Lead(II) azide
Bleidiazid
Colorless acicular crystals
Fixed
- 4.763 g · cm -3 ( α - form)
- 4,845 g · cm -3 ( β - form)
- 4.38 g · cm -3 ( 91.5 % pure with dextrin )
Decomposition above 250 ° C.
Poorly in water ( 230 mg · l -1)
Risk
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Lead azide is the lead salt of hydrazoic acid. It is explosive and is used as a primary explosive.
History
Lead azide was first prepared in 1891 as well as silver azide and Quecksilberazid by Theodor Curtius. The meaning of the compound was detected by the military trial Office in Berlin early in 1907 and a detonator was patented on the basis of this salt by Lothar Wohler. Due to the high tendency to spontaneous explosions went to the general international application several decades.
Representation and extraction
The preparation is carried out in batch or continuous process by reacting aqueous solutions of sodium azide and lead nitrate. It is important to avoid the formation of large crystals, since low mechanical stress such as the breaking of crystal needles already can cause an explosion. For this reason, dextrin, polyvinyl alcohol or any other crystal growth are added interfering substances so that a product with a technical Bleiazidgehalt 92-96 % results.
Properties
Lead azide forms colorless crystals which are relatively resistant to heat and humidity and slightly hygroscopic. It is practically insoluble in water. Lead azide occurs in four polymorphic modifications. These are an orthorhombic α - form, β - form, a monoclinic, a monoclinic form and a γ - δ triclinic form. The compound has explosive properties, particularly the mechanical sensitivity to shock, impact and friction is relevant. It is relatively heat stable and decomposes only above 315 ° C. The decay products are finely divided lead and nitrogen. Important explosion indicators are:
- Heat of explosion: 1639 kJ · kg -1.
- Detonation velocity: 4630 m · s-1 at the density of 3.0 g · cm -3 and 5180 m · s-1 at the density of 4.0 g · cm -3
- Normal gas volume: 308 l · kg -1.
- Specific energy: 380 kJ · kg -1
- Deflagration: 315-360 ° C
- Lead block expansion: 11 cm3 · g -1
- Impact sensitivity: 2,5 - 4 N · m ( pure) or 3 to 6.5 N · m ( technically )
- Friction sensitivity: 0.1 N
Pure lead azide is also very high electrostatically sensitive and ignites easily by field breakdown. The data relate to the common α - form. The β - form is much more sensitive. Destruction of lead azide can be carried out in an aqueous solution of 8% sodium nitrite and 15% nitric acid or in a 10% sodium hydroxide solution.
Use
Lead azide has become since its first use as a primary explosive ( SN -Martin ) to the main initiating explosive, which has the previously used mercury fulminate nearly completely. Despite lower energy content and lower density, it has greater initial force is less sensitive to impact than mercury fulminate and also has a much greater stability at higher temperatures and moisture. The introduction of aluminum to the detonator production and thus a significant cost reduction was possible only through the use of lead azide, mercury fulminate as forms an amalgam with aluminum.
Toxicity
Lead azide is toxic and toxic to reproduction. It is classified as carcinogenic category 2 and a germ cell mutagen in the 3A category.