and exploded in 5 sec at 289°. The heat of detonation is 360 cal/g (Ref 6) and -50.8 kcal/mol (Ref 14). These investigators consider Zn(Ns)a a rather weak expl approaching in its expl props, the alkaline earth azides which are not as powerful as the heavy metal azides... 

The K salt is rather sensitive to expln by shock, heat or friction. Crysts may expl violently when broken or rubbed in an agate mortar. When heated rapidly in air the subst detonates with a sharp expln, but less violently than the heavy metal azides. On expln in air, a spectacular flame is produced with the liberation of much heat and the formation of numerous products ... 

Heavy Metals. Contact of aqueous solutions with heavy metals (brass, Cu, and Pb) may lead to the formation of explosive heavy metal azides (e.g., in plumbing lines).7... 

Typical reaction conditions for the formation of lanthanide azides are described in Eq. (5) [72-73], Like other heavy metal azides, lanthanide azides can be... 

Sodium azide and other heavy metal azides are too dangerous to be handled by this procedure and should be treated as explosives. Sodium amide and potassium amide can be destroyed by standard procedures. Many other compounds in which nitrogen is linked to a metal should be disposed of as potential explosives. 

Fidlar patented a flashlegs propellent explosive formed of nitrocellulose, starch, and dinitrotoluene. Grotta patented compound detonators with a priming charge formed of mercury fulminate, a heavy-metal azide, and a secondary charge formed of a mixture of equal amounts of Tetryl and potassium chlorate. 

Heavy metal azides have more covalent structures and detonate upon heating or mechanical stress. Lead diazide Pb(N3)2 is used as an initiator for explosives. Like chlorine, azide can act as a ligand in complexes. The complex azido anions are often more stable than binary azide complexes (see Ammonia N-donor Ligands). Dinuclear azide complexes are found with two coordination modes of the azide ion, (56) and (57) depicted in Scheme 20. 

Violent reaction with benzoyl chloride combined with KOH, Bt2, barium carbonate, CS2, Cr(OCl)2, Cu, Pb, HNO3, BaCOs, H2SO4, hot water, (CH3)2S04, dibromomalononitrile, sulfuric acid. Incompatible with acids, ammonium chloride + trichloroacetonitrile, phosgene, cyanuric chloride, 2,5-dinitro-3-methylbenzoic acid + oleum, trifiuroroacryloyl chloride. Reacts with heavy metals (e.g., brass, copper, lead) to form dangerously explosive heavy metal azides, a particular problem in laboratory equipment and drain traps. When heated to decomposition it emits very toxic fumes of NOx and Na20. See also AZIDES. 

The physical and chemical properties of the inorganic azides have been extensively reviewed [4-11], Richter [12] has discussed the chemical classification of azides as (i) stable ionic azides, (ii) heavy-metal azides and (iii) unstable covalent azides. This classification is based on the percentage ionic character of the metal-azide bond, tabulated as formal ionicities in [12]. For example, the Na-Nj and Ba-Nj bonds are 70% ionic, but Pb-N, is only 34% and H-Nj is 22%. Bertrand et al. [13] have reviewed the photochemical and thermal behaviour of organometallic azides. Richter has also given [12] an excellent review of the methods of preparation of HNj and other azides. He criticizes early workers for inadequate purification and characterization of their starting materials and their neglect of allowance for the possible formation of hydrates (e.g., barium azide may be present as Ba(N3)2.1. SHjO below 284 K, forms a monohydrate between 284 and 325.5 K and is anhydrous above 325.5 K). 

The behaviour of the heavy-metal azides on heating is less predictable, some being extremely unstable and hence of value as primary explosives. Nevertheless it has been stated [18] that "All solid inorganic azides can be thermally decomposed at controllable and measurable rates". Azides which detonate under well-defined conditions have become model systems for the development of theories of "fast" reactions in solids [9,10]. 

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