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Lead(II) nitrate is an inorganic compound with the chemical formula Pb(NO3)2. It is a colourless crystal or white powder and, unlike most other lead(II) salts, it is soluble in water. Known since the Middle Ages by the name plumb dulcis, its main use has been as raw material in the production of pigments in lead paints. Lead(II) nitrate production from either metallic lead or lead oxide in nitric acid was small-scale, for direct use into other lead compounds only, until commercialisation in the 19th century in Europe and the United States. In the 20th century, it was used in industry as a heat stabiliser in nylon and polyesters, and in coatings of photothermographic paper.
Lead(II) nitrate is toxic, an oxidising agent, and it is categorised as probably carcinogenic to humans by the International Agency for Research on Cancer. Therefore, it must be handled and stored with the appropriate safety precautions to prevent inhalation, ingestion and skin contact. Due to its hazardous nature, lead(II) nitrate has limited applications outside the laboratory.
Since the Middle Ages, lead(II) nitrate has been produced as a raw material for the production of coloured pigments in lead paints, such as chrome yellow (lead(II) chromate), chrome orange (lead(II) hydroxide chromate) and similar lead compounds. These pigments were used for dyeing and printing calico and other textiles.
In 1597, the German alchemist Andreas Libavius first described the compound, coining the medieval names of plumb dulcis and calx plumb dulcis, meaning "sweet lead", because of its taste. Although originally not understood during the following centuries, the decrepitation property of lead(II) nitrate saw its use in matches and special explosives such as lead azide.
The production process was and still is chemically straightforward, effectively dissolving lead in aqua fortis (nitric acid), and subsequently harvesting the precipitate. However, the production remained small-scale for many centuries, and the commercial production of lead(II) nitrate as raw material for the manufacture of other lead compounds was not reported until 1835.
Lead(II) nitrate readily dissolves in water to give a clear colourless solution. This reacts with soluble iodides such as potassium iodide to produce a precipitate of the bright orange-yellow lead(II) iodide. This reaction is often used to demonstrate precipitation, because of the striking colour change observed:
The similar reaction takes place in the solid phase, in which lead nitrate and potassium iodide are finely ground and mixed in a mortar.
Apart from lead(II) nitrate, lead(II) acetate is the only other common soluble lead compound. Nearly all other lead compounds are insoluble in water, even when coupled with commonly very soluble anions. E.g., lead(II) chloride, lead(II) bromide and lead(II) iodide, collectively known as lead halides, are weakly soluble in water (less than 0.01 mole per litre) at room temperature, and only slightly more closer to the boiling point. This means that lead(II) nitrate has particular importance as a starting point for the production of insoluble lead compounds via double decomposition.
Hot solutions of lead halides can be brought to precipitation on cooling to create feathery, iridescent crystals suspended in water, the colour of which crystal depends on the particular halide (chloride = white, bromide = buff, iodide = yellow). These crystals appear suddenly, when the solutions dip below the recrystallisation temperature. This effect is used for demonstration of solubility in classrooms.
When concentrated sodium hydroxide solution is added to lead(II) nitrate solution, basic nitrates are formed, even well past the equivalence point. Up through the half equivalence point, Pb(NO3)2·Pb(OH)2 predominates, then after this point Pb(NO3)2·5Pb(OH)2 is formed. No simple Pb(OH)2 is formed up to at least pH 12.
The crystal structure of solid lead(II) nitrate has been determined by neutron diffraction. The compound crystallises in the cubic system with the lead atoms in a face-centered cubic system. Its space group is Pa3Z=4 (Bravais lattice notation), with each side of the cube with length 784 picometres.
The black dots represent the lead atoms, the white dots the nitrate groups 27 picometre above the plane of the lead atoms, and the blue dots the nitrate groups the same distance below this plane. In this configuration, every lead atom is bonded to 12 oxygen atoms (bond length: 281 picometre). All N–O bond lengths are identical, at 127 picometre.
Research interest in the crystal structure of lead(II) nitrate was partly based on the possibility of free internal rotation of the nitrate groups within the crystal lattice at elevated temperatures, but this did not materialise.
Lead(II) nitrate is associated with interesting supramolecular chemistry because of its coordination to nitrogen and oxygen electron-donating compounds. The interest is largely academic, but with several potential applications. For example, combining lead nitrate and pentaethylene glycol (EO5) in a solution of acetonitrile and methanol followed by slow evaporation produces a new crystalline material [Pb(NO3)2(EO5)]. In the crystal structure for this compound, the EO5 chain is wrapped around the lead ion in an equatorial plane similar to that of a crown ether. The two bidentate nitrate ligands are in trans configuration. The total coordination number is 10, with the lead ion in a bicapped square antiprism molecular geometry.
The complex formed by lead(II) nitrate, lead(II) perchlorate and a bithiazole bidentate N-donor ligand is binuclear, with a nitrate group bridging the lead atoms with coordination number of 5 and 6. One interesting aspect of this type of complexes is the presence of a physical gap in the coordination sphere, i.e., the ligands are not placed symmetrically around the metal ion. This is potentially due to a lead lone pair of electrons, also found in lead complexes with an imidazole ligand.
This type of chemistry is not always unique to lead nitrate; other lead(II) compounds such as lead(II) bromide also form complexes, but the nitrate is frequently used because of its solubility properties and its bidentate nature.
Oxidation and decrepitation
Lead(II) nitrate is an oxidising agent. Depending on the reaction, this may be due to the Pb2+(aq) ion, which has a standard reduction potential (E0) of -0.125 V, or the nitrate ion, which under acidic conditions has an E0 of +0.956 V.
When heated, lead(II) nitrate crystals decompose to lead(II) oxide, dioxygen and nitrogen dioxide, accompanied by a crackling noise. This effect is referred to as decrepitation.
Because of this property, lead nitrate is sometimes used in pyrotechnics such as fireworks.
The compound can be obtained by dissolving metallic lead in aqueous nitric acid:
More commonly, lead(II) nitrate is obtained by dissolving lead oxide, which is readily available as a mineral, in aqueous nitric acid:
Since Pb(NO3)2 has very low solubility in nitric acid, the lead(II) nitrate is crystallised directly from the solution. Most commercially available lead(II) nitrate, as well as laboratorium-scale material, is produced accordingly. Supply is in 25 kg bags, or smaller, and in laboratory containers.
In nitric acid treatment of lead-containing wastes, e.g., in the processing of lead-bismuth wastes from lead refineries, impure solutions of lead(II) nitrate are formed as by-product. These solutions are reported to be used in the gold cyanidation process.
Due to the toxic nature of lead(II) nitrate, there is a preference for using alternatives in industrial applications. In the formerly major application of lead paints, it has largely been replaced by titanium dioxide. Current applications of lead(II) nitrate include use as a heat stabiliser in nylon and polyesters, as a coating for photothermographic paper, and in rodenticides.
On a laboratory scale, lead(II) nitrate provides a reliable source of dinitrogen tetroxide. When the salt is carefully dried, and heated in a steel vessel, it produces nitrogen dioxide along with dioxygen.
The gases are condensed and fractionally distilled to give N2O4.
To improve the leaching process in the gold cyanidation, lead(II) nitrate solution is added. Although a bulk process, only limited amounts (10 to 100 gram lead(II) nitrate per tonne gold) is required. Both the cyanidation itself, as well as the use of lead compounds in the process, are deemed controversial due to the compounds' toxic nature.
Lead(II) nitrate is toxic, and ingestion may lead to acute lead poisoning, as is applicable for all soluble lead compounds. All inorganic lead compounds are classified by the International Agency for Research on Cancer (IARC) as probably carcinogenic to humans (Category 2A). They have been linked to renal cancer and glioma in experimental animals and to renal cancer, brain cancer and lung cancer in humans, although studies of workers exposed to lead are often complicated by concurrent exposure to arsenic. Lead is known to substitute for zinc in a number of enzymes, including δ-aminolevulinic acid dehydratase (porphobilinogen synthase) in the heme biosynthetic pathway and pyrimidine-5′-nucleotidase, important for the correct metabolism of DNA.
To prevent inhalation, ingestion and exposure to skin, lead(II) nitrate must be handled in a fume cupboard, with face, body and hand protection. Special instructions for handling are included in all Material Safety Data Sheets (MSDS). After use, all material and its containers must be disposed of as hazardous waste. Spillage and release to the environment must be avoided.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lead(II)_nitrate". A list of authors is available in Wikipedia.|