Lead and its compounds

Lead is a heavy dull grey soft ductile metallic element belonging to group 14 (formerly IVB) of the periodic table. The main ore is the sulphide galena (PbS); other minor sources include anglesite (PbSO₄), cerussite (PbCO₃) and litharge (PbO). The metal is extracted by roasting the ore to give the oxide, followed by the reduction with carbon. Silver is also recovered from the ores. Lead has a variety of uses including building construction, lead-plate accumulators, bullets, and shot, and is a constituent of such alloys as solder, pewter, bearing metals, type metals, and fusible alloys. Chemically, it forms compounds with the +2 and +4 oxidation states, the lead(II) state being the more stable.

Lead-acid accumulator is an accumulator in which the electrodes are made of lead and the electrolyte consists of dilute sulphuric (V1) acid. The electrodes are usually cast from a lead alloy containing 7-12% of antimony (to give increased hardness and corrosion resistance) and a small amount of tin (for better casting properties). The electrodes are coated with a paste of lead (II) oxide (PbO) and finely divided lead; after insertion into the electrolyte a forming current is passed through the cell to convert the PbO on the negative plate into a sponge of finely divided lead. On the positive plate the PbO is converted to lead (IV) oxide (PbO₂). The equation for the overall reaction during discharge is:

PbO₂ + 2H₂SO₄ + Pb  2PbSO₄ + 2H₂O.

The reaction is reversed during charging. Each cell gives an e.m.f. of about 2 volts and in motor vehicles a 12-volt battery of six cells is usually used. The lead-acid battery produces 80-120 kJ per kilogram.

Lead (ll) carbonate is a white solid, PbCO₃, insoluble in water; rhombic; r.d. is 6.6. It occurs as the mineral cerussite, which is isomorphous with aragonite and may be prepared in the laboratory by the addition of cold ammonium carbonate solution to a cold solution of a lead (II) salt (acetate or nitrate). It decomposes at 315°C to lead (II) oxide and carbon dioxide.

Lead (ll) carbonate hydroxide (white lead; basic lead carbonate) is a

p

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owder, 2PbCO₃.Pb(OH)₂, insoluble in water, slightly soluble in aqueous carbonate solutions; r.d. is 6.14. It decomposes at 400^CC. Lead (II) carbonate hydroxide occurs as the mineral hydroxycerussite (of variable composition). It was previously manufactured from lead in processes using spent tanning bark or horse manure, which released carbon dioxide. It is currently made by electrolysis of mixed solutions (e.g. ammonium nitrate, nitric acid, sulphuric (V1) acid and acetic acid) using lead anodes. For the highest grade product the lead must be exceptionally pure (known in the trade as corroding lead) as small amounts of metallic impurity impart grey or pink discolorations. The material was used widely in paints both for art work and for commerce, but it has the disadvantage of reacting with hydrogen sulphide in industrial atmospheres and production of black lead sulphide. The poisonous nature of lead compounds has also contributed to the declining importance of this material