edited by DARRELL H. BEACH The Culver Academies Culver, Indiana 4651 1
chemical of the month Lead Helen M. Stone Ben L. Smith High School Greensboro Public Schools Greensboro.NC 27407 Lead, Pb, from the Latin plumbum History
Lead is easily reduced from its ore and melts a t readily accessible temperatures thereby making lead a prime candidate for the first metal recovered from its ore by man. A lead figurine cast in Egypt as early as 3000 B.C. is displayed a t the British Museum. The Egyptians also used lead for making cosmetics, dishes, and fishing weights. Lead is mentioned in the Old Testament as part of the "dross" or "slag" from the smelting of silver. Not only the chemical symbol fur lead, Pb, but also many English words, such as plumb, plumber, and d u m b line are derived from the Latin word for lead. ~ 1 u m bum. Roman plumbers installed the world's first pllmbing svstem, an intricate network of lead wives. Lead was also used by the artisans of the Middle Ages in-the making of stained . glass wmaows. The toxicity of lead was recognized in ancient times. Nicander, a Greek poet and physician, wrote an explicit description of lead poisoning in the second century B.C. Pliny, a Greek historian, warned that breathing lead fumes could be fatal, yet recommended that copper wine vessels be lined with lead to avoid imparting the bitter taste of copper to wine. Only the ruling classes could afford the luxury of lead plumbing and leaded wine casks, and, as a consequence, only they suffered the madness and lowered fertility that result from lead poisoning. Chemical analysis of human bones from that era reveals an increasina- concentration of bone lead as Roman
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For Benjamin Franklin's opinions on the toxicity of lead see his letter reproduced in the March 1981 issue of THIS JOURNAL, page 274.
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Journal of Chemical Education
civilization reached its peak, and many historians list lead poisoning as a factor contributing to the fall of the Roman Empire (I ). Chronolor!ical lavers of snow strata wermanentlv hound in annular polar ice sheets serve as a monitor of lead use through the centuries. An analvsis of core samdes from Camo Centurv. (;rernl.llld. :nc,\r.; lh.11 Iwd vonctmtoitwfli n,ir from t j . 5 ng d 1% kg uf we i t 8 AIM! 13.1'. 1,) 5 k 1 nx ( t i PI) kg d i < , ( ,( t t I%;:>. Thi>. A w i rhn~:~tmuq]hericl h d in rhr northern ilemisphcw ~ n c r e a ~ eillfl-ttrld d d u r ~ n y hat 2hOO w a r p e r i d . A L i - ~ d d ~ i r f the lndustrinl l t t . v ~ . f ~ incrrnsr cwnc~dedu ith r h .~rlwnt ricm in 1-51,: flrr nesr :m,mrtrk incrrw~et r r ~ ~ r r iltlrmg td the 1!%ifl', with rhr intnduetiul d l r a d d r:iiolinr 111. l w -~ 1!1:L , , S i---concentrations have decreased due to government control of automotive emissions and are expected to decrease further as the government begins to enforce the October 1980 maximum concentration of 0.5 g Pblgal of gasoline ( 3 ) . In more recent history, lead has been associated with the making of laws and the makinr! of fortunes. One of the first puhlic ht.alth l a w i n this c c ~ u n t nwas paswd hy the State of \Ia.~whu.rtts i n I-?:{ ;and f < d u d r he ~~>coilcncl rquipn~enr in t hc, fl~itillingo i run1 ur ~ d l e a dconraincm for the shipping of rum.'l'hi mdinmce reiulted rrum the a~rmplaint-.olNorth brings with him to the numerous awards, citations, and 24 years of secondary school teaching. He received his B.A. in Chemistry from Central Michigan University in 1957, his MA. from the University 01 Michigan in 1963, and his Ed.D. from the University of Sarosota (formerlyLaurence University)in 1973. Dr. Beach is presently a Master Instructor of Chemistry and the Eugene C. Eppley Chairhalder of Chemistry at the Culver Academies. He D m d l H. Beach
JOURNAL,
was the
1477 recinient ....r.. .. nf . the .. . Outstandino -~ .
Chemistry Teacher from the Purdue ACS Section and was the 1979 regional MCA Award winner. He has also been an NSF and GE Fellow. Dr. Beach has authored three books and has over two dozen other publications. He has served on the ACS-NSTA Level I1 Examination Comminee since 1968 and has been chairman ol the group since 1974.
Carolina imbibers who developed the "dry gripes" from drinkine" Massachusetts rum. Perhaus as a result of these tainted imports, North Carolina today is one of the heaviest producers of illicit moonshine whiskey, which is often leadcontaminated from distilling it through automobile radiators or lead tuhine, from locally -. therehv causinr - the "dry..pripes" . produced spirits.' Lead azide, Ph(Nn)?,was one of the chemicals selected by to the establishment of t i e Nobel Prizes (4). TWII Nobel Prizes in Chemistry have been awarded to lead researchers, one to Theodore Richards in 1914, and one to Frederick Soddy in 1921. Method of Preparation
Lead may he recovered from its chief ore, galena (PbS), by roasting it in air to uroduce the oxide which is then reduced with carbon in a small blast furnace
cipitate exposed to S2-(aq) will gradually convert to the more insoluble PhS (5,6). Lead is relatively insoluble in sulfuric and hydrochloric acid, hut will dissolve in hot, concentrated nitric acid. Lead is slowly oxidized and carhonated in moist air and may he attacked hy either pure water or weak acids in the presence of oxygen to form lead hydroxide. As a result, neither foods nor beverages should he stored in antiaue imurouerlv . uewter. . . . " dazed earthenware, or opened soldered cans. Lead is resistant to hard water, hydrogen . . fluoride, hrine, and most solvents and is used industrially for storage and transport of many solvents, including cold sulfuric acid (6). Uses
The Charlotte Observer (6-8-80)reports that U S . industry uses 1.3 million tons of lead annually. At the present time, the chief industrial product is the lead storage battery consisting of P b and P h 0 2 electrodes dipping into a strong solution of HzSOa. As the battery discharges, the following overall cell reaction occurs Pb!s)
Carhon monoxide, CO, may also form in the reduction process. In an alternative process the lead sulfide is partially oxidized in air, resulting in a mixture of PhO, PbS, and PbS04. These products are smelted in the absence of air so that PhS itself reduces the other two products: PbS + ZPbO PhS + PhSO*
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3Pb(l) + SOn(g) 2Pb!l)
+ 2SO2(g)
This crude lead contains impurities such as antimony, copper, and silver. Most impurities are oxidized and float to the surface of the molten lead where they are skimmed off. Silver is usually extracted into molten zinc in which silver is 270 times more soluble than it is in lead. High purity lead may he ohtained by further refining with electrolysis (5). Physical Properties and Structure
lustrous bluish-white to silvery-gray appearance, but tarnishes quickly. It has a hardness of 1on the Moh's scale, indicating that it is very soft and malleable, and is easily cast, rolled, or extruded. Lead melts a t 327.4" C and boils a t 1740' C. Its density is 11.34 g cm-" and its specific heat a t 20" C is 0.031 g callg. There are four important naturally-occurring isotopes of lead with the following masses and frequencies: 204 (1.4%), 206 (25.2%), 207 (21.7%), and 208 (52.7). The last three isotopes arise from the three series of naturally-occurring radioactive elements: uranium series, Phzffi, thorium series, PhzoS.actinium series. PbZo7.The averaee atomic weieht of naturally occurring lead is 207.2, a weighted average of the 27 isotopes with masses raneine from 194 to 214. The electrode potential of the d i v a l e n t i o n - ~ b ~is+ -1.12 V and of the tetravalent ion Pb4+ is 0.80 V (6). Pb2+ is a Group I cation which forms a white precipitate with the C1- ion
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PbClz(s)
Hot water dissolves PbC12;the presence of the resulting Ph2+ ion is confirmed by the formation of a yellow precipitate with the Cr042- ion Pb2+(aq)+ CrOF(aq)
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ZPbS04+ 2Hz0
As the battery charges, the reverse reaction occurs, reproducing HzS04 solution, thus allowing a mechanic to test the specific gravity of the resulting solution in order to determine if the cell is functioning properly. In 1968 the petroleum industry was the second largest user of lead. Tetraethyl lead, Pb(C?H5)a, . . . is the best anti-knock additive ever found for gasoline and increases its efficiency as a fuel by 6% (7). However, this use has declined durine the past decade as the government has tightened emission Standards and as gasoline usage has declined. In 1978,7.4 million hbllday were used; in 1980, 6.6 million bbllday were used
\-,.
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Other major uses of lead include paints and pigments. Red lead, Pb304, is used as a rust-inhibitor; white lead or basic lead carbonate. 2Ph(CO&.Ph(OH)?. . . -. was once widelv used in exterior as was ba& lead sulfate; however, a t the present time, titanium has replaced lead in most of these paints. Lead chromates are used as rust-inhibitors and in pigments; lead dioxide is used by the dyeing and chemical industries and also in the manufacture of lightning arrestors (8). Litharge or lead monoxide, PbO, has numerous uses, including the fonnation of lead silicates in the glazing of pottery. Lead is also used in making ball bearings, industrial greases, percussion caps, and radiation shielding. Sportsmen use lead shot for hunting and lead weights for fishing. Fortunately, the use of lead arsenates in insecticides has decreased as a result of government fiat. Contrary to popular belief, pencil lead, or plumbago, is made of graphite, not lead. Amount Produced in the United States
The world production of lead was about Z1/2 million tons in 1977. Of this production, the United States consumed 5070, but one-third of this consumption was reclaimed scrap, or secondary lead (6). Approximatelv 600,000 tons of lead produced is lost to the atmosphere, the soil, or the water (7j. Current Price
Chemical Properties
Pb2+(aq)+ 2ClF!aq)
+ 2HzS04(aq)+ 2PhO;.
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PbCrOA
Most divalent anions precipitate Phz+(aq). The most insoluble of these precipitates is PhS; therefore, any P h pre-
In 1946 Lange's "Handbook of Chemistry" listed the wholesale nrice of lead a t 561lh: in 1966 Van Nostrand Reinhold Co. li&d it a t 16~Ilb; in 1980 two chemical supply houses listed lead shot a t $6.25fih-one of these houses listed ACS reagent-grade granular lead metal a t $17.30fib. The October 1980 issue of Omni reports that 32,000 tons of lead worth $15 million are discarded annually by U S . industries which indicates that the present industrial price of lead is 230Ilb. General Comments
Serious health hazards, particularly to young children, result from the absorption of air-borne lead through the lung Volume 58 Number 9
Seotember 1981
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and nutrient-borne lead through the gut. Both the placental barrier of the unborn and the hlood-brain barrier of the newly born are nermeable to lead. and. conseauentlv, children are much more easily poisoned by lead than b e ad&. Of the lead ahsorhed, 92% is stored in cartilaginous tissue such as bone and teeth thereby making the tooth a permanent marker of lifetime exposure to lead. Dr. H. L. Needleman and his colleagues utilized this relationship in their definitive study linking the lead content of the deciduous teeth of primary schno
