Cobalt

COBALT, a metallic chemical element closely allied to iron and nickel (symbol Co, atomic number 27, atomic weight Though the atomic weight of cobalt is slightly greater than that of nickel, the properties of cobalt indicate it should precede nickel in the periodic classification, a conclusion confirmed by the fact that 28 is the atomic number of nickel (q.v., also ATOMIC NUM BER). The term cobalt is met with in the writings of the alchem ists, and was used to designate in general substances which resem bled metallic ores but yielded no metal on smelting. The term was particularly applied to two ores, cobaltite and smaltite, both of which contain cobalt combined with arsenic. These ores closely resemble metals and were termed "kobold" (goblin, compare Greek, K6j3aXos), by the Saxony miners, because they gave off poisonous arsenical fumes on smelting and yielded no silver, despite their silver-white appearance.

Occurrence.—The metal was first prepared in an impure con dition by G. Brandt in 1733, who found it to be magnetic and very infusible. It is found free in nature, though only in small quantities, chiefly in meteorites and its presence has been detected in the sun and many stars. In combination cobalt is fairly widely distributed, but it occurs seldom except in association with other metals, such as iron, nickel, copper, manganese, antimony and bismuth. It is remarkable that practically all the ores of cobalt are arsenical and consist of arsenides, arseno-sulphides or arsenates. But it has been found as monoxide, CoO, along with manganese in wad, as sulphide, CoS, in jaipurite, and as hydrated sulphate or vitriol, in bieberite. The crimson hy drated arsenate, erythrite, is probably a hydro lytic oxidation product of the arsenide, as the sulphate is of the sulphide. The chief source of the ores smelted in Great Britain is Queensland, Australia, where smaltite, occurs associ ated with erythrite. The cobalt speiss, resulting from the roasting of the ore to form oxide and arsenate, contains from 4o to 6o% of cobalt with only very small amounts of nickel, iron and traces of other metals. Smaltite is also found at Cobalt, Ontario, Canada, but contains only about Io to 20% of cobalt in the roasted speiss, together with larger amounts of nickel, and varying amounts of copper, silver and the platinum metals, chiefly palladium, plati num and rhodium. Considerable quantities of cobaltiferous resi dues result from the production of nickel and copper in Ontario, and are either worked up for cobalt or smelted without purifica tion for alloy purposes.

Production and Properties.

The process used for extrac tion of cobalt from speiss is essentially the same whatever the origin of the ore. A good speiss should contain enough cobalt sesquioxide to yield sufficient chlorine with hydrochloric acid to oxidize all the arsenic to arsenate, and sufficient iron to combine with all the arsenic to form ferric arsenate, which is precipitated with milk of lime. Lead, bismuth and copper, if present, are removed as sulphides by saturation with sulphuretted hydrogen. The resulting solution is oxidized and fractionally precipitated with bleaching-powder liquor, cobalt being precipitated first as sesquioxide, leaving nickel chloride in solution. The nickel may then be removed as dioxide by further addition of bleach liquor, or as carbonate by addition of washing soda solution. The black cobalt sesquioxide, thus obtained, is dried and extracted with dilute hydrochloric acid to remove traces of nickel, again dried, mixed to a stiff paste with starch and roasted in thin layers. The reduction to metal is effected partly by carbon from the starch and partly by carbon monoxide from the furnace gases.

Cobalt, a silver-white metal, slightly harder than iron or nickel, is magnetic at all temperatures up to 1,15o° C, and then becomes non-magnetic. The melting point is 1,480° and the boiling point 2,415°, both temperatures being intermediate between the corre sponding temperatures for nickel and iron. The density of the metal varies from about 8.7 to about 8.9. Cobalt in a finely divided condition occludes up to about 200 times its volume of hydrogen, which is readily given up again on heating to 200° in vacuo.

Polished cobalt is not appreciably affected by exposure to air or water, and is only superficially oxidized at a red heat. It dis solves in nearly all dilute acids evolving hydrogen, but is scarcely attacked by moderately concentrated nitric acid. It is not attacked by alkaline solutions or fused alkalis. Heated in chlorine, bromine or iodine vapour, the metal forms the blue dichloride, green di bromide or black di-iodide, respectively. At Soo° in ammonia a nitride, is formed which decomposes into cobalt and nitro gen at 600°. The metal is obtained in a pyrophoric condition (spontaneously inflammable) by reduction of its oxides at 250°. The cold pyrophoric metal decomposes acetylene with incan descence, forming carbon, hydrogen and benzene. Finely divided cobalt, even when not pyrophoric, decomposes carbon monoxide above 35o°, forming carbon and oxygen. At 15o° under a pres sure of upwards of 4o atmospheres, the reduced metal com bines with carbon monoxide yielding the tetracarbonyl, in orange crystals, which decompose at 6o° at atmospheric pres sure by loss of one-quarter of their carbon monoxide to form the black tricarbonyl, At red heat the metal decomposes steam, and at 15o° burns in nitric oxide to form cobaltous oxide, CoO.

Oxides.

Cobalt is known in three conditions of oxidation, represented by the oxides, CoO, and Co02. A fourth oxide is also known, which crystallizes in black octahedra, or CoO, or Co(Co02)2, and resembles the spinels in form and composition. The ordinary salts of commerce are cobaltous salts derived from the monoxide. A few simple and many complex salts are known derived from the sesquioxide. The dioxide gives rise to a few complex salts, and has slight acidic properties, com bining with bases to form cbbaltites, such as Cobalt thus possesses at least four valency electrons, of which two are relatively feebly bound and give rise to cobaltous salts. In the bivalent condition cobalt has slight basic properties, the salts hav ing an acidic reaction and being slightly hydrolysed by water. This accords with its position as a transition element of the 4th period. (See PERIODIC LAW.) Salts.—The metal is occasionally found native and is never found combined in nature with a higher valency than two. Cobalt ous salts of nearly all the known acids have been prepared, of which the chloride, nitrate, sulphate, carbonate, acetate and linole ate are in fairly extensive commercial use. Nearly all soluble cobaltous salts are red in the hydrated condition, while the in soluble salts are usually pink. On dehydration many of the red salts assume an intense blue colour, and this fact is made use of in the preparation of "sympathetic" inks. The hydrated sulphate, cobalt vitriol, is a noteworthy exception, the anhydrous salt being pink. Most cobaltous salts combine with ammonia forming hexammines, for example though pentammines and tetrammines are also known. Similar complex compounds are formed with many organic bases. A very large number of cobalt ous double salts are known, such as double sulphates, double nitrates, double carbonates and double cyanides. Potassium cobalt ous sulphate belongs to the series where R" may be Co, Fe, Mn or Cr. Double nitrates of the form have been used in the separation of the tri valent "rare earth" elements. Potassium cobalto-carbonate, forms a pink solution in water, which becomes blue on heating, the pink colour returning on cooling. Potassium cobaltocyanide, K4[Co(CN)e], decomposes water on boiling with liberation of hydrogen and formation of caustic potash and co balticyanide, [ Co (CN) . On rapid oxidation by air, the cobalticyanide yields hydrogen peroxide, caustic potash and the cobalticyanide, but on slow oxidation no hydrogen peroxide results, any hydrogen peroxide formed being utilized to oxidize the cobaltocyanide.

Almost the only simple cobaltic salt that is known is the fluor ide, a green powder, decomposed by water to form brown cobaltic hydroxide. Cobaltic double and complex salts are, how ever very numerous. The alums, as form blue octahedra, which are unstable in solution and yield cobaltous and alkali sulphates and oxygen. The cobaltinitrites of the stronger bases only are known. The sodium salt is freely soluble in water, whereas the potassium, ammonium and barium salts are insoluble. Sodium cobaltinitrite is used as a test for potassium, yielding with potassium salts very insoluble yellow precipitates of the disodium potassium salt, or the sodium dipotassium salt, [ Co (NO,) e] . The tripotas sium salt cannot be obtained in the presence of sodium salts. The cobalticyanides are much more stable than the cobaltocyanides, or even the f erro- and ferricyanides. On treatment with concen trated acids, the cobalticyanides are not decomposed but yield the complex acid, hydrogen cobalticyanide, [ Co (CN) e] which is colourless and becomes anhydrous at zoo° C. Cobalti cyanides of nearly all the metallic bases have been prepared, including those of the "rare earth" metals. Yttrium may be sepa rated from erbium owing to the fact that the yttrium salt is about thrice as soluble as the erbium salt.

On treatment of cobaltous salts with an oxidizing agent in the presence of ammonia, complex cobaltic ammines are formed. Cobaltous chloride, for example, dissolved in excess of ammonia solution, is oxidized by hydrogen peroxide in the presence of ammonium chloride to form a mixture of cobaltammines, in which the number of ammonia molecules per atom of cobalt varies from four to six, pentammines predominating. If much ammonia is present, the two tetrammines, (NH3)4] and are converted, on warming, into pen tammines. The main product consists of the three pentammines, purpureo-cobalt chloride, roseo-cobalt chlo ride, and hydroxo-purpureo-cobalt chloride, [CoOH(NH3),] The roseo and hydroxo salts are both con verted into the purpureo salt if much ammonium chloride be present and the liquid be boiled. The hexammine, [Co(NH3)e]C13, luteo-cobalt chloride, is invariably present to the extent of 2 or 3% and becomes the chief product if the reaction takes place under a pressure of a few atmospheres. In the foregoing series of ammines, the ammonia may be replaced by equivalents of ethylenediamine and other organic bases and the chlorine by other acid radicals. Cobaltic diammines and triammines are also known, particularly in combination with the nitrite radical, f or example, ammonium diamminocobaltinitrite (Erdmann's salt), and triamminocobaltic nitrite, which does not give rise to any ions on solution.

In all complex cobaltic compounds six non-ionizing groups can be discerned attached or co-ordinated to the cobalt atom, which is accordingly said to possess the co-ordination number six. (See AMMINES.) Some of these co-ordinated compounds have been shown to be capable of existing in optically-active, isomeric forms, proving, as in the case of carbon, that the valency direc tions are not planar but spatial. In the case of carbon, the at tached atoms or groups are disposed tetrahedrally about the car bon atom, and give rise to optically active isomerides according to the laws of symmetry. In the case of cobalt, the disposition of groups about the cobalt atom is octahedral, and as many as five isomeric forms have been proved to exist for certain co-ordi native groupings. (See also STEREOCHEMISTRY.) Detection and Estimation.—Cobalt can be detected, even in very minute amounts, by the formation of an intensely blue borate on heating a cobalt salt in a borax bead. In solution, co baltous salts can be detected by the green colour resulting on treatment with sodium bicarbonate and hydrogen peroxide, and by the red colour or precipitate resulting on treatment with an acetic acid solution of a-nitroso– f3-naphthol. In group analysis, cobalt is precipitated as sulphide by means of ammonium sul phide. It may be separated from nickel owing to the fact that cobalticyanides are stable to hypobromites, whereas nickelocy anides are decomposed by hypobromites to form nickel dioxide. The intense blue colour and solubility of cobaltous thiocyanate in organic solvents may be used as a test for cobalt. The solu bility of sodium cobaltinitrite and the insolubility of the potassium salt may be used as a test for either potassium or cobalt. After separation from other metals, cobalt may be precipitated as oxide by persulphates or hypobromites, as sulphide by ammonium sulphide, or as carbonate. When estimated by the a-nitroso j3-naphthol method, the precipitate may be dried and weighed, but in the other cases it is usual to ignite to sulphate with sulphuric acid, or to ignite and reduce to metal with pure hydrogen. The anhydrous sulphate method when available (in the absence of non-volatile metals and non-volatile acids) is rapid and accurate. Cobalt may also be estimated after electro-deposition from an acid solution of the double ammonium sulphate. The thiocya nate test has been adapted for the colorimetric estimation of small amounts of cobalt.

Uses.—Metallic cobalt is used industrially in the manufacture of ferrous and non-ferrous alloys. In some cases cobalt speiss is smelted directly to yield a ferro-nickel-cobalt alloy, which is used in the manufacture of special steels. Steels containing much cobalt and tungsten are used in the manufacture of permanent magnets for telephony and other purposes. Non-ferrous alloys, containing over 5o% of cobalt with varying amounts of tungsten, chromium and occasionally molybdenum, are used in the produc tion of high-speed cutting tools, which retain their hardness at a red heat. The high-speed tool alloy, stellite, has a composition approximating to cobalt 55, tungsten 15 to 25, chromium 15 to 25, and molybdenum 5%. Cochrome is a ferrous alloy containing cobalt 6o, chromium 14 to 16, iron 14 to 16%, similar to nichrome in composition and properties but with cobalt instead of nickel. It is used for electrical heating elements. Cobalt compounds are used to an increasing extent in industry, the linoleates, oleates, resinates and acetates being used as "driers" in the paint and varnish industry. Smalt is a complex potash aluminium cobalt silicate or glass, made by roasting impure cobalt oxide with quartz and potassium carbonate. It is a valuable and very permanent pigment of an intense blue colour. Potassium cobaltinitrite has long been used as an artists' pigment of great stability under the name of cobalt yellow. This pigment has also been used in the ceramic industry for producing a pure blue colour on porcelain after baking. Cobalt oxide, alone and in conjunction with other oxides, is in common use in the production of blues, purples, yellows and reds in the china trade. Cobalt ultramarine consists of the oxides of cobalt, aluminium and zinc ; cobalt green of cobalt and zinc oxides; turquoise green of cobalt, zinc and chromium oxides ; and cobalt red of cobalt and magnesium oxides.

Cobalt compounds have no therapeutic value and are non-toxic, the toxicity even of the complex cobaltamines being no greater than that of the ammonia they contain.

See J. N. Friend, Text-book of Inorganic Chemistry, vol. ix., part i (1926) . (J. D. M. S.)