Azides of Lead

Lead forms various azides in its tetravalent state like the other IVA elements, but is the only member of the group to also form a normal azide in its lower (Pb ) oxidation state. 

Pb(N3)2 is better known than almost any inorganic azide in fact, the number of publications written on it is surpassed only by sodium azide and HN3. This is undoubtedly because lead azide has been used for nearly 60 years as an effective, if sometimes problematic, primary explosive. It is not the first azide to be considered for military use the feasibility of silver azide was explored in Germany as early as 1904. In 1911, lead azide was suggested as a military explosive by Wohler [224], and being technically and economically superior [275] to mercury fulminate, has gradually replaced the latter. 

The chemistry of making lead azide is quite simple. It is precipitated in a metathetic reaction by mixing solutions of a lead salt and sodium azide  

In practice, however, there are factors such as purity, polymorphism, particle form, etc., which are of great technological concern as they may influence the explosive behavior and the storage compatibility of the product. Controlling these factors is essential, and thanks to continued research, our knowledge of the underlying principles has advanced in at least some areas. Others lag behind, owing in part to a lack of appropriate study materials. From the viewpoint of 

Lead azide is a white, crystalline solid which is sparingly soluble in water [276] its solubility may be enhanced by complexation [157,276]. Hydrolysis at room temperature is negligible [197]. The compound is somewhat photosensitive and, unless handled under yellow light, appears buff or gray [175,176]. It is highly sensitive to impact, heat shock, and friction and explodes with high brisance [197]. Thermal decomposition leads to explosion above 345 C [2,176]. 

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Introduction of oxygen into a Minisci-type reaction mixture leads to formation of amino ketones (Scheme 94). The mixed acetate/azide of lead(IV) with styrene in acetonitrile at -20 C yields phen-acyl azide (60%). One example of azirine formation has already been discussed (Scheme 87). Other related syntheses from vinyl azides are included in a recent review. ... 

The azides of group fVA display properties which are, for the most part, symptomatic of covalent element-to-azide bonds. Evidence of ionic influence appears late in the group and remains small, for example in the ability to form an azido complex of Sn(IV). Even the azides of lead, the most electropositive IVA metal, are predominantly covalent, although the ionic bonding component is sufficiently strong to form the only divalent normal azide of the group. 

As noted in the introduction, the small band gap azides of lead, silver, and thallium exhibit many similar properties which differentiate them from the large band gap azides. Barium azide may be an intermediate case since with irradiation it shows properties similar to both groups of materials. The small band gap azides in question detonate while barium azide deflagrates but will not sustain detonation. When the small band gap azides, barium azide, and silver and lead halides are exposed to radiation, decomposition appears to take place in both the metal and anion sublattices. Apparently, colloidal metal is formed from the metal sublattice [7,8,81-84] and, in addition, nitrogen [85,86] or halogen gas [87,88] is liberated from the anion sublattice. The relationship... 

Figure 7. Rates of slow thermal decomposition for the azides of lead, thallium, and sodium as a function of temperature, determined under the same experimental conditions.

Spontaneous explosions of LA occur without apparent reason during the course of its crystallization. This phenomenon is known but not very well tmderstood. These explosions are not exclusive to azides of lead but have been observed also in the case of cadmium, cupric, and mercuric [15]. In the case of lead azide, it has been frequently attributed to the formation of p-lead azide or the growth of large crystals to a point where internal stresses become important [12, 18, 34]. Both of these theories have httle or no experimental support [18]. 

Sinha, S.K. Study of basic azides of lead by thermometric titration. In Hansson, J. (ed.) Proceedings of 3rd Symposium on Chemical Problems Connected with the Stability of Explosives, pp. 16-32. Sektionen for detonik och Forbranning, Ystad (1973)... 

The organometallic azides of lead(IV) are more stable and have been widely studied The types R3PbN3 and R2Pb(N3)2 are obtained by the action of hydrazoic acid upon R3PbOH or R2PbO. 

Many different explosives were tested. Attanpts were made to produce explosives in World War I that would also produce toxic gases or fumes. Other explosives that used cheap and plentiful raw materials were also in demand. Finally, many of the fuses or detonators in shells malfunctioned, and explosives were sought for use that would more surely detonate on impact bnt not detonate npon handling or firing. Lyconite, various chlorates and perchlorates, azides of lead, strontium and thallium, and hydrazine nitfate were tested. Much of this work occurred at the AUES, but many of these private companies also had laboratories where this experimentation took place. 

Lead azide is not readily dead-pressed, ie, pressed to a point where it can no longer be initiated. However, this condition is somewhat dependent on the output of the mixture used to ignite the lead azide and the degree of confinement of the system. Because lead azide is a nonconductor, it may be mixed with flaked graphite to form a conductive mix for use in low energy electric detonators. A number of different types of lead azide have been prepared to improve its handling characteristics and performance and to decrease sensitivity. In addition to the dextrinated lead azide commonly used in the United States, service lead azide, which contains a minimum of 97% lead azide and no protective colloid, is used in the United Kingdom. Other varieties include colloidal lead azide (3—4 pm), poly(vinyl alcohol)-coated lead azide, and British RE) 1333 and RE) 1343 lead azide which is precipitated in the presence of carboxymethyl cellulose (88—92). 

Ma.nufa.cture. Lead azide is typically made from sodium azide [26628-22-8] in small (eg, 5 kg) batches buffered by the reaction solutions of lead nitrate or lead acetate ... 

Many investigations are reported on azides of barium, calcium, strontium, lead, copper, and silver in the range 100 to 200°C (212 to 392°F). Time exponents were 6 to 8 and activation energies of 30 to 50 kcal/g mol (54,000 to 90,000 Btu/lb mol) or so. Some difficulties with reproducibility were encountered with these hazardous materials. 

Lead azide, Pb(N3)2, is used as a detonator in car airbags. The impact of a collision causes PMN to be converted into an enormous amount of gas that fills the airbag. A125°C, a saturated solution of lead azide is prepared by dissolving 25 mg in water to make 100.0 mL of solution. What is Kv for lead azide ... 

The reaction of Lead Azide (LA) with Cu (see Table) deserves special comment, Although this reaction is relatively slow, even in the presence of w, some forms of Cu Azide are so sensitive that they create a serious hazard even in minute quantities, particularly when in contact with LA. For this reason, AJ and stainless steel containers are now used exclusively. PicArsn requires that all new fuze designs contain no Cu or Cu alloys, with the possible exception of the electrical system. Even here, the Cu must be coated for protection against the formation of hydrazoic acid. Another prohibition involves the use of Pb thiocyante in contact with A1 (Refs 4, 5 6)... 

The work of Avrami et al on the impact sensy of Lead Azide-water (LA-w) mixts contg various expls and drying agents, conducted with the standard PicArsn impact test app, is of interest. 

H. Jackson, Impact Sensitivity of Lead Azide in Various Solid and Liquid Media ,... 

Not detonated by 0.35g of Lead Azide and 0.2g of Tetryl, or by 0.5g of Mercury Fulminate 6 minutes Good... 

Four samples of Lead Styphnate were analyzed by dispersing in acet and were found to have average diameters of 15.9 y with a standard deviation of 0.4p. The reproducibility for Tetracene, with average diameter of 35 y, was 2y. Methanol was found to be a satisfactory dispersant for Lead Azide with average particle diameter of 12y... 

C.C. Cornells, Investigation of Lead Azide Memory Effect , Technical Report No 226, Mason and Hanger-Silas Mason Co, Inc, (1974)... 

L. Avrami et al, Preliminary Studies on Pulsed Electric Field Breakdown of Lead Azide , PATR 4991 (1976) 128) D.P. Tetz, Teledyne... 

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

Mass effects due to some ions in salts. It is generally observed that there is a greater instability amongst compounds containing heavy atoms compared with elements in the first periods of the periodic tabie.This can be observed by analysing enthalpies of formation of ammonia, phosphine, arsine and stibine (see previous table for the last three). In the same way, it is easier to handle sodium azide than lead azide, which is a primary explosive for detonators. It is exactly the same with the relatively highly stable zinc and cadmium thiocyanates and the much less stable mercury thiocyanate. 

Two crystallographic forms of lead azide are important, the ordinary alpha form which is orthorhombic and the beta form which is monoclinic. The densities of these forms are 4-71 and 4-93 respectively. It was for many years believed that the beta form is the more sensitive to friction and impact and accounted for detonations which have occurred in the manufacture and handling of the substance. It is now known that the beta form is in fact no more sensitive than the alpha. Even the alpha form, when present as large crystals, is very sensitive and conditions can arise (particularly when the formation of the lead azide is controlled by diffusion effects) where spontaneous detonation occurs. Although with modern knowledge these hazards can be avoided, pure lead azide is nevertheless a dangerous compound and is now made only for military purposes. 

Commercially, lead azide is usually manufactured by precipitation in the presence of dextrine, which considerably modifies the crystalline nature of the product. The procedure adopted is to add a solution of dextrine to the reaction vessel, often with a proportion of the lead nitrate or lead acetate required in the reaction. The bulk solutions of lead nitrate and of sodium azide are, for safety reasons, usually in vessels on the opposite sides of a blast barrier. They are run into the reaction vessel at a controlled rate, the whole process being conducted remotely under conditions of safety for the operator. When precipitation is complete, the stirring is stopped and the precipitate allowed to settle the mother liquor is then decanted. The precipitate is washed several times with water until pure. The product contains about 95% lead azide and consists of rounded granules composed of small lead azide crystals it is as safe as most initiating explosives and can readily be handled with due care. 

Many other methods of making lead azide in a safe form have been described, but the only one to have found commercial importance consists of replacing the dextrine by a small proportion of gelatine. When properly made this form of lead azide is as safe to handle as the dextrinated form and has improved sensitiveness to flame. It can therefore be used by itself in electric and delay detonators, but not in plain detonators as it is not ignited with certainty by safety fuse. 

The original initiating explosive used by Nobel and all manufacturers for many years was mercury fulminate. This had the disadvantage of decomposing slowly in hot climates, particularly under moist conditions. For this reason mercury fulminate is no longer widely used. In most countries it has been replaced by a mixture of dextrinated lead azide and lead styphnate. In the U.S.A. some detonators are made containing diazodinitrophenol. 

The introduction of lead azide led to a difficulty in the choice of metal for the detonator tube. Under moist conditions, lead azide and copper can react to form cuprous azide on the inner wall of the tube and thus in a particularly dangerous position. Therefore with plain detonators, which cannot be sealed, copper cannot be used when lead azide is employed. Such detonators are usually made from aluminium tubes, or occasionally zinc. 

The initiating explosive used must ignite with certainty from the spit of a safety fuse. It must be remembered that the intensity of the spit can be reduced if the safety fuse is not cut squarely and also that the fuse may in practice not always be fully inserted into the detonator. Lead azide by itself is not sufficiently easily ignited to give a satisfactory plain detonator and it is therefore used in admixture with lead styphnate, which is very readily ignited by flame. The proportions of such mixtures vary from 25 to 50% of lead styphnate. Mercury fulminate and diazodinitrophenol are sufficiently sensitive to flame not to require such additives. 

The normal initiating charge in a British detonator is lead azide modified with gelatine. In the case of plain detonators a small proportion of lead styphnate is added to the azide to ensure satisfactory ignition from safety fuse. 

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