552 EXTRACTION OF COBALT. It is first roasted at a low but gradually rising temperature, in order to expel the greater portion of the arsenic; after which it is dissolved in aqua regia, and evaporated to dryness to expel the excess of acid; it is then redissolved in water, and a current of sulphuretted hydrogen is transmitted through the solution. Bismuth, copper, and the remainder of the arsenic are thus precipitated as sulphides. The filtered liquid is boiled to expel the excess of the gas, and a slight excess of nitric acid is added to the boiling liquid, to convert the ferrous into ferric salts; when cold, it is diluted and supersaturated with ammonia; the iron is precipitated as peroxide, carrying with it a little cobalt, but the bulk of the cobalt remains dissolved, with any nickel which the ore may have contained. The exact separation of cobalt from nickel is tedious. Two methods have been proposed, one by Rose, the other by Liebig (737). Rose's method is the following:-The two metals are thrown down from the ammoniacal liquid as sulphides, by the addition of sulphide of ammonium. The sulphides are redissolved in nitric acid, the solution is then largely diluted, and acted upon by a current of chlorine; after this it is digested in a closed vessel for 12 hours upon powdered carbonate of barium. The chlorine converts the cobalt into sesquioxide, which is gradually precipitated by the barium, and remains mixed with the excess of carbonate of barium employed. This precipitate is again dissolved in hydrochloric acid; the barium is removed by adding sulphate of sodium, and the cobalt precipitated as protoxide by caustic soda the precipitate must then be well washed with boiling water, and reduced in a current of hydrogen gas, which leaves the metal in the form of a black, highly magnetic powder. When nickel is to be separated from cobalt for purposes of analysis, T. H. Henry recommends the substitution of a solution of bromine for chlorine gas in the foregoing process. Bromine may be used instead of chlorine in many analogous cases with great convenience. : If oxide of cobalt be reduced in a crucible lined with charcoal, a carbide of cobalt is formed, which may be obtained in a well-fused button. The crucible may be lined with charcoal for this purpose by dipping it into water, and filling it completely with charcoal finely powdered, and sufficiently moistened to render it coherent when firmly beaten into the crucible; a cylindrical cavity is then scooped out of the middle of the mass, and its interior is carefully smoothed with a glass rod, after which the crucible is allowed to dry slowly. Cobalt nearly pure may be pro PROPERTIES OF COBALT-ARSENIDES OF COBALT. 553 cured by heating the oxalate in a covered porcelain crucible, enclosed in a second earthen one, with the cover luted down ; the crucibles are then exposed for an hour to the most intense heat of a forge: a well-fused button of cobalt may generally be obtained in this manner. Properties.-Metallic cobalt is nearly as infusible as iron. It is of a reddish-grey colour, is hard, and strongly magnetic. Deville states that by reducing the oxalate in a crucible lined with lime, he obtained a metallic button which yielded a wire of a tenacity nearly double that of an iron wire of the same diameter. It is dissolved slowly, with evolution of hydrogen, by hydrochloric and diluted sulphuric acids, and it is freely attacked by nitric acid; when exposed to the atmosphere, it becomes slowly converted into oxide. Cobalt is not used in the metallic state in the arts. Many of the compounds of cobalt are remarkable for the beauty and brilliancy of their colour, and are used as pigments. The alloys of cobalt are unimportant. Its compounds with arsenic are interesting, as they supply the greater part of the cobalt employed in the arts. Tin-white cobalt (CoAs,), when pure, contains 28.57 per cent. of cobalt and 71°43 of arsenic; but portions of the cobalt are frequently displaced by nickel and iron. The purest specimens of this mineral are obtained from Tunaberg; the ore from this locality is the best material to employ in preparing the compounds of cobalt. Arsenide of cobalt melts at a moderate red heat. Bright white cobalt, or cobalt glance (Єo§„‚¤oAs,), corresponds in composition to mispickel: it crystallizes in cubes, octohedra, or dodecahedra, and contains 35°54 per cent. of cobalt, 45 18 of arsenic, and 19:28 of sulphur. These minerals are violently decomposed by nitric acid or by aqua regia, and are readily attacked when heated in a current of gaseous chlorine. They are also decomposed when roasted in a current of air. (720) OXIDES OF COBALT.-There are two well-marked oxides of cobalt, the protoxide, Coe, which is the salifiable base of the metal, and the sesquioxide, Єo,,; these two oxides are capable of uniting with each other in different proportions. According to Schwarzenberg, an acid oxide, Єo, may be obtained in combination, by strongly igniting the protoxide or the carbonate with hydrate of potash, in which case a crystalline compound is formed, which when dried at 212°, consists of K,→, 3 ¤ ̧ ̧. 3 H2O. Protoxide (Co=75, or CoO=375).—This oxide, when dried at a low temperature, is of a greenish-grey colour; when heated 554 PROTOXIDE OF COBALT-ZAFFRE-SMALT. to dull redness in the air it absorbs oxygen, and becomes black, forming an oxide (Co) corresponding to the black or magnetic oxide of iron, but if more strongly heated it again loses oxygen, and becomes reconverted into the protoxide, which is of a brown colour, and which may be cooled in a current of carbonic anhydride without absorbing oxygen (Russell). Protoxide of cobalt is soluble in acids, and forms solutions which, when concentrated, are of a beautiful blue colour, but they become pink on dilution. The oxide forms an important article of commerce, from its employment for the production of a blue colour in painting on porcelain. When describing the preparation of nickel, a process will be detailed which furnishes the oxide of cobalt fit for this purpose (729). Protoxide of cobalt combines with bases as well as with acids. If fused with hydrate of potash it forms a blue compound, which is decomposed by the free addition of water; when heated with nitrate of magnesium, a pale pink residue, formed by the combination of the magnesia with oxide of cobalt, is obtained; with alumina it forms the blue pigment known as Thénard's blue, and with oxide of zinc the compound constitutes Rinman's green. The zaffre of commerce is a very impure oxide of cobalt, procured by imperfectly roasting cobalt ore, mingled with 2 or 3 times its weight of siliceous sand. Smalt is a beautiful blue glass coloured by oxide of cobalt; it is chiefly manufactured in Saxony. In preparing smalt, the cobalt ore is first roasted; but the roasting is arrested at a particular stage, the object being to oxidize the cobalt, whilst the nickel, copper, and iron remain in combination with arsenic and sulphur; it is necessary to leave a sufficient amount of arsenic in the mass to retain these metals, as the admixture of a very small quantity of the oxides either of iron, nickel, or copper with the glass, seriously injures the purity of its colour. From 4 to 5 parts of the roasted ore in powder are next mingled with 10 parts of ground calcined quartz and 4 parts of carbonate of potassium, and the mixture is slowly melted in pots arranged in a furnace resembling that used in making ordinary glass. The oxide of cobalt combines with the fused silicate of potassium; a deep blue glass is thus formed, whilst the mixed arsenides and sulphides of nickel, copper, and iron fuse, and collect at the bottom of the pot, in the form of a brittle mass, of metallic appearance, commonly known as speiss. The pot is then skimmed, and the glass is ladled out, and poured into cold water, by which means it is split into innumerable fragments: the speiss is cast into ingots and used in the manufacture of nickel. SMALT-THENARD'S BLUE-RINMAN'S GREEN. 555 The broken glass is stamped to powder, and subsequently ground between granite stones, which are caused to revolve under water, in a vessel through which a gentle stream of water is continually flowing. The water as it flows carries off with it the powdered smalt in suspension: it is made to pass through a number of depositing vessels, so arranged that the overflow from the first shall pass into the second, that from the second into the third, and so on each of these vessels is successively larger than the one which precedes it, so that the period for which the washings are retained in each goes on progressively increasing, and the particles deposited progressively increase in the minuteness of their subdivision; the colour becoming less intense, the greater the degree of subdivision of its particles. Smalt is used largely by paper-stainers, to produce a blue colour, and it is employed to some extent by laundresses, for correcting the yellow tinge in linen. Another valuable pigment into the composition of which cobalt enters is of a pale blue colour, and is known as Thénard's blue. The most approved method of preparing it consists of precipitating nitrate of cobalt by means of phosphate of potassium, and mixing the precipitate whilst still moist with four or five times its bulk of the gelatinous mass obtained by adding carbonate of sodium to a dilute solution of alum perfectly free from iron. The mixture is dried and then exposed to a dull red heat in a covered crucible. The brilliancy of the colour is much impaired by the reducing action of the combustible gases of the fuel. The best preventive of this effect is found to consist in placing a little red oxide of mercury at the bottom of each crucible; by the decomposition of this oxide an atmosphere of oxygen is obtained, and the metallic mercury is dissipated in vapour (Regnault, Cours Elém. de Chimie, vol. iii. p. 150). Rinman's green is a pigment of analogous composition, containing oxide of cobalt combined with oxide of zinc. Hydrated oxide of cobalt (Єo, H2O) is precipitated by the addition of solution of potash or of soda to solutions of any of its salts. The pale blue precipitate which is first formed is a basic salt of cobalt, but if an excess of alkali be used, it quickly becomes violet, and finally rose-coloured, which is the true colour of the hydrated oxide: these changes occur most rapidly if the liquid be warmed. It becomes of a dingy green if exposed while moist to the air, owing to the gradual absorption of oxygen. The hydrated protoxide is readily dissolved by a solution of carbonate of am 556 SESQUIOXIDE OF COBALT-BASES WHICH CONTAIN AMMONIA. monium, and also by excess of ammonia, especially in the presence of a neutral salt of ammonium. 49 2 2 Sesquioxide of cobalt (Єo‚Ð ̧=166, or Co,O=83) may be prepared by suspending the hydrated protoxide of the metal in water, and transmitting a current of chlorine through the liquid; chloride of cobalt is formed and dissolved, whilst a black hydrated sesquioxide of cobalt is precipitated, Є04, 3 H2→. The reaction may be thus expressed : 3 (Єo,H,O) +Cl,=¤¤¿Ð ̧, 3 H ̧Ð+¤¤Cl ̧. If the oxide of cobalt be suspended in a solution of potash instead of in pure water, the whole of the cobalt is converted into sesquioxide. It may be rendered anhydrous by a careful application of heat, but if strongly heated it becomes converted into a black oxide (Єo, Єo,,), corresponding with the magnetic oxide of iron. This magnetic oxide is sometimes deposited in small, hard, anhydrous, brilliant, steel-grey octohedra when a pure aqueous solution of chloride of roseocobaltia (721) is boiled. In this form it is insoluble in nitric acid, in hydrochloric acid, and in aqua regia: it is but slowly attacked by heating it with oil of vitriol or with acid sulphate of potassium. The basic powers of the sesquioxide are extremely feeble. Cold sulphuric, nitric, hydrochloric, phosphoric, and acetic acids dissolve the hydrated oxide, but the salts are gradually converted at ordinary temperatures into those of the protoxide, and this change is immediately effected if the solutions are heated. (721) AMMONIACAL COMPOUNDS OF COBALT.-When a solution of a salt of cobalt in ammonia is exposed to the air, it absorbs oxygen rapidly, although the hydrated protoxide of cobalt alone exhibits this tendency to a small extent only. If the hydrated oxide be dissolved in a solution of chloride of ammonium containing free ammonia, the absorption of oxygen proceeds quickly, and a remarkable violet-red colour gradually developes itself in the liquid. If at this stage the liquid be supersaturated with hydrochloric acid in the cold, a heavy brick-red crystalline powder is precipitated, the chloride of roseocobaltia (ЄoCl, 5 Н ̧Ñ‚Í‚ ̧Ð, or Co2Cl3, 5 H,N, 2 HO, Genth and Gibbs); and this compound, if boiled, is converted into a purple precipitate of chloride of purpureocobaltia (EoCl, 5 H,N, Genth and Gibbs), which separates in crystals, leaving the solution nearly colourless: this precipitate may be dissolved by heating it in water slightly acidulated with hydrochloric acid, and as the liquid cools, beautiful ruby-red octohedral crystals are formed (F. Claudet). This remarkable compound is quite insoluble in boiling hydrochloric acid, and may be employed as a means of obtaining chemically pure cobalt: at a |