lead

sounding lead; lead line; a navigation device used to measure water depth from shipboard.

There are three types of leads: manual, mechanical, and hydroacoustic (sonic depth finder). The manual device consists of a lead or cast-iron conical or pyramidal weight (3.5-5 kg) attached to a line that is divided into meters or feet by marks of various kinds and colors. With a manual lead, it is possible to measure depths to 50 m at speeds no faster than 3-5 knots (5-9 km/hr). The operational principle of the mechanical lead is the measurement of the hydrostatic pressure in a tube that is sealed off at the upper end, open at the lower, and immersed in the water. The mechanical lead measures depths from 10 to 200 m at speeds of less than 16 knots (28 km/hr). The mechanical devices used on oceanographic vessels for the measurement of very great depths are called bathymeters (fathometers, depthometers).

Pb, a chemical element in group IV of Mendeleev’s periodic system. Atomic number, 82; atomic mass, 207.2. A heavy, bluish gray metal, lead is very ductile and soft; it can be cut with a knife and scratched with a fingernail. Natural lead is composed of five stable isotopes, with mass numbers 202 (trace), 204 (1.5 percent), 206 (23.6 percent), 207 (22.6 percent), and 208 (52.3 percent). The last three isotopes are the final products in the radioactive transformations of ²³⁸U, ²³⁵U, and ²³²Th. A large number of lead radioisotopes are formed in nuclear reactions.

Historical survey. Lead was known to the civilizations of Mesopotamia, Egypt, and other ancient countries around 6000–7000 B.C. It was used to make statues, household articles, and writing tablets. The Romans were known to have used lead for water pipes. Alchemists called lead Saturn and used the symbol for the planet to designate Pb. Lead compounds, such as lead ash (PbO) and white lead [2PbC0₃. Pb(OH)₂], were used in ancient Greece and Rome as constituents of medicines and dyes. With the invention of firearms, lead was used in making bullets. The toxicity of lead was noted as early as the first century A.D. by the Greek physician Dioscorides and by Pliny the Elder.

Distribution in nature. The content of lead in the earth’s crust (clarke) is equal to 1.6 × 10^-3 percent by weight. The formation of approximately 80 lead-containing minerals, the principal one being galena (PbS), in the earth’s crust is associated, for the most part, with the formation of hydrothermal deposits. A large number (approximately 90) of secondary minerals are formed in the oxidation zones of complex-metal ores. These minerals include sulfates (anglesite, PbSO₄), carbonates (cerussite, PbCO₃), and phosphates [pyromorphite, Pb₅(PO₄)₃Cl].

Lead is for the most part dispersed in the biosphere; the concentration is low in living matter (5 × 10^-5 percent) and in sea-water (3 × 10^-9 percent). Lead is partially sorbed from natural waters by clay and is precipitated by hydrogen sulfide; hence lead is accumulated in sea silts mixed with hydrogen sulfide and in the dark-colored clays and schists formed from silt.

Physical and chemical properties. Lead crystallizes in a face-centered cubic lattice (a = 4.9389 angstroms [Å]) and has no allotropic forms. It has an atomic radius of 1.75 A and ionic radii of 1.26 A for Pb²⁺ and 0.76 Å for Pb⁴⁺. Lead has a density of 11.34 g/cm³ at 20°C, a melting point of 327.4°C, a boiling point of 1725°C, and a specific heat capacity at 20°C of 0.128 kilojoule/kg°K (0.0306 calorie/g°C). Its thermal conductivity is 33.5 watts/m°K (0.08 calorie/cm.sec.°C), thermal expansion coefficient is 29.1 × 10^-6 at room temperature, and Brinell hardness is 25–40 meganewtons/m²(2.5–4 kilograms force/mm²). Lead has a tensile strength of 12–13 meganewtons/m², a compressive strength of approximately 50 meganewtons/m², and the specific ultimate elongation of 50–70 percent. Cold working does not increase the mechanical properties of lead because the recrystallization temperature lies below room temperature (approximately - 35°C at deformations of 40 percent and higher). Lead is diamagnetic, with a magnetic susceptibility of -0.12 × 10^-6; at 7.18°K, lead becomes a superconductor.

The electronic configuration of the outer subshells in a Pb atom is 6s² 6p², in accordance with which the atom exhibits oxidation states of +2 and +4. Lead displays relatively little chemical activity. The metallic luster of freshly cut lead gradually disappears upon exposure to air because of the formation of a very thin PbO film, which shields the metal from further oxidation. Lead combines with oxygen to yield the oxides Pb₂O, PbO, PbO₂, Pb₃O₄, and Pb₂O₃.

In the absence of O₂, water does not react with lead at room temperature, but Pb decomposes hot water vapor to yield lead oxide and hydrogen. The hydroxides corresponding to the oxides PbO and PbO₂, namely Pb(OH)₂ and Pb(OH)₄, are amphoteric in nature.

The compound PbH₄, combining lead and hydrogen, is obtained in small quantities upon the action of dilute hydrochloric acid on Mg₂Pb. PbH₄, a colorless gas, readily decomposes to yield Pb and H₂. Lead combines with halogens upon heating to form the halides PbX₂ (X is a halogen), which are all sparingly soluble in water. Halides with the formula PbX₄ are also obtained; these include lead tetrafluoride (PbF₄), in colorless crystals, and lead tetrachloride (PbCl₄), an oily yellow liquid. Both compounds decompose readily to yield F₂ or Cl₂, and both are hydrolyzed by water. Lead does not react with nitrogen. Lead azide [Pb(N₃)₂] is obtained upon the interaction of solutions of sodium azide (NaN₃) and Pb(II) salts; Pb(N₃)₂ occurs as colorless, acicular crystals, sparingly soluble in water and decomposing upon impact or heating to yield Pb and N₂ with an explosion. Sulfur reacts with lead upon heating to form lead sulfide (PbS)—an amorphous black powder. Lead sulfide may also be obtained by passing hydrogen sulfide into solutions of Pb(II) salts. PbS occurs naturally as lead glance, or galena.

In the electromotive force series, Pb ranks higher than hydrogen (normal electrode potentials of –0.126 volt [V] for Pb ⇆ Pb²⁺ + 2 e and +0.65 V for Pb ⇆ Pb⁴⁺ + 4e). However, lead does not displace hydrogen from dilute hydrochloric and sulfuric acids because of the H₂ overvoltage on Pb and the formation of protective films, composed of the poorly soluble chloride PbCl₂ and sulfate PbSO₄, on the metal surface. Concentrated H₂SO₄ and HCL react with Pb upon heating to produce soluble complex compounds with the compositions Pb(HSO₄)₂ and H₂[PbCl₄]. Nitric, acetic, and certain organic acids (for example, citric) dissolve lead with the formation of Pb(II) salts. Lead salts are classified according to water solubility as soluble (acetate, nitrate, and chlorate), sparingly soluble (chloride and fluoride), and insoluble (sulfate, carbonate, chromate, phosphate, molybdate, and sulfide). Pb(IV) salts may be obtained by the electrolysis of solutions of Pb(II) salts strongly acidified with H₂SO₄. The most important Pb(IV) salts are the sulfate Pb(SO₄)₂ and the acetate Pb(C₂H₃O₂)₄. Pb(IV) salts tend to combine with excess negative ions to form complex anions, for example, plumbates (PbO₃)^2- and (PbO₄)^4-, chloroplumbates (PbCl₆)^2-, and hydroxyplumbates [Pb(OH)₆]^2-. Concentrated caustic alkali solutions react with Pb upon heating to yield hydrogen and hydroxyplumbites of the type X₂[Pb(OH)₄].

Production. Metallic lead is obtained by the oxidative roasting of PbS with subsequent reduction of PbO to crude Pb (lead bullion), which is then refined (purified). Oxidative roasting of the concentrate is carried out in continuous traveling-grate sinter machines. The following reaction predominates during the roasting of PbS:

2PbS + 3O₂ = 2PbO + 2SO₂

In addition, a small quantity of the sulfate PbSO₄ is obtained, which is converted into the silicate PbSiO₃ with the addition of silica sand to the charge. The sulfides of other metals (Cu, Zn, Fe), present as admixtures, are also oxidized at the same time.

Instead of yielding a powdery sulfide mixture, the roasting process gives an agglomerate—a porous, sintered, solid mass composed chiefly of the oxides PbO, CuO, ZnO, and Fe₂O₃. Pieces of the agglomerate are mixed with coke and limestone and the entire mixture is then placed in a water-jacketed furnace; air is fed under pressure from below into the furnace by means of tubes (tuyeres). The coke and carbon monoxide reduce the PbO to Pb at rather low temperatures (below 500°C). At higher temperatures, the following reactions occur:

CaCO₃ = CaO + CO₂

2PbSiO₃ + 2CaO + C = 2Pb + 2CaSiO₃ + CO₂

The Zn and Fe oxides are partially converted into ZnSiO₃ and FeSiO₃, which together with CaSiO₃ form a slag, which floats to the surface. Lead oxides are reduced to metal. Crude lead contains 92–98 percent Pb, the remainder being admixtures of Cu, Ag (sometimes Au), Zn, Sn, As, Sb, Bi, and Fe. Admixtures of Cu and Fe are removed by liquidation. Air is passed through the molten metal in order to remove Sn, As, and Sb. The extraction of Ag (and Au) involves the addition of Zn, which forms a crust made from compounds of Zn and Ag (and Au), which are lighter than Pb and fuse at 600°-700°C. The Zn excess is removed from the molten Pb by the passage of air, water vapor, or chlorine. Ca or Mg, which are added to molten Pb in order to remove Bi, form relatively infusible Ca₃Bi₂ and Mg₃Bi₂. Thus, the refined lead contains 99.8–99.9 percent Pb. Further purification is effected by electrolysis, which yields a purity of no less than 99.99 percent.

Uses. Lead is widely used in the manufacture of lead-acid storage batteries and in the design of plant equipment that must be resistant to aggressive gases and liquids. Lead exhibits very strong absorption of both gamma and X rays, as a result of which it is used as a shielding material in, for example, containers for the storage of radioactive substances and X-ray laboratory equipment. Large amounts of lead are used in the manufacture of sheaths for electric cables, which protect the cables from corrosion and mechanical damage. Many alloys are prepared on a lead base. The oxide PbO is added to crystal and optical glass to obtain materials with a high refractive index. Minium, chromate (chrome yellows), and basic lead carbonate (white lead) are pigments with limited use. Lead chromate is an oxidizing agent used in analytical chemistry. Azide and styph-nate (trinitrorescorcinate) are explosive primers. Tetraethyl-lead prevents knocking in internal combustion engines. Lead acetate serves as an analytical reagent for detecting H₂S. The isotopes ²⁰⁴Pb (stable) and ^2l2Pb (radioactive) are used as iso-topic tracers.

In the organism. Plants absorb lead from the soil, water, and atmospheric precipitation. Lead enters the human body in food (approximately 0.22 mg), water (0.1 mg), and dust (0.08 mg). The permissible daily Pb intake level for humans is 0.2–2 mg. Lead is excreted primarily in feces (0.22–0.32 mg) and, to a lesser extent, in urine (0.03–0.05 mg). The human body contains an average of 2 mg lead; in certain cases this figure reaches 200 mg. Inhabitants of industrially developed countries have a higher lead content than do those of agrarian countries, and urban dwellers have a higher content than do country dwellers. The major lead deposits in the body are found in the skeleton (90 percent of all the lead in the organism); 0.2–1.9 micrograms per gram μg/g) is accumulated in the liver, 0.15–0.40 micrograms per milliliter (μg/ml) in the blood, 24 μg/g in the hair, and 0.005–0.15 μg/ml in mother’s milk. Lead is also present in the pancreas, kidneys, brain, and in other organs. The concentration and distribution of lead in animal organisms are similar to the levels established for humans. An increased lead level in the environment is matched by an increased accumulation in the bones, hair, and liver. The biological functions of lead have not yet been determined.

Poisoning. Lead poisoning can occur during ore extraction, lead smelting, and the manufacture of lead paints as well as in printing, cable manufacture, the manufacture of ceramics, and the preparation and use of tetraethyllead. Poisoning in the home, which is rare, results from the ingestion of food that has been stored for a long time in earthenware vessels coated with a glaze containing minium or litharge. Lead and its inorganic compounds, in aerosol form, enter the organism primarily through the respiratory tract and, to a lesser degree, through the gastrointestinal tract and skin. In the blood, lead is circulated in highly dispersed colloid form—phosphate and albuminate. It is generally excreted through the intestine and kidneys. As lead poisoning develops, there is a disruption of porphyrin, protein, carbohydrate, and phosphate metabolism and a deficiency of vitamins C and B₁. Functional and organic changes in the central and autonomic nervous systems occur, and there is a toxic effect from lead on the bone marrow. Poisoning may be latent (carriage) and can occur in mild, intermediate, or severe forms.

The most specific symptoms of lead poisoning are a slate blue line at the edge of the gums, a pale earthy tinge to the integument, reticulocytosis and other changes in the blood, increased porphyrin content in urine, and a lead concentration in the urine of 0.04–0.08 mg per liter or more. Damage to the nervous system is manifested by asthenia and, in pronounced forms, encephalopathy and by paralysis (mainly of wrist and finger extensors) and polyneuritis. Lead colic is characterized by sharp stomach cramps and constipation, lasting from a few hours to two to three weeks; often the colic is accompanied by nausea, vomiting, and a rise in arterial pressure and body temperature (up to 37.5°–38°C). Possible effects of chronic poisoning include damage to the liver and cardiovascular system and disruption of endocrine functions, which with women can lead to miscarriage, dysmenorrhea, and menorrhagia. The suppression of immunobiologic reactivity increases the general morbidity rate.

Treatment. Lead poisoning is treated with both specific (com-plexone-forming) and general restorative (glucose, vitamins) agents. Physical therapy and treatment in sanatoriums and health spas (Piatigorsk, Matsesta, Sernovodsk) are also recommended. Preventive measures include the substitution of less toxic substances for lead (zinc and titanium white for white lead) and the automation and mechanization of operations in lead production. There should also be effective exhaust ventilation, protective gear for individual workers, a therapeutic diet, supplementary vitamins, and preliminary and periodic medical examinations.

Lead preparations are used in medicine (only externally) as astringents and antiseptics. Examples are diluted lead subace-tate solution (for inflammation of skin and mucosa) and simple and complex lead plasters (for boils and suppurative inflammations of the skin).