Preparation of Lead Compounds An Exercise in Applied Chemistry Michael Laing, . David Williams-Wynn, and Saroj Subramofley University of Natal, Durban 4001. South Africa Lead is an essential commodity in the modern industrial world. In terms of volume of production i t ranks fifth among the metals after iron, copper, aluminum, and zinc. Durban is the site of a plant that manufactures a wide varietv of lead products, including "red lead", PbsO4, well knownis a pigment in anticorrosion paints and as an oxidizing agent in incendiary mixtures. w e take our second-year BS students to visit this plant. While there the students collect their own samples of "red lead", from which they will make a range of simple lead compounds, each of which has commercial or industrial significance. These compounds are Pb(N03)2, PbOz, P b C r 0 4 , P b S , PbO, basic lead carbonate: Pb(OH)z.2PhC03, lead tetraacetate, and lead metal itself. As part of the lecture course, the importance of each compound is described: used in gold refining, in dyeing and printing, and in pyrotechnics. PbOn used in plates for lead-acid accumulators, in the manufacture of dyes and pigments, and in pyrotechnics. PbCrO4 used .lam. . as a paint pigment and for decorating poreeoccurs in nature as the mineral galena; synthetic PbS material used in ,.elazine .. earthenware. I'hO urrd i n the manufacture of glass nnd remmirs, pigments, and in rubher; used in orsaying. used asa powerful vet aderriveoxrduing agent m I'brCH-COO], organic syntheses. Pb(OH)$2PhCO3used as the most important white paint pigment before the introduction of Ti09 Ph(N0h
We try to emphasize the special significance of some of these compounds (produced in small volumes) to industrial processes worth many millions of dollars per year, specifically: the use of lead nitrate in the final step of producing gold solution by zinc dust, metal by reduction of the [AU(CN)~]the use of lead carbonate in the pyrometallurgical concentration of platinum metals in their refining process, and the use of ultra-pure lead (as litharge) in the assaying of gold. The compounds that the students make are thus not only of some interest in the academic sense, showing oxidation states +2, +4, and 0, but also the compounds are of great economic importance. T o make things interesting, we point out the attractive variety of colors-white, yellow, red, black, and silver-of these compounds ( I ) . The importance of both descriptive chemistry (2) and industrial chemistry (3) in the curriculum for the bachelor's degree has been emphasized recently. We wholeheartedlv concur. o n e must bear id mind that lead and its compounds are toxic if ingested; therefore, they must be handled judicionsIv. The volatilitv of the comnounds described in this exoeriment is very low, thus inhalation poses no danger. Being inoreanic. these comoounds are not absorbed throueh the s k i n either. Thus, t i e only potential danger to an gndergraduate is the possibility of taking in some material directly by mouth. Washing the hands with soap and water after the experiment is satisfactorv protection. Details of the toxicitv ofiead compounds are riadily available (4). Laboratory Methods of Preparatlon The procedures for the preparation of each compound
follow. T h e work can be completed in three hours. Preparation of Lead Nitrate, Pb(N03)~and Lead Dioxide, PbO* Weigh out 3 g of Phs04 into a 250-mL beaker. Add 150 mL of 4 M nitric acid. Boil gently with stirring until the orange material has decomposed.The red lead reacts leaving a black insoluble residue of lead dioxide.
Filter through #542 filter paper, and wash with hot water. Divide the filtrate into three equal volumes of about 50 mL. Evaporate one purrmn m a porrclarn haarn until white crystals of lead nitratr begin tofurm. Alhu tt,ruol, rhen chill in ice. Filter using ~ 5 4 filter 2 paper in a small Duchner funnel. Wash the cr\,,tal\ with ethanol, and dw in air. Weieh the crvstals. then transfkr to a soecimen tube. and record yr&'yirlo of l'esd n/trate. \Vaah thr lead dioxirlr w i t h cthanol.nnd dry innir. Weigh rhelead dioxide, trnnsier t o n specimen ruhe, and r e n d your yield. Alternative Preparation of Pb(N03)~ Take one-third of the lead dioxide,put intoa 100-mLbeaker, add 50 mL of 4 M nitric acid, andwarm toabout 70 "C. Slowly add "10volume" hydrogen peroxide until the black solid dissolves. (Ahout 50 mL normally suffices.) Bubbles of oxygen gas are liberated.
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(This can be considered as a mutual reduction of two strone oxidizkg agknti: Ph,l\'~ PMll,, H 2 0 r H10 with the simucanrous prudurtion uf uxygen.) Evaporate the rolution until crystals oi lrad nitrate I e y i n t u form and iiltcr a3 ahovr. Preparation of Basic Lead Carbonate, Pb(O&-2PbC03 To the second 50-mL aliquot of lead nitrate solution add saturated rodi~mcarbonatr solut:on unril no more whirr preripicnte of basic lead carbonate forms. Filtrr through #h12 filter paper on a Hilchnrr lunnrl, waqh wirh hot water and rhen wirhethannl.anddry in air. Weigh the basic lead carbonate, transfer to a specimen tube, and record your yield. Preparation of Lead Chromate, PbCrO, To the third 50-mL aliquot of lead nitrate solution add ammonia until neutral. If a precipitate forms this is Pb(OH)%; add acetic acid until it redissolves. Add 10% sodium chromate solution until no more yellow precipitate of lead chromate forms. Filter through #542 filter paper on a Biiehner funnel, wash with hot water and then with ethanol, and dry in air. Weigh the lead chromate, transfer to a specimen tube, and record your yield. Preparation of Lead Sulfide, PbS Take half of the crystalline lead nitrate prepared earlier, weigh, and dissolve in 15 mL of water in a test tube. Add 5 mL of 1 M thioacetamide solution. and allow it to react slowlv bv heatine in a beaker of boilineu water.'^ ~ . r e c.i ~ i t aoft eblack lead bulide is formed. Filter, wash, and dry as above. Weigh, transfer to a specimen tube, and calculate your yield. ~
Preparation of Lead Monoxide, PbO Take a third of the lead dioxide prepared earlier, weigh into a silica crucible, and heat strongly in a fume cupboard. A yellow-red solid, lead monoxide, is formed. Weigh, transfer to a specimen tuhe, and calculate your yield. Preparation of Lead Tetraacetate, Pb(CH3C00h Put 10 mL of glacial acetic acid and 2 mL of acetic anhydride Volume 64
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(handle with care) into a 50-mL conical flask. Warm to 50 'Con a hot plate and add 3 g of red lead (Phs04)in several small portions. After each addition gently swirl the flask to dissolve the solid, keeping the temperature below 65 "C.Seal with a rubber stopper, and cool in ice. Filter the crystalson #542 filter paper in a Bfichner funnel, and suck dry. Do not wash. Weigh, transfer to a specimen tube, and calculate the yield.
Preparation of Lead Metal, Pb Take about 100 mg of the basic lead carbonate prepared earlier. mix with an equal amount of carbon powder on a watch glass. Pour the mixture into a heap on a charcoal block and heat it to redness using a Pasteur pipet as a blowpipe. A bead of lead metal forms.
Acknowledgment W e t h a n k Noel Drew, Managing Director of Cookson
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Journal of Chemical Education
Chemicals (Associated Lead Manufacturers), for his generosity a n d hospitality over t h e years.
Literature Clted 1.
la1 Latimer. W. M.; Hildohrsnd. J. H. ReierrnceXnoAof l n n r ~ o n i cChemistry: MaemilIan: New York. 1940: pp330-334 (h) Brirew, H . T . Ceneroi Chemistry for Cnliezea, 4th ed.: Hnughton Mifnin: Boston, 1949: pp 712-717: (e) Moody. B. Cnmpaialiur lnorpnnic Chemklis. 2nd ed.; Arnold: London. 1!169: pp 277-286: Id) Blown, G.I. lnlroduriion i n lnnrmnic Chemiatrv: Loneman: London, 197% np 207-216: (el 1 . b
