Amongst the soluble lead salts, that best known and most often applied in practical chemistry islead 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 sometimesmet 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 PbCl2and 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, orwhite 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, orbasic lead carbonate, after being dried at 120°, has a composition Pb(OH)2,2PbCO3.[53]It may be obtained by adding a solution of sodiumcarbonate 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 vesself; 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 intof. Into the latter carbonic anhydride is delivered from the generator D, and forms a precipitate of white lead.[53 bis]
see captionFig.82.—Manufacture of white lead.
Fig.82.—Manufacture of white lead.
In order to mark the transition from lead oxide, PbO, into lead dioxide PbO2(plumbic anhydride), it is necessary to direct our attention to the intermediate oxide, orred lead, Pb3O4.[54]In the artsit is used in considerable quantities, because it forms a very durable yellowish-red paint used for colouring the resins (shellac, colophony, &c.) composing sealing wax. It also forms a very good cheap oil paint, used especially for painting metals, more particularly because drying oils—for instance, hemp seed, linseed oils—very quickly dry with red lead and with lead salts. Red lead is prepared by slightly heating massicot, for which purpose two-storied stoves are used. In the lower story the lead is turned into massicot, and in the higher one, having the lower temperature (about 300°), the massicot is transformed into red lead. Frémy and others showed the instability of red lead prepared by various methods, and its decomposition by acids, with formation of lead dioxide, which is insoluble in acids, and a solution of the salts of lead oxide. The artificial production (synthesis) of red lead by double decomposition was most important. For this purpose Frémy mixed an alkaline solution of potassium plumbate, K2PbO3(prepared by dissolving the dioxide in fused potash),[54 bis]with an alkaline solution of lead oxide. In this way a yellow precipitate of minium hydrate is formed, which, when slightly heated, loses water and turns into bright red anhydrous minium Pb3O4.
Minium is the first and most ordinary means of producinglead dioxide, or plumbic anhydride, PbO2,[55]because when red lead istreated with dilute nitric acid it gives up lead oxide, and PbO2remains, on which dilute nitric acid does not act. The composition of minium is Pb3O4, and therefore the action of nitric acid on it is expressed by the equation: Pb3O4+ 4HNO3= PbO2+ 2Pb(NO3)2+ 2H2O. The dioxide may also be obtained by treating lead hydroxide suspended in water with a stream of chlorine. Under these conditions the chlorine takes up the hydrogen from the water, and the oxygen passes over to the lead oxide.[56]When a strong solution of lead nitrate is decomposed by the electric current, the appearance of crystalline lead dioxide is also observed upon the positive pole; it is also found in nature in the form of a black crystalline substance having a specific gravity of 9·4. When artificially produced it is a fine dark powder, resisting the action of acids, but nevertheless when treated with strong sulphuric acid it evolves oxygen and forms lead sulphate, and with hydrochloric acid it evolves chlorine. The oxidising property of lead dioxide depends of course on the facility of its transition into the more stable lead oxide, which is easily understood from the whole history of lead compounds. In the presence of alkalis it transforms chromium oxide into chromic acid, whilst lead chromate, PbCrO4, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis. The oxidising action of lead dioxide on sulphurous anhydride is most striking, as it immediately absorbs it, with formation of lead sulphate.This is accompanied by a change of colour and development of heat, PbO2+ SO2= PbSO4. When triturated with sulphur the mixture explodes, the sulphur burning at the expense of the oxygen of the lead dioxide.Tetrachloride of lead, PbCl4, belongs to the same class of lead compounds as PbO2. This chloride is formed by the action of strong hydrochloric acid upon PbO2, or, in the cold, by passing a stream of chlorine through water containing PbCl2in suspension. The resultant yellow solution gives off chlorine when heated. With a solution of sal ammoniac (Nicolukin, 1885) it gives a precipitate of a double salt, (NH4)2PbCl6(very slightly soluble in a solution of sal ammoniac), which when treated with strong sulphuric acid (Friedrich, 1890) gives PbCl4as a yellow liquid sp. gr. 3·18, which solidifies at -18°, and when heated gives PbCl2+ Cl2. It is not acted upon by H2SO4like SnCl4. Tetrafluoride of lead (Brauner) belongs to the same class of compounds, it easily forms double salts and decomposes with the evolution of fluorine (Chapter II., Note 49 bis).[56 bis]
Amongst the elements of the second and third groups it was observed that the elements were more basic in the even than in the uneven series. It is sufficient to remember calcium, strontium, and barium in the even, and magnesium, zinc, and cadmium in the uneven series. In addition to this, in the even series, as the atomic weight increases, in the same type of oxidation the basic properties increase (the acid properties decrease); for example, in the second group, calcium, strontium, barium. The same also appears in the fourth and all the following groups. In the even series of the fourth group titanium, zirconium, cerium, and thorium are found. All their highest oxides, RO2, even the lightest, titanic oxide, TiO2, have more highly developed basic properties than silica, SiO2, and in addition to this the basic properties are more distinctly seen in zirconium dioxide, ZrO2, than in titanic oxide, TiO2, although the acid property of combining with bases still remains. In the heaviest oxides, cerium dioxide, CeO2, and thorium dioxide, ThO2, no acid properties are observed, these being both purely basic oxides. In Chapter XVII. (Note43) we already pointed out this higher oxide of cerium. As the above-mentioned elements are rather rare in nature, have but little practical application, and do not present any new forms of combination, it is unadvisable to dwell on them in this treatise.
Titaniumis found in nature in the form of its anhydride or oxide, TiO2, mixed with silicon in many minerals, but the oxide is also foundseparately in the form of semi-metallicrutile(sp. gr. 4·2). Another titanic mineral is found as a mixture in other ores, known astitanic iron ore(in the Thuensky mountains of the southern Ural; it is known asthuenite), FeTiO3. This is a salt of ferrous oxide and titanic anhydride. It crystallises in the rhombohedric system, has a metallic lustre, grey colour, sp. gr. 4·5. The third mineral in which titanium is found in considerable quantities in nature isspheneortitanite, CaTiSiO5= CaO,SiO2,TiO2, sp. gr. 3·5, colour yellow, green, or the like, crystallises in tablets. The fourth, but rare, titanic mineral isperoffskite, calcium titanate, CaTiO3; it forms blackish-grey or brown cubic crystals, sp. gr. 4·02, and occurs in the Ural and other localities. It may be prepared artificially by fusing sphene in an atmosphere of water vapour and carbonic anhydride. At the end of the last century Klaproth showed the distinction between titanic compounds and all others then known.[57]
The comparatively rare elementzirconium, Zr = 90, is very similar to titanium, but has a more basic character. It is rarer in nature than titanium, and is found principally in a mineral calledzircon, ZrSiO4= ZrO2.SiO2, crystallising in square prisms, sp. gr. 4·5. It has considerable hardness and a characteristic brownish-yellow colour, andis occasionally found in the form of transparent crystals, as a precious stone called hyacinth.[58]Metallic zirconium was obtained, by Berzelius and Troost, by the action of aluminium on potassium zirconofluoride in the same way that silicon is prepared; it forms a crystalline powder, similar in appearance to graphite and antimony, but having a very considerable hardness, not much lustre, sp. gr. 4·15. In many respects it resembles silicon; it does not fuse when heated, and even oxidises with difficulty, but liberates hydrogen when fused with potash. When fused with silica it liberates silicon. With carbon in the electrical furnace it forms ZrC2, with hydrogen it gives ZrH2(like CaH2, Winkler, Vol. I., p. 621); hydrochloric and nitric acids act feebly on it, but aqua regiaeasily dissolves it. It is distinguished from silicon by the fact that hydrofluoric acid acts on it with great facility, even in the cold and when diluted, whilst this acid does not act on silicon at all.
The very similar elementthorium(Th = 232) was distinguished by Berzelius from zirconium. It is very rarely met with, inthoriteandorangeite, ThSiO4,2H2O. The latter is isomorphous with zircon (sp. gr. 4·8).[59]