Lead tetroxide
| Lead tetroxide | |
|---|---|
| Red lead powder | |
| lead(II,IV) oxide | |
| Identifiers | |
| CAS number | 1314-41-6 7px |
| ChemSpider | 21169908 7px |
| Jmol-3D images | Image 1 |
| |
| Molecular formula | Pb3O4 2PbO·PbO2 |
| Molar mass | 685.598 g/mol |
| Appearance | orange red powder |
| Density | 8.3 g/cm3 |
| Melting point |
500°C |
| Solubility in water | insoluble |
| Structure | |
| Crystal structure | Tetragonal, tP28 |
| Space group | P42/mbc, No. 135 |
| Hazards | |
| Main hazards | toxic from lead presence |
| Flash point | none |
| Autoignition temperature |
none |
| 14px (verify) (what is: 10px/10px?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | |
| Infobox references | |
Lead tetroxide, also called minium, red lead or triplumbic tetroxide, is a bright red or orange crystalline or amorphous pigment. Chemically, red lead is lead tetroxide, Pb3O4, or 2PbO·PbO2.
This compound's Latin name minium originates from the Minius River in northwest Spain where it was first mined. Natural minium is uncommon, forming only in extreme oxidizing conditions of lead ore bodies. The best specimens known come from Broken Hill, New South Wales, Australia, where they formed as the result of a mine fire.[1]
Lead tetroxide is used in the manufacture of batteries, lead glass and rust-proof primer paints.
Contents
Structure
Lead tetroxide has a tetragonal crystal structure at room temperature, which transforms to an orthorhombic (Pearson symbol oP28, Space group = Pbam, No 55) form at temperature 170 K. This phase transition only changes the symmetry of the crystal and slightly modifies the interatomic distances and angles.[2]
- Red-lead-unit-cell-3D-balls.png
Unit cell of tetragonal Pb3O4
- Red-lead-3D-balls.png
Part of tetragonal red lead's crystal structure
Preparation
Lead tetroxide is prepared by calcination of lead(II) oxide (also called litharge) in air at about 450 to 480 °C:
- 6 PbO + O2 → 2 Pb3O4
The resulting material is contaminated with lead(II) oxide. If a pure compound is desired, PbO can be removed by a potassium hydroxide solution:
- PbO + KOH + H2O → K[Pb(OH)3] (aq)
Another method of preparation relies on annealing of lead carbonate (cerussite) in air:
- 6 PbCO3 + O2 → 2 Pb3O4 + 6 CO2
Yet another method is oxidative annealing of white lead:
- 3 Pb2CO3(OH)2 + O2 → 2 Pb3O4 + 3 CO2 + 3 H2O
In solution, lead tetroxide can be prepared e.g. by reaction of potassium plumbate with lead acetate, yielding yellow insoluble lead tetroxide monohydrate, Pb3O4·H2O, which can be turned into the anhydrous form by gentle heating:
- K2PbO3 + 2 Pb(OCOCH3)2 + H2O → Pb3O4 + 2 KOCOCH3 + 2 CH3COOH
Reactions
Red lead is virtually insoluble in water and in alcohol. However, it is soluble in hydrochloric acid present in the stomach, and is therefore toxic when ingested. It also dissolves in glacial acetic acid and a diluted mixture of nitric acid and hydrogen peroxide.
When heated to 500 °C, it decomposes to lead(II) oxide and oxygen. At 580 °C, the reaction is complete.
- 2 Pb3O4 → 6 PbO + O2
Nitric acid dissolves the lead(II) oxide component, leaving behind the insoluble lead(IV) oxide:
- Pb3O4 + 4 HNO3 → PbO2 + 2 Pb(NO3)2 + 2 H2O
With iron oxides and with elemental iron, lead tetroxide forms insoluble iron(II) and iron(III) plumbates, which is the basis of the anti-corrosive properties of lead-based paints applied to iron objects.
Use
Lead tetraoxide is most often used as a pigment for primer paints for iron objects. Due to its toxicity, its use is being limited. In the past, it was used in combination with linseed oil as a thick, long-lasting anti-corrosive paint. The combination of minium and linen fibres was also used for plumbing, now replaced with PTFE tape. Currently it is mostly used for manufacture of glass, especially lead glass. It finds limited use in some amateur pyrotechnics as a relatively potent oxidizer.
Red lead was also used for engineer's scraping, before being supplanted by Engineer's blue. In traditional Chinese medicine, red lead is used to treat ringworms and ulcerations, though the practice is limited due to its toxicity.
Physiological effects
When inhaled, lead tetroxide irritates lungs. In case of high dose, the victim experiences a metallic taste, chest pain, and abdominal pain. When ingested, it is dissolved in the gastric acid and absorbed, leading to lead poisoning. High concentrations can be absorbed through skin as well, and it is important to follow safety precautions when working with lead-based paint.
Long-term contact with lead tetroxide may lead to accumulation of lead compounds in organisms, with development of symptoms of acute lead poisoning. Chronic poisoning displays as agitation, irritability, vision disorders, hypertension, and also a grayish facial hue.
Lead tetroxide was shown to be carcinogenic for laboratory animals. Its carcinogenicity for humans was not proven.
History
Lead tetroxide was used as a red pigment in ancient Rome, where it was prepared by calcination of white lead. In the ancient and medieval periods it was used as a pigment in the production of illuminated manuscripts, and gave its name to the minium or miniature, a style of picture painted with the colour. As a finely divided powder, it was also sprinkled on dielectric surfaces to study Lichtenberg figures.
See also
- Lead(II) oxide, PbO
- Lead dioxide (Lead(IV) oxide), PbO2
References
External links
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