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Minerals; Inorganic; Column Four
Proved by Hartlaub, Trinks, Hering and Nenning. Allen's Encyclop. Mat. Med., Vol. VIII, 1.
Description of the substance
Rarely found free in nature, lead is present in several minerals; but all are of minor significance except the sulfide, PbS (galena, or lead glance), which is the major source of lead production throughout the world. Lead may be extracted by roasting the ore and then smelting it in a blast furnace or by direct smelting without roasting. Additional refining removes impurities present in the lead bullion produced by either process. Almost half of all refined lead is recovered from recycled scrap. When freshly cut, lead oxidizes quickly, forming a dull gray coating, formerly thought to be lead suboxide, Pb2O, but now recognized as a mixture of lead and lead monoxide, PbO, which protects the metal from further corrosion. Similarly, although lead is soluble in dilute nitric acid, it is only superficially attacked by hydrochloric or sulfuric acids because the insoluble chloride or sulfate coatings that are formed prevent continued reaction. Because of this general chemical resistance, considerable amounts of lead are used in roofing, as coverings for electric cables placed in the ground or underwater, and as linings for water pipes and conduits and structures for the transportation and processing of corrosive substances. Lead has many other applications; the largest is in the manufacture of storage batteries. It is used in ammunition (shot and bullets) and as a constituent of various low-melting alloys, such as solder, type metal, and pewter. In the construction of large buildings, lead sheets are used in the walls to block the transmission of sound; and pads of lead and asbestos are used in the foundations to absorb the vibrations caused by street traffic and other sources. Because lead effectively absorbs electromagnetic radiation of short wavelengths, it is used as a protective shielding around nuclear reactors, particle accelerators, X-ray equipment, and containers used for transporting and storing radioactive materials. Lead and its compounds are toxic and are retained by the body, accumulating over a long period of time-a phenomenon known as cumulative poisoning-until a lethal quantity is reached. In children the accumulation of lead may result in cognitive deficits; in adults it may produce progressive renal disease. See also lead poisoning. Lead has four stable isotopes, all of which are the end products of the radioactive decay of other elements; their relative abundances are: lead-204, 1.48 percent; lead-206, 23.6 percent; lead-207, 22.6 percent; and lead-208, 52.3 percent. More than 20 radioactive isotopes have been reported.Compounds. Lead shows valences of +2 and +4 in its compounds. Among the many important lead compounds are the oxides: lead monoxide, or lead(II) oxide, PbO; lead dioxide, or lead(IV) oxide, PbO2; and trilead tetroxide, Pb3O4. Lead monoxide exists in two modifications, litharge and massicot. Litharge, or alpha lead(II) oxide, is a red or reddish yellow solid, has a tetragonal crystal structure, and is the stable form at temperatures below 488° C (910° F). Massicot, or beta lead(II) oxide, is a yellow solid and has an orthorhombic crystal structure; it is the stable form above 488° C. Both forms are insoluble in water but dissolve in acids to form salts containing the Pb2+ ion or in alkalies to form plumbites, which have the PbO22- ion. Litharge is produced by air oxidation of lead. Except for tetraethyllead [Pb(C2H5)4], an organic compound that has been used as a gasoline antiknock additive, litharge is the most important commercial compound of lead; it is used in large amounts directly and as the starting material for the preparation of other lead compounds. Considerable quantities of lead(II) oxide are consumed in manufacturing the plates of lead-acid storage batteries. High-quality glassware contains as much as 30 percent litharge, which increases the refractive index of the glass and makes it brilliant, strong, and resonant. Litharge is also employed as a drier in varnishes and in making sodium plumbite, which is used for removing malodorous thiols (a family of organic compounds containing sulfur) from gasoline. Lead(IV) oxide, found in nature as the brown-to-black mineral plattnerite, is commercially produced from trilead tetroxide by oxidation with chlorine. It decomposes upon heating and yields oxygen and lower oxides of lead. Lead(IV) oxide is used as an oxidizing agent in the production of dyestuffs, chemicals, pyrotechnics, and matches and as a curing agent for polysulfide rubbers. Trilead tetroxide (known as red lead, or minium) is produced by further oxidation of lead(II) oxide. It is the orange-red to brick-red pigment commonly used in corrosion-resistant paints for exposed iron and steel. It also reacts with iron(III) oxide to form a ferrite used in making permanent magnets. Another economically significant compound is lead(II) acetate, Pb(C2H3O2)2, a water-soluble salt made by dissolving litharge in strong acetic acid. The common form, the trihydrate [Pb(C2H3O2)2*3H2O], called sugar of lead, is used as a mordant in dyeing and as a drier in certain paints. In addition, it is utilized in the production of other lead compounds and in gold cyanidation plants, where it primarily serves to precipitate soluble sulfides from solution. Various other salts, most notably basic lead carbonate, basic lead sulfate, and basic lead silicate, were once widely employed as pigments for white exterior paints. Since the mid-20th century, however, the use of such so-called white lead pigments has decreased substantially because of a concern over their toxicity and attendant hazard to human health. The use of lead arsenate in insecticides has virtually been eliminated for the same