red lead

Lead tetroxide
IUPAC name	dilead(II) lead(IV) oxide
Identifiers
CAS number	[1314-41-6]
Properties
Molecular formula	Pb3O4 2PbO.PbO2
Molar mass	685.598 g/mol
Appearance	orange red powder
Density	8.3 g/cm3
Melting point	500°C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Unit cell of Pb₃O₄

Part of red lead's crystal structure

Red lead, also called minium, lead tetroxide or triplumbic tetroxide, is a bright red or orange crystalline or amorphous pigment. Its 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]. A map and list of known occurrences is available here.

The melting point of lead tetroxide is 500 °C, at which it decomposes to lead(II) oxide and oxygen.

Chemically red lead is lead tetroxide, Pb₃O₄, or 2PbO.PbO₂. It is used in the manufacture of batteries, lead glass and rust-proof primer paints.

Red lead is virtually insoluble in water. However, it is soluble in hydrochloric acid present in the stomach, and therefore it is toxic when ingested. It is also insoluble in alcohol. It dissolves in hydrochloric acid, glacial acetic acid, and diluted mixture of nitric acid and hydrogen peroxide.

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 lead white:

3 Pb₂CO₃(OH)₂ + O₂ → 2 Pb₃O₄ + 3 CO₂ + 3 H₂O

In solution, lead tetroxide can be prepared eg. by reaction of potassium plumbate with lead acetate:

K₂PbO₃ + 2 Pb(OCOCH₃)₂ + H₂O → Pb₃O₄ + 2 KOCOCH₃ + 2 CH₃COOH

yielding yellow insoluble lead tetroxide monohydrate, Pb₃O₄.H₂O, which can be turned into the anhydrous form by gentle heating.

Chemical properties

With iron oxides and with elementary 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.

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

Use

Lead tetroxide is most often used as a pigment for undercoat paints for iron objects. Due to its toxicity its use is being limited. In past it was used in combination with linseed oil as a thick, long-protecting anticorrosive paint. Also combination of minium and linen fibres was 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.

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 gets dissolved in the gastric acid and gets 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.

Minium from a mine fire at Broken Hill, Australia

History

Lead tetroxide was used as a red pigment in ancient Rome, where it was prepared by calcination of lead white. 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. It was first isolated as a pure compound by Arabic chemists and was clearly described by Muhammad ibn Zakarīya Rāzi (Rhazes) in the early 10th century.^[2]

References

External links

• National Pollutant Inventory - Lead and Lead Compounds Fact Sheet
• Minium mineral data