General Description
Odorless gray or yellow green or red-brown solid. Sinks in water.
Reactivity Profile
LITHARGE has weak oxidizing or reducing powers. Redox reactions can however still occur. The majority of compounds in this class are slightly soluble or insoluble in water. If soluble in water, then the solutions are usually neither strongly acidic nor strongly basic. These compounds are not water-reactive. Aluminum carbide is oxidized with incandescence on warming with lead oxide, [Mellor, 1946, Vol. 5, 872]. Mixtures of lead oxide with aluminum powder(as with other metals: sodium, zirconium) give a violent explosions, [Mellor, 1946, Vol. 5, 217, 1941].
Health Hazard
General symptoms of lead poisoning (delayed). Inhalation or ingestion causes abdominal pain (lead colic), metallic taste in mouth, loss of weight, pain in muscles, and muscular weakness. Dust may irritate eyes.
Description
Lead(II) oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure. Both forms occur naturally as rare minerals. The red form is known as “Litharge” and the yellow form is known as “Massico”.
Physical properties
The oxide exhibits two crystalline modifications, the reddish or orange-red alpha form, known as litharge, and the yellow beta form, massicot. The alpha form constitutes tetragonal crystals while the beta modification is a yellow amorphous powder of orthorhombic crystal structure. The alpha form is stable at ordinary temperatures, converting to the beta form when heated at 489°C; density 9.35 g/cm3 (beta form); Moh’s hardness 2 (alpha form); the oxide melts at 888°C; vaporizes at 1,472°C with decomposition; vapor pressure 1 torr at 943°C and 5 torr at 1,039°C; practically insoluble in water (the solubility of alpha form is 17 mg/L at 20°C and that of beta form 23 mg/L at 22°C); insoluble in ethanol; soluble in dilute nitric acid and alkalies.
Production Methods
Lead monoxide is obtained commercially by two processes, Barton process and the Ball Mill process. The Ball-Mill process involves reaction of molten lead with oxygen or air, and in the Barton process atomized molten lead is stirred in a mechanical furnace above 550°C. The molten metal splashed by the stirring paddle comes in contact with air fed into the cover of the furnace through a pipe, thus forming a mist of finely divided lead monoxide. The mist also contains a small amount of unreacted lead. The mist is passed through an upright shaft where a major portion of unreacted lead falls back into the furnace. It is then rapidly cooled and collected in condensing chambers. The crude product may contain 1 to 3% lead. It is finely ground and sold. The remaining lead in the crude product may be converted into the lead monoxide by stirring the molten mass in presence of air for several hours. The hot product is then cooled rapidly to a temperature below 300°C to prevent any formation of lead tetroxide, Pb3O4.
In an alternate process, a variation of the above method, molten lead is atomized in a shaft furnace. An air stream carries the very finely divided metal into the hot zone of the shaft furnace where the metal evaporates and oxidizes producing very finely divided lead monoxide. The product is passed through the cold zone of the furnace and cooled rapidly. The product obtained is a yellow powdery material, the beta form of lead oxide, massicot, consisting of orthorhombic crystals.
The red lead oxide (the tetragonal alpha modification) is obtained by slow cooling of the lead monoxide melt. The solidified mass may contain the red alpha form of the oxide resulting from slow cooling of the melt, under an outer layer of yellow beta form that may result from the rapid cooling of the outer portion.
Lead monoxide also is produced by a modified Ball Mill process in which high purity lead balls placed in the mill are partially oxidized to produce black or grey oxide. Both the red and yellow form of the oxide may be prepared by alkaline dehydration of lead hydroxide, Pb(OH)2.
Flammability and Explosibility
Notclassified
Purification Methods
Higher oxides are removed by heating under vacuum at 550o with subsequent cooling under vacuum. It is red at room temperature but becomes yellow at high temperatures (~480o) reversibly. [Ray & Ogg J Am Chem Soc 78 5994 1956, Kwestroo et al. J Inorg Nucl Chem 29 39 1967.]
