KNO2  .3H20, is best known . It may be prepared by the addition of

potassium nitrite to an acetic acid solution of cobalt chloride . The yellow precipitate obtained is washed with a solution of potassium acetate and finally with dilute alcohol . The reaction proceeds according to the following equation: 2CoC12+10KNO2+ 4HNO2=Co2(NO2)6•6KNO2+4KC1+2N0+2H20 (A . Stromeyer, Annalen, 1855, 96, p . 220) . This salt may be used for the separation of cobalt and nickel, since the latter metal does not form a similar double nitrite, but it is necessary that the alkaline earth metals should be absent, for in their presence nickel forms complex nitrites containing the alkaline earth metal and the alkali metal . A sodium cobaltinitrite is also known . Cobalt nitrate, Co(NO3)2.6H2O, is obtained in dark-red mono-clinic tables by the slow evaporation of a solution of the metal, its hydroxide or carbonate, in nitric acid . It deliquesces in the air and melts readily on heating . By the addition of excess of ammonia to its aqueous solution, in the complete absence of air, a blue precipitate of a basic nitrate of the composition 6CoO•N203.5H2O is obtained . By boiling a solution of cobalt carbonate in phosphoric acid, the acid phosphate CoHPO4.3H2O is obtained, which when heated with water to 250° C. is converted into the neutral phosphate Cos(PO4)2.2H20 (H .

Debray,

Ann. de chimie, 1861, [3] 61, p . 438) . Cobalt ammonium phosphate, CoNH4PO4.12H2O, is formed when a soluble cobalt salt is digested for some time with excess of a warm solution of ammonium phosphate . It separates in the form of small rose-red crystals, which decompose on boiling with water . Cobaltous cyanide, Co(CN)2.3H20, is obtained when the carbonate is dissolved in hydrocyanic acid or when the acetate is precipitated by potassium cyanide . It is insoluble in dilute acids, but is readily soluble in excess of potassium cyanide . The double cyanides of cobalt are analogous to those of iron . Hydrocobaltocyanic acid is not known, but its potassium salt, K4Co(CN)3, is formed when freshly precipitated cobalt cyanide is dissolved in an ice-cold solution of potassium cyanide . The liquid is precipitated by alcohol, and the washed and dried precipitate is then dissolved in water and allowed to stand, when the salt separates in dark-coloured crystals . In alkaline solution it readily takes up oxygen and is converted into potassium cobalticyanide, K3Co(CN)s, which may also be obtained by evaporating a solution of cobalt cyanide, in excess of potassium cyanide, in the presence of air, 8KCN+2Co(CN)2+H2O+O= 2K3Co(CN)4+2KHO . It forms monoclinic crystals which are very soluble in water . From its aqueous solution, concentrated hydrochloric acid precipitates hydrocobalticyanic acid, H3Co(CN)6, as a colourless solid which is very deliquescent, and is not attacked by concentrated hydrochloric and nitric acids .

For a description of the various•salts of this acid, see P . Wesselsky, Berichte, 1869, 2, p . 588 . Cobaltammines . A large number of cobalt compounds are known, of which the empirical composition represents them as salts of cobalt to which one or more molecules of ammonia have been added . These salts have been divided into the following

series : Diammine Series, [Co(NH3)2]X4M . In these salts X=NO2 and M =one atomic proportion of a monovalent metal, or the equivalent quantity of a divalent metal . Triammine Series, [Co(NH3)s]X2 . Here X = Cl, NO3, NO2, 4SO4, &c . Tetrammine Series . This group may be divided into the •Praseo-salts [R2Co(NH3)4]X, where X=C1 . Croceo-salts [(NO2)2Co(NH3)4]X, which may be considered as a subdivision of the praseo-salts .

Tetrammine purpureo-salts [RCo(NH3)4•H2O]X2 . Tetrammine roseo-salts [Co(NH3)4•(H2O)2]X3 . Fuseo-salts [Co(NH3)4]OH•X2 . Pentammine Series . Pentammine purpureo-salts [R•Co(NH3)s]X2 where X = Cl, Br, NO3, NO2, SO4, &c . Pentarntnine roseo-salts [Co(NH3)s•H2O]X2 . Hexammine or Luteo Series [Co(NH3)6]X3 . The hexammine salts are formed by the oxidizing

action of air on dilute ammoniacal solutions of cobaltous salts, especially in presence of a large excess of ammonium chloride . They form yellow or bronze-coloured crystals, which decompose on boiling their aqueous solution . On boiling their solution in caustic alkalis, ammonia is liberated . The pentammine purpureo-salts are formed from the luteo-salts by loss of ammonia, or from an air slowly oxidized ammoniacal cobalt salt solution, the precipitated luteosalt being filtered off and the filtrate boiled with concentrated acids . They are violet-red in colour, and on boiling or long standing with dilute acids they pass into the corresponding roseo-salts .

The pentammine nitrito salts are known as the xanthocobalt salts and have the

general formula [NO2•Co•(NH3)s]X2 . They are formed by the action of nitrous fumes on ammoniacal solutions of cobaltous salts, or purpureo-salts, or by the mutual reaction of chlorpurpureosalts and alkaline nitrites . They are soluble in water and give characteristic precipitates with platinic and auric chlorides, and with potassium ferrocyanide . The pentammine roseo-salts can be obtained from the action of concentrated acids, in the cold, on air-oxidized solutions of cobaltous salts . They are of a reddish colour and usually crystallize well ; on heating with concentrated acids are usually transformed into the purpureo-salts . Their alkaline solutions liberate ammonia on boiling . They give a characteristic pale red precipitate with sodium pyrophosphate, soluble in an excess of the precipitant; they also form arecipitates on the addition ofplatinic chloride and potassium ferrocyanide . For methods of preparation of the tetrammine and triammine salts, see O . Dammer's Handbuch der anorganischen Chemie, vol . 3 (containing a complete account of the preparation of the cobaltammine salts) . The diammine salts are prepared by the action of alkaline nitrites on cobaltous salts in the presence of much ammonium chloride or nitrate; they are yellow or brown crystalline solids, not very soluble in cold water . The above series of salts show striking differences in their behaviour towards reagents; thus, aqueous solutions of the luteo chlorides are strongly ionized, as is shown py their high electric conductivity; and all their chlorine is precipitated on the addition of silver nitrate solution .

The aqueous solution, however, does not show the

ordinary reactions of cobalt or of ammop)a, and so it is to be presumed that the salt ionizes into [Co(NH3)s] and 3C1' . The purpureo chloride has only two-thirds of its chlorine precipitated on the addition of silver nitrate, and the electric conductivity is much less than that of the luteo chloride; again in the praseosalts only one-third of the chlorine is precipitated by silver nitrate, the conductivity again falling; while in the triammine salts all ionization has disappeared . For the constitution of these salts and of the " metal ammonia " compounds generally, see A . Werner, Zeit. fur anorg . Chemie, 1893 et seq., and Berichte, 1895, et seq . ; and S . Jorgensen, Zeit. fur anorg . Chemie, 1892 et seq . The oxycobaltammines are a series of compounds of the general type [Co203•H2(NH3)10]Xs first observed by L . Gmelin, and subsequently examined by E . Fremy, W . Gibbs and G .

Vortmann (Monatshefte fur Chemie, 1885, 6, p . 404) . They result from the cobaltammines by the

direct taking up of oxygen and water . On heating, they decompose, forming basic tetrammine salts . The atomic weight of cobalt has been frequently determined, the earlier results not being very concordant (see R . Schneider, Pog . Ann., 1857, 101, p . 387; C . Marignac, Arch . Phys . Nat . [2], I, p .

373; W . Gibbs, Amer . Jour . Sci . [2], 25, p . 483; J . B .

Dumas, Ann . Chim . Phys., 1859 [3], 55, p . 129; W . J .

Russell, Jour . Chem . Soc., 1863, 16, p . 51) . C . Winkler, by the analysis of the chloride, and by the action of iodine on the metal, obtained the values 59.37 and 59.07, whilst W . Hempel and H . Thiele (Zeit. f. anorg . Chem., 1896, II, p . 73), by reducing cobalto-cobaltic oxide, and by the analysis of the chloride, have obtained the values 58.56 and 58.48 . G . P .

Baxter and others deduced the value 58.995 (0 =16) . Cobalt salts may be readily detected by the formation of the black sulphide, in alkaline solution, and by the blue colour they produce when fused with borax . For the quantitative determination of cobalt, it is either weighed as the oxide, Co3O4, obtained by ignition of the precipitated monoxide, or it is reduced in a current of hydrogen and weighed as metal . For the quantitative separation of cobalt and nickel, see E . Hintz (Zeit. f. anal . Chem., 1891, 30, p . 227), and also NICKEL .