Oxides.—Lead combines with oxygen to form five oxides, viz. PbO, and The suboxide, is the first product of the oxidation of lead, and is also obtained as a black powder by heating lead oxalate to 300° out of contact with air. It ignites when heated in air with the formation of the monoxide ; dilute acids convert it into metallic lead and lead monoxide, the latter dissolving in the acid. The monoxide, PbO, occurs in nature as the mineral lead ochre. This oxide is produced by heating lead in contact with air and removing the film of ox ide as formed. It is manufactured in two forms, known as "massi cot" and "litharge." The former is produced at temperatures below, the latter at temperatures above the fusing-point of the oxide. The liquid litharge when allowed to cool solidifies into a hard stone-like mass, which, however, when left to itself, soon crumbles up into a heap of resplendent daik yellow scales known as "flake litharge." "Buff" or "levigated litharge" is prepared by grinding the larger pieces under water. Litharge is much used for the preparation of lead salts, for the manufacture of oil var nishes, of certain cements, and of lead plaster, and for other pur poses. Massicot is the raw material for the manufacture of "red lead" or "minium." Lead monoxide is dimorphous, occurring as cubical dodecahe dra and as rhombic octahedra. Its specific gravity is about 9; it is sparingly soluble in water, but readily dissolves in acids and molten alkalis ; it is used in making flint glass and glazes. A. yellow and a red modification have been described. The corre sponding hydrate, PbO,Pb(OH)2, is obtained as a white crystal line precipitate by adding ammonia to a solution of lead nitrate or acetate. It dissolves in an excess of alkali to form plum bites of the general formula Pb(0M)2. It absorbs carbon diox ide from the air when moist.
Lead dioxide, also known as puce oxide, occurs in nature as the mineral plattnerite, and may be most conveniently prepared by heating mixed solutions of lead acetate and bleaching powder until the original precipitate blackens. The solution is filtered, the precipitate well washed, and, generally, is put up in the form of a paste in well-closed vessels. It is also obtained by passing chlorine into a suspension of lead oxide or carbonate, or of magnesia and lead sulphate, in water ; or by treating the ses quioxide or red oxide with nitric acid. The formation of lead dioxide by the electrolysis of a lead solution, the anode being a lead plate coated with lead oxide or sulphate and the cathode a lead plate, is the fundamental principle of the storage cell (see ACCUMULATOR). Heating or exposure to sunlight reduces it to the red oxide; it fires when ground with sulphur, and oxidizes ammonia to nitric acid, with the simultaneous formation of am monium nitrate. It oxidizes a manganese salt (free from chlorine) in the presence of nitric acid to a permanganate; this is a very delicate test for manganese. It forms crystallizable salts with potassium and calcium hydrates, and functions as a weak acid forming salts named plumbates. The Kassner process formerly used for the manufacture of oxygen depended upon the formation of calcium plumbate, by heating a mixture of lime and litharge in a current of air, decomposing this substance into cal cium carbonate and lead dioxide by heating in a current of car bon dioxide, and then decomposing these compounds with the evolution of carbon dioxide and oxygen by raising the tempera ture.
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 with this oxide, which may, however, be removed by repeated digestion with lead acetate. Acids decompose it into lead di oxide and monoxide, and the latter may or may not dissolve to form a salt ; red lead may, therefore, be regarded as lead ortho plumbate, It is chiefly used as a pigment and in the manufacture of flint glass.