Lead

Quadrivalent Lead.—If a suspension of lead dichloride in hydrochloric acid be treated with chlorine gas, a solution of lead tetrachloride is obtained; by adding ammonium chloride ammo nium plumbichloride, is precipitated, which on treatment with strong sulphuric acid yields lead tetrachloride, as a translucent, yellow, highly refractive liquid. It freezes at —15° to a yellowish crystalline mass; on heating it loses chlorine and forms lead dichloride. With water it forms a hy drate, and ultimately decomposes into lead dioxide and hydro chloric acid. It combines with alkaline chlorides—potassium, rubidium and caesium—to form crystalline plumbichlorides. By dissolving red lead, in glacial acetic acid and crystallizing the filtrate, colourless monoclinic prisms of lead tetra-acetate, are obtained. This salt gives the corresponding chloride, fluoride and phosphate by treatment with the appro priate acid.

These salts are like those of tin ; and the resemblance to this metal is clearly enhanced by the study of the alkyl compounds. Here compounds of bivalent lead have not yet been obtained; by acting with magnesium ethyl bromide (see GRIGNARD REAGENTS) on lead chloride, lead tetraethyl, Pb(C2H3)4, is obtained, with the separation of metallic lead. This compound is finding much use under the trade mark "ethyl" as an "anti-knock" in internal combustion engines. See ORGANO-METALLIC COMPOUNDS.

Lead sesquioxide, is obtained as a reddish-yellow amor phous powder by carefully adding sodium hypochlorite to a cold potash solution of lead oxide, or by adding very dilute ammonia to a solution of red lead in acetic acid. It is decomposed by acids into a mixture of lead monoxide and dioxide, and may thus be regarded as lead metaplumbate, Red lead, minium or triplumbic tetroxide, is a scarlet crystalline powder of specific gravity 8.6-9.1, obtained by heating very finely divided pure massicot or lead carbonate at 400° for 24 hours in a rever beratory furnace with free access of air; the brightness of the colour depends in a great measure on the roasting. Pliny men tions it under the name of minium, but it was confused with cin nabar and the red arsenic sulphide. On heating, it assumes a finer colour, but then turns violet and finally black, regaining, however, its original colour on cooling. On ignition, it loses oxygen and forms litharge. Commercial red lead is frequently contaminated I% or even less of antimony or copper. Boiling concentrated sulphuric acid converts lead into sulphate, with evolution of sul phur dioxide. Dilute nitric acid readily dissolves the metal,

with formation of nitrate Pb(NO3)2. Lead has the peculiar property of making a lustrous black streak on paper.

Lead Alloys.

Lead unites readily with almost all other met als; hence, and on account of its being used for the extraction of (for instance) silver, its alchemistic name of saturnus. Alloys with the following metals may be named:— Antimony.—An alloy of 83 parts of lead and 17 of antimony is used as type metal ; other proportions are used, however, and other metals added besides antimony (e.g., tin, bismuth) to give the alloy certain properties.

Arsenic renders lead harder. An alloy made by addition of about of arsenic has been used for making shot.

Bismuth and Antimony.—An alloy consisting of 9 parts of lead, 2 of antimony and 2 of bismuth is used for stereotype plates.

Bismuth and Tin.—These triple alloys are noted for their low fusing points. An alloy of 5 of lead, 8 of bismuth and 3 of tin fuses at C, i.e., below the boiling-point of water (Rose's metal). An alloy of 15 parts of bismuth, 8 of lead, 4 of tin and 3 of cadmium (Wood's alloy) melts below 7o° C.

Tin unites with lead in any proportion with slight expansion, the alloy fusing at a lower temperature than either component. It is used largely for soldering.

"Pewter" (q.v.) may be said to be substantially an alloy of the same two metals, but small quantities of copper, antimony and zinc are frequently added.

Lead and Radioactivity.

Lead is the end product of sev eral series of radioactive decay (see RADIOACTIVITY), and has been proved to be identical with radium-D. The atomic weight of lead therefore depends upon the series from which it is de rived, and atomic weights as low as 206.0 have been found for lead derived from uranium, as in uraninite, and from several min erals in the Belgian Congo.

Compounds of Lead.

Lead generally functions as a bivalent element of distinctly metallic character, yielding a definite series of salts derived from the oxide PbO. At the same time, however, it forms a number of compounds in which it is most decidedly quadrivalent; and thus it shows relations to carbon, silicon, ger manium and tin.

Hydrides.—In 1919, F. Paneth found that traces of a hydride were produced by dissolving a lead-magnesium alloy in acids or by electrolytic reduction of solutions of lead salts. In 1925, E. J. Weeks obtained a hydride, as a grey deposit on aluminium foil placed in a solution of an alkali plumbite.