Although acids thus have directly but little effect on lead, and this is one of its most important practical properties, yet when air has free access, lead (like copper) very easily reacts with many acids, even with those which are comparatively feeble. The action of acetic acid on lead is particularly striking and often applied in practice. If lead be plunged into acetic acid it does not change at all and does not pass into solution, but if part of the lead be immersed in the acid, and the other part remain in contact with the air, or if lead be merely covered with a thin layer of acetic acid in such a way that the air is practically in contact with the metal, then it unites with the oxygen of the air to form oxide, which combines with the acetic acid and forms lead acetate, soluble in water. The formation of lead oxide is especially marked from the fact that with a sufficient quantity of air not only is the normal lead acetate formed but also the basic salts.[47]
When oxidising in the presence of air,[48] when heated or in the presence of an acid at the ordinary temperature, lead forms compounds of the type PbX2. Lead oxide, PbO, known in industry as litharge, silberglätte (this name is due to the fact that silver is extracted from the lead ores of this kind) and massicot. If the lead is oxidised in air at a high temperature, the oxide which is formed fuses, and on cooling is easily obtained in fused masses which split up into scales of a yellowish colour, having a specific gravity of 9·3; in this form it bears the name of litharge. Litharge is principally used for making lead salts, for the extraction of metallic lead, and also for the preparation of drying oils—for instance, from linseed oil.[49] When oxidised carefully and slightly heated, lead forms a powdery (not fused) oxide known under the name of massicot. It is best prepared in the laboratory by heating lead nitrate, or lead hydroxide. It has a yellow colour, and differs from litharge in the greater difficulty with which it forms lead salts with acids. Thus, for instance, when massicot is moistened with water it does not attract the carbonic acid of the air so easily as litharge does. It may, however, be imagined that the cause of the difference depends only on the formation of dioxide on the surface of the lead oxide, on which the acids do not act. In any case lead oxide is comparatively easily soluble in nitric and acetic acids. It is but slightly soluble in water, but communicates an alkaline reaction to it, since it forms the hydroxide. This hydroxide is obtained in the shape of a white precipitate by the action of a small quantity of an alkali hydroxide on a solution of a lead salt. An excess of alkali dissolves the hydroxide separated, which fact demonstrates the comparatively indistinct basic properties of lead oxide. The normal lead hydroxide, which should have the composition Pb(OH)2, is unknown in a separate state, but it is known in combination with lead oxide as Pb(OH)2,2PbO or Pb3O2(OH)2. The latter is obtained in the form of brilliant, white, octahedral crystals when basic lead acetate is mixed with ammonia and gently heated. The basic qualities of this hydroxide are shown distinctly by its absorbing the carbonic anhydride of the air. When an alkaline solution of the hydroxide is boiled, it deposits lead oxide in the form of a crystalline powder.
Lead oxide forms but few soluble salts—for instance, the nitrate and the acetate. The majority of its salts (sulphate, PbSO4; carbonate, PbCO3; iodide, PbI2, &c.) are insoluble in water. These salts are colourless or light yellow if the acid be colourless. In lead oxide the faculty of forming basic salts, PbX2nPbO or PbX2nPbH2O2, is strongly developed. A similar property was observed in magnesium and also in the salts of mercury, but lead oxide forms basic salts with still greater facility, although double salts are in this case more rarely formed.[50]
Amongst the soluble lead salts, that best known and most often applied in practical chemistry is lead nitrate, obtained directly by dissolving lead or its oxide in nitric acid. The normal salt, Pb(NO3)2, crystallises in octahedra, dissolves in water, and has a specific gravity of 4·5. When a solution of this salt acts on white lead or is boiled with litharge, the basic salt, having a composition Pb(OH)(NO3), is formed in crystalline needles, sparingly soluble in cold water but easily dissolved in hot water, and therefore in many respects resembling lead chloride. When the nitrate is heated, either lead oxide is obtained or else the oxide in combination with peroxide.
Lead chloride, PbCl2, is precipitated from the soluble salts of lead when a strong solution is treated with hydrochloric acid or a metallic chloride. It is soluble in considerable quantities in hot water, and therefore if the solutions be dilute or hot, the precipitation of lead chloride does not occur, and if a hot solution be cooled, the salt separates in brilliant prismatic crystals. It fuses when heated (like silver chloride), but is insoluble in ammonia. This salt is sometimes met with in nature, and when heated in air is capable of exchanging half its chlorine for oxygen, forming the basic salt or lead oxychloride, PbCl2PbO, which may also be obtained by fusing PbCl2 and PbO together. The reaction of lead chloride with water vapour leads to the same conclusion, showing the feeble basic character of lead 2PbCl2 + H2O = PbCl2,PbO + 2HCl. When ammonia is added to an aqueous solution of lead chloride a white precipitate is formed, which parts with water on being heated, and has the composition Pb(OH)Cl,PbO. This compound is also formed by the action of metallic chlorides on other soluble basic salts of lead.[51]
Lead carbonate, or white lead, is the most extensively used basic lead salt. It has the valuable property of ‘covering,’ which only to a certain extent appertains to lead sulphate and other white powdery substances used as pigments. This faculty of ‘covering’ consists in the fact that a small quantity of white lead mixed with oil spreads uniformly, and if such a mixture be spread over a surface (for instance, of wood or metal) the surface is quickly covered—that is, light does not penetrate through even a very thin layer of superposed white lead; thus, for example, the grain of the wood remains invisible.[52] White lead, or basic lead carbonate, after being dried at 120°, has a composition Pb(OH)2,2PbCO3.[53] It may be obtained by adding a solution of sodium carbonate to a solution of one of the basic salts of lead—for instance, the basic acetate—and likewise by treating this latter with carbonic acid. For this purpose the solution of basic acetate is poured into the vessel f; it is prepared in the vat A, containing litharge, into which the pump P delivers the solution of the acetate, which remains after the action of carbonic anhydride on the basic salt. In A a basic salt is formed having a composition approaching to Pb4(OH)6(C2H3O2)2; carbonic anhydride, 2CO2, is passed through this solution and precipitates white lead, Pb2(OH)2(CO3)2, and normal lead acetate, Pb(C2H3O2)2, remains in the solution, and is pumped back into the vat A containing lead oxide, where the normal salt is again (on being agitated) converted into the basic salt. This is run into the vessel E, and thence into f. Into the latter carbonic anhydride is delivered from the generator D, and forms a precipitate of white lead.[53 bis]
Fig. 82.—Manufacture of white lead.