Lead - chemistry.

Lead - chemistry. I INTRODUCTION Lead, symbol Pb (Latin plumbum, a lead weight), dense, bluish-gray metallic element that was one of the first known metals. The atomic number of lead is 82; the element is in group 14 (or IVa) of the periodic table (see Periodic Law). Lead was mentioned in the Old Testament (see Bible). It was used by the Romans for making water pipes, soldered with an alloy of lead and tin. II PROPERTIES Metallic lead is a soft, malleable, ductile metal. When gently heated it can be forced through annular holes or dies. It has low tensile strength and is a poor conductor of electricity. A freshly cut surface has a bright silvery luster, which quickly turns to the dull, bluish-gray color characteristic of the metal. Lead melts at 327.46°C (621.43°F), boils at 1749°C (3180°F), and has a specific gravity of 11.35; the atomic weight of lead is 207.2. Lead is soluble in nitric acid but is little affected by sulfuric or hydrochloric acids at room temperature. In the presence of air, it slowly reacts with water to form lead hydroxide, which is slightly soluble. Lead is toxic when taken internally; although ordinary water usually contains salts that form a coating on pipes, inhibiting the formation of soluble lead hydroxide, pipes used for carrying drinking water should not contain lead. Lead occurs naturally in eight isotopic forms, of which four are stable and four radioactive. The stable isotopes, lead-206, lead-207, and lead-208, are, respectively, the end products of the uranium, actinium, and thorium series of radioactive decay; lead-204, also stable, has no natural radioactive precursors (see Radioactivity). III OCCURRENCE Lead is widely distributed all over the world in the form of its sulfide, the ore galena. Lead ranks about 36th in natural abundance among elements in Earth's crust. Ores of secondary importance are cerussite and anglesite. The principal method of extracting lead from galena is to roast the ore--that is, convert it to the oxide, and reduce the oxide with coke in a blast furnace (see Metallurgy). Another method is to roast the ore in a reverberatory furnace until part of the lead sulfide is converted to lead oxide and lead sulfate. The air supply to the furnace is then cut off and the temperature raised; then the original lead sulfide combines with the lead sulfate and lead oxide to form metallic lead and sulfur dioxide. Waste material, such as battery scrap, recovered from various industrial processes, is also smelted and constitutes an important source of lead. Because galena often has other minerals associated with it, the crude lead, or pig lead, that is obtained from the smelting processes contains metals such as copper, zinc, silver, and gold as impurities. The recovery of precious metals from lead ores is often as important economically as the production of lead itself. Silver and gold are recovered by the Parkes process, whereby a small amount of zinc stirred into molten lead dissolves the precious metals. This molten alloy then rises to the surface of the lead as an easily removed scum, and the zinc is removed from the silver or gold by distillation. Pig lead is often purified by stirring molten lead in the presence of air. The oxides of the metallic impurities rise to the top and are skimmed off. The purest grades of lead are refined electrolytically. IV USES Lead is used in enormous quantities in storage batteries and in sheathing electric cables. Large quantities are used in industry for lining pipes, tanks, and X-ray apparatus. Because of its high density and nuclear properties, lead is used extensively as protective shielding for radioactive material. Among numerous alloys containing a high percentage of lead are solder, type metal, and various bearing metals. A considerable amount of lead is consumed in the form of its compounds, particularly in paints and pigments. V PRODUCTION Principal sources of lead are found in Australia, the United States, Canada, Mexico, Peru, Serbia, and Russia. The United States consumes about half of the world production of lead, and it formerly produced about one-third of the world supply. Since World War II ended in 1945, the richest veins of galena have been exhausted, and U.S. output has been greatly reduced. VI COMPOUNDS OF LEAD Basic lead carbonate, (PbCO3)2 · Pb(OH)2, called white lead, has been used for over 2000 years as a white pigment. It is also used in ceramic glazes and in making other pigments. In recent years, however, because of the dangers of lead poisoning, the use of lead-based paints for interior use has largely been discontinued. The socalled Dutch process is the oldest method still in use for making white lead. In this process earthenware pots containing lead gratings and acetic acid are wrapped in tanbark (small pieces of bark that are rich in tannin); the reaction of the fermenting tanbark and the acetic acid is allowed to process the lead over a period of 90 days. More rapid processes, such as electrolysis or forcing hot air and carbon dioxide through large rotating cylinders containing powdered lead and acetic acid, are now industrially important. Lead monoxide, or litharge (PbO), a yellow, crystalline powder formed by heating lead in air, is used in making flint glass, as a drier in oils and varnishes, and in the manufacture of insecticides. Red lead, or minium (Pb3O 4), a scarlet, crystalline powder formed by oxidizing lead monoxide, is the pigment in paint used as a protective coating for structural ironwork and steelwork. Lead chromate, or chrome yellow (PbCrO4), a crystalline powder used as a yellow pigment, is prepared by the reaction of lead acetate and potassium bichromate. Chrome red, orange chrome yellow, and lemon chrome yellow are some of the pigments obtained from lead chromate. Lead acetate (Pb(C2H3 O2)2 · 3H2O), a white, crystalline substance called sugar of lead because of its sweet taste, is prepared commercially by dissolving litharge in acetic acid. It is used as a mordant in dyeing, as a paint and varnish drier, and in making other lead compounds. Lead tetraethyl (Pb(C2H5)4) is the chief constituent of the antiknock compound added to gasoline to prevent premature detonation in internal-combustion engines; it is considered a significant contributor to air pollution. All gasoline-powered automobiles now manufactured in the United States are required to operate on unleaded gasoline. VII LEAD POISONING Lead taken internally in any of its forms is highly toxic; the effects are usually felt after it has accumulated in the body over a period of time. The symptoms of lead poisoning are anemia, weakness, constipation, colic, palsy, and often a paralysis of the wrists and ankles. Flaking lead-based paints and toys made from lead compounds are considered serious hazards for children. Children are especially at hazard from lead, even at levels once thought safe. Lead can reduce intelligence, delay motor development, impair memory, and cause hearing problems and troubles in balance. In adults, one lead hazard at levels once thought safe is that of increased blood pressure. Present-day treatment of lead poisoning includes the administration of calcium disodium ethylenediaminetetraacidic acid, or EDTA, a chelating agent; lead is removed from the body by displacing the calcium in EDTA and forming a stable complex that is excreted in the urine. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.