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For other uses, see Lead (disambiguation).
For "Pb" as an abbreviation, see PB.

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Lead (IPA: /lɛd/) is a chemical element in the periodic table that has the symbol Pb (Template:Lang-la) and atomic number 82. A soft, heavy, toxic and malleable poor metal, lead is bluish white when freshly cut but tarnishes to dull gray when exposed to air. Lead is used in building construction, lead-acid batteries, bullets and shot, and is part of solder, pewter, and fusible alloys. Lead has the highest atomic number of all stable elements - although the next element, bismuth, has a half life so long (longer than the est. age of the universe) it can be considered stable. Like mercury, another heavy metal, lead is a potent neurotoxin which accumulates in soft tissues and bone over time.

Notable characteristics

Lead has a dull lustre and is a dense, ductile, very soft, highly malleable, bluish-white metal that has poor electrical conductivity. This true metal is highly resistant to corrosion. Because of this property, it is used to contain corrosive liquids (e.g. sulfuric acid). Lead can be toughened by adding a small amount of antimony or other metals to it. Lead is the only metal in which there is zero Thomson effect. Lead is also poisonous. All lead, except 204Pb, is the end product of a complex radioactive decay (see isotopes of lead below).

History

Lead has been used by humans for at least 7000 years, because it is widespread, easy to extract and easy to work with. It is highly malleable and ductile as well as easy to smelt. In the early Bronze Age lead was used with antimony and arsenic. Lead was mentioned in the Book of Exodus (15:10). Alchemists thought that lead was the oldest metal and associated it with the planet Saturn. Lead pipes that bear the insignia of Roman emperors are still in service and many Roman "pigs" (ingots) of lead figure in Derbyshire lead mining history and in the history of the industry in other English centres. Lead's symbol Pb is an abbreviation of its Latin name plumbum for soft metals; originally it was plumbum nigrum, where plumbum candidum was tin. The English word "plumbing" also derives from this Latin root.

Lead is also very toxic, and lead poisoning was recognized even by the ancients. Similarly, in the Twentieth Century, the use of lead in paint pigments was ended because of the danger of lead poisoning, especially to children.[1][2][3] By the mid-1980s, a significant shift in lead end-use patterns had taken place. Much of this shift was a result of the U.S. lead consumers' compliance with environmental regulations that significantly reduced or eliminated the use of lead in nonbattery products, including gasoline, paints, solders, and water systems. Recently, lead use is being further curtailed by the European Union's RoHS directive. Lead may still be found in harmful quantities in stoneware, vinyl (such as that used for tubing and the insulation of electrical cords), and brass manufactured in China.

Occurrence

Lead ore

Native lead does occur in nature, but it is rare. Currently lead is usually found in ore with zinc, silver and (most abundantly) copper, and is extracted together with these metals. The main lead mineral is galena (PbS), which contains 86.6% lead. Other common varieties are cerussite (PbCO3) and anglesite (PbSO4). The largest current source of lead, however, is recycling, primarily of automobile batteries.[citation needed]

In mining, the ore is extracted by drilling or blasting and then is crushed and ground. The ore is then treated using extractive metallurgy. The Froth flotation process separates the lead and other minerals from the waste rock (tailings) to form a concentrate. The concentrate, which can range from 50% to 60% lead, is dried and then treated using pyrometallurgy. The concentrate is sintered before being smelted in to produce a 97% lead concentrate. The lead is then cooled in stages which causes the lighter impurites (dross) to rise to the surface where they can be removed. The molten lead bullion is then refined by additional smelting with air being passed over the lead to form a slag layer containing any remaining impurities and producing 99.9% pure lead.

Lead ore (galena) is found commonly in north-east Wales. The North-east Wales Orefield was by far the most important source of lead and zinc in Wales and second in national importance only to the North Pennine Orefield. Lead mining dates back to at least Roman times and continued until well into the 20th century. Galena can be found today in the spoil heaps associated with mining activity. Galena is present in steeply-dipping fissure veins and in pipes and is in Mississippi Valley-type lead-zinc-fluorite and copper-dolomite associations. The mineralisation occurs in the upper parts of the Loggerheads and Cefn Mawr Formations of the Carboniferous Limestone.[4]

Isotopes

Main article: Isotopes of lead

Lead has four stable, naturally occurring isotopes: 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%), and 208Pb (52.4%). 206Pb, 207Pb and 208Pb are all radiogenic, and are the end products of complex decay chains that begin at 238U, 235U and 232Th, respectively. The corresponding half-lives of these decay schemes vary markedly: 4.47 × 109, 7.04 × 108 and 1.4 × 1010 years, respectively. Each is reported relative to 204Pb, the only non-radiogenic stable isotope. The ranges of isotopic ratios for most natural materials are 14.0 - 30.0 for 206Pb/204Pb, 15.0 - 17.0 for 207Pb/204Pb, and 35.0 - 50.0 for 208Pb/204Pb, although numerous examples outside these ranges are reported in the literature.

Because lead is radiogenic and formed from the decay of most of the heavier elements that formed billions of years ago, it is much more common and much cheaper than most heavy elements. The cost has been further lowered in recent years with the phasing out of lead in many processes, including gasoline and paint.

Health effects

Main article: Lead poisoning

Lead is a poisonous metal that can damage nervous connections (especially in young children) and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. The historical use of lead acetate (also known as sugar of lead) by the Roman Empire as a sweetener for wine is considered by some to be the cause of the dementia which affected many of the Roman Emperors. At one point in time, some lead compounds, because of their sweetness, were used by candy makers. Although this has been banned in industrialized nations, there was a 2004 scandal involving lead-laced Mexican candy being eaten by children in California. Even in these cases however, it should be noted that the lead is not an additive but merely a contaminant that enters incidentally via for instance, metal particles accumulated during grinding processes.[5]

The concern about lead's role in cognitive deficits in children has brought about widespread reduction in its use (lead exposure has been linked to schizophrenia). Lead-white paint has been withdrawn from sale in industralised countries. The yellow lead chromate is still in use; for example, Holland Colours Holcolan Yellow. Many older houses may still contain substantial lead in their old paint; see also lead paint: it is generally recommended that old paint should not be stripped by sanding, as this generates inhalable dust.

Lead salts used in pottery glazes have on occasion caused poisoning, when acid drinks, such as fruit juices, have leached lead ions out of the glaze.[citation needed] It has been suggested that what was known as "Devon colic" arose from the use of lead-lined presses to extract apple juice in the manufacture of cider. Lead is considered to be particularly harmful for women's ability to reproduce. For that reason many universities do not hand out lead-containing samples to women for instructional laboratory analyses.[citation needed]

Lead as a soil contaminant is a widespread issue, since lead may enter soil through (leaded) gasoline leaks from underground storage tanks or through a wastestream of lead paint or lead grindings from certain industrial operations.

Biochemistry of lead poisoning

In Medicine, lead inhibits α-aminolevulinate (ALA) dehydratase and ferrochelatase, preventing both porphobilinogen formation and the incorporation of iron into protoporphyrin IX, the final step in heme synthesis. Inhibition of both of these steps results in ineffective heme synthesis and subsequent microcytic (hemoglobin-poor) anemia.[citation needed]

Processing of metal from ore

The principal ores of lead are galena (PbS), anglesite (PbSO4), and cerussite (PbCO3). Most ores contain less than 10% lead, and ores containing as little as 3% lead can be economically exploited. Ores are crushed and concentrated by froth flotation typically to 70% or more. Sulfide ores are roasted, producing primarily lead oxide and a mixture of sulfates and silicates of lead and other metals contained in the ore.[6]

Lead oxide from the roasting process is reduced in a coke-fired blast furnace.[7] This converts most of the lead to its metallic form. Three additional layers separate in the process and float to the top of the metallic lead. These are slag (silicates containing 1.5% lead), matte (sulfides containing 15% lead), and speiss (arsenides of iron and copper). These wastes contain concentrations of copper, zinc, cadmium, and bismuth that can be recovered economically, as can their content of unreduced lead.[6]

Metallic lead that results from the roasting and blast furnace processes still contains significant contaminants of arsenic, antimony, bismuth, zinc, copper, silver, and gold. The melt is treated in a reverberatory furnace with air, steam, and sulfur, which oxidizes the contaminants except silver, gold, and bismuth. The oxidized contaminants are removed by drossing, where they float to the top and are skimmed off.[6][8]

Most lead ores contain significant concentrations of silver, resulting in the smelted metal also containing silver as a contaminant. Metallic silver as well as gold is removed and recovered economically by means of the Parkes process.[9][6][8]

Desilvered lead is freed of bismuth according to the Betterton-Kroll process by treating it with metallic calcium and magnesium, which forms a bismuth dross that can be skimmed off.[6][8]

Very pure lead can be obtained by processing smelted lead electolytically by means of the Betts process. The process uses anodes of impure lead and cathodes of pure lead in an electrolyte of silica fluoride.[6][8]

Descriptive chemistry

Various oxidized forms of lead are easily reduced to the metal. An example is heating PbO with mild organic reducing agents such as glucose. A mixture of the oxide and the sulfide heated together without any reducing agent will also form the metal.[9]

2PbO + PbS   →   3 Pb + SO2

Metallic lead is attacked only superficially by air, forming a thin layer of oxide that protects it from further oxidation. The metal is not attacked by sulfuric or hydrochloric acids. It does, however, dissolve in nitric acid with the evolution of nitric oxide gas to form dissolved Pb(NO3)2.

3 Pb + 8 H+ + 8 NO3–   →   3 Pb2+ + 6 NO3– + 2 NO + 4H2O

When heated with nitrates of alkali metals, metallic lead oxidizes to form PbO (also known as litharge), leaving the corresponding alkali nitrite. PbO is representative of lead's II oxidation state. It is soluble in nitric and acetic acids, from which solutions it is possible to precipitate halide, sulfate, chromate, carbonate (PbCO3), and basic carbonate (Pb3(OH)2(CO3)2) salts of lead. The sulfide can also be precipitated from acetate solutions. These salts are all poorly soluble in water. Among the halides, the iodide is less soluble than the bromide, which, in turn, is less soluble than the chloride.[10]

The II oxide is also soluble in alkali hydroxide solutions to form the corresponding plumbite salt.[9]

PbO + 2OH– + H2O   →   Pb(OH)42–

Chlorination of plumbite solutions causes the formation of lead's IV oxidation state.

Pb(OH)42– + Cl2   →   PbO2 + 2 Cl– + 2 H2O

Lead dioxide is representative of the IV state, and is a powerful oxidizing agent. The chloride of this oxidation state is formed only with difficulty and decomposes readily into the II chloride and chlorine gas. The bromide and iodide of IV lead are not known to exist.[10] Lead dioxide dissolves in alkali hydroxide solutions to form the corresponding plumbates.[9]

PbO2 + 2 OH– + 2 H2O   →   Pb(OH)62–

Lead also has an oxide that is a hybrid between the II and IV oxidation states. Red lead (also called minium) is Pb3O4.

Lead readily forms an equimolar alloy with sodium metal that reacts with alkyl halides to form organometallic compounds of lead such as tetraethyl lead.

Applications

  • Lead is a major constituent of the lead-acid battery used extensively in car batteries.
  • Lead is used as a coloring element in ceramic glazes, notably in the colors red and yellow.
  • Lead is used as projectiles for firearms and fishing sinkers because of its density, low cost compared to alternative products and ease of use due to relatively low melting point.[11]
  • Lead is used in some candles to treat the wick to ensure a longer, more even burn. Because of the dangers, European and North American manufacturers use more expensive alternatives such as zinc.[12]
  • Lead is used as shielding from radiation.
  • Molten lead is used as a coolant, eg. for lead cooled fast reactors.
  • Lead glass is composed of 12-28% lead oxide. It changes the optical characteristics of the glass and reduces the transmission of radiation.
  • Lead is the traditional base metal of organ pipes, mixed with varying amounts of tin to control the tone of the pipe.
  • Lead is used as electrodes in the process of electrolysis.
  • Lead is used in solder for electronics, although this usage is being phased out to reduce the amount of environmentally unfriendly waste.
  • Lead is used in high voltage power cables as sheathing material to prevent water diffusion into insulation.
  • Lead is used for the ballast keel of sailboats. Its high weight-to-volume ratio allows it to counterbalance the heeling effect of wind on the sails while at the same time occupying a small volume and thus offering the least underwater resistance.
  • Lead is added to brass to reduce machine tool wear.
  • Lead sheets are used as roofing material.
  • Lead is frequently used in scuba diving weight belts to counteract the diver's natural buoyancy and that of his equipment.

Former applications

  • Lead was used as a pigment in lead paint for white as well as yellow and red colors. It was discontinued because of the dangers of lead poisoning. However, lead chromate is still in use.
  • Lead was used for plumbing in Ancient Rome
  • Lead was used for joining cast iron water pipes and used as a material for small diameter water pipes until the early 1970s.
  • Tetraethyl lead was used in leaded fuels to reduce engine knocking; however, this is no longer common practice in the Western world due to health concerns.[13]
  • Lead was used to make bullets for slings.
  • Lead was formerly used as a component of toys, though due to many toy safety regulations, this has been stopped.
  • Lead was used in car body filler, which was used in many custom cars in the 1940s-60's. Hence the term Leadsled.
  • Lead is a superconductor at 7.2 K and IBM tried to make a Josephson Computer out of lead-alloy.[14]

Contrary to popular belief, pencil 'leads' have never been made from lead. The term comes from the Roman stylus, called the penicillus, which was made of lead.[15] When the pencil originated as a wrapped graphite writing tool, the particular type of graphite being used was named plumbago (lit. "act for lead"; "leadmocku").

Phrases

A "lead pipe cinch" is something that is absolutely certain. In the 19th century a horse saddle was safe when it was well "cinched". The "lead pipe" qualifier is an obscure "intensifier"[16].

See also

Literature

  • Keisch, B., Feller, R. L., Levine, A. S., and Edwards, R. R.: Dating and Authenticating Works of Art by Measurement of Natural Alpha Emitters. In: Science, 155, No. 3767, p. 1238-1242, 1967.
  • Keisch, B: Dating Works of Art Through their Natural Radioactivity: Improvements and Applications. In: Science, 160, p. 413-415, 1968.
  • Keisch, B: Discriminating Radioactivity Measurements of Lead: New Tool for Authentication. In: Curator, 11, No. 1., p. 41-52, 1968.

References

  1. ^ "NSW Multicultural Health Communication Service". NSW Health. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. ^ "Download: Lead paint: Cautionary note". Queensland Government. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  3. ^ "Lead Paint Information". Master Painters, Australia. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  4. ^ Davies, J.R., Wilson, D. & Williamson, I.T. (2004). The geology of the country around Flint. Memoir of the British Geological Survey, Sheet 108. (England and Wales). British Geological Survey, Keyworth.
  5. ^ "Toxic Treats ( Lead candy from Mexico)". Orange County Register. 2006-06-29. Retrieved 2007-04-07.
  6. ^ a b c d e f Samans, Carl H. Engineering Metals and their Alloys MacMillan 1949
  7. ^ "Primary Extraction of Lead Technical Notes". LDA International. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  8. ^ a b c d "Primary Lead Refining Technical Notes". LDA International. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  9. ^ a b c d Pauling, Linus General Chemistry, W.H. Freeman 1947 ed.
  10. ^ a b Brady, James E. and Holum, John R. Descriptive Chemistry of the Elements John Wiley and Sons
  11. ^ Dr. Rooney, Corinne. "Contamination at Shooting Ranges" (PDF). The Lead Group, incorporated. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  12. ^ Randerson, James (2002). "Candle pollution". NewScientist.com (2348). Retrieved 2007-04-07. {{cite journal}}: Unknown parameter |month= ignored (help)
  13. ^ "Countries where Leaded Petrol is Possibly Still Sold for Road Use, as of 22nd February 2007". The Lead Group, incorporated. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  14. ^ Henkels, W. H.; Geppert, L. M.; Kadlec, J.; Epperlein, P. W.; Beha, H. (1985). "Josephson 4 K-bit cache memory design for a prototype signal processor". Harvard University. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  15. ^ "A history of pencils". www.pencils.com. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  16. ^ Quinion, Michael. "lead pipe chich". World Wide Words. Retrieved 2007-02-19. {{cite web}}: Cite has empty unknown parameter: |1= (help)
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