It rapidly oxidizes when fused with caustic soda, but is scarcely acted upon by caustic potash. Hydrochloric and sulphuric acids are almost without action on the metal, but it dissolves readily in dilute nitric acid. Nickel salts are antiseptic ; they arrest fermen tation and stop the growth of plants. Nickel carbonyl, however, is extremely poisonous. On the toxic properties of nickel salts see A. Riche and Laborde, Jour. Pharm. Chem., 1888 (5), 17, PP. I, 59, 97.
Nickel is used for the manufacture of domestic utensils, for crucibles, coinage, plating and for the preparation of various alloys, such as German silver, nickel steels such as invar (nickel, 35.7%; steel, 64.3%), which has a negligible coefficient of thermal expansion, and constantan (nickel, 45%; copper, 55%), which has a negligible thermal coefficient of electrical resistance.
Nickel Oxides.—Several oxides of nickel are known. The monoxide, NiO, occurs naturally as bunsenite, and is obtained artificially when nickel hydroxide, carbonate, nitrate or sulphate is heated. It may also be prepared by the action of nickel on water, by the reduction of the oxide with hydrogen at about 200° C., or by heating nickel chloride with sodium carbonate and extracting the fused mass with water. It is a green powder which becomes yellow when heated. It dissociates at a red heat, and is readily reduced to the metal when heated with carbon or in a current of hydrogen. It is readily soluble in acids, forming salts. The hydroxide, is obtained in the form of a greenish amorphous powder when nickel salts are precipitated by the caus tic alkalis. It is readily soluble in acids and in an aqueous solution of ammonia. Nickel sesquioxide, is formed when the nitrate is decomposed by heat at the lowest possible temperature. It is a black powder, the composition of which is never quite definite, but approximates to the formula given above. When heated with oxy acids it dissolves, with evolution of oxygen, and with hydrochloric acid it evolves chlorine. Numerous hydrated forms of the oxide have been described. A peroxide, has been obtained as a salt of barium, by heating the monoxide with anhydrous baryta in the electric furnace. G. Pellini and D. Meneghini ob tained a greyish green powder by adding an alcoholic solution of potassium hydrate to nickel chloride and hydrogenperoxide at — 5o°. It has all the reactions of hydrogenperoxide and S. Tanatar
regards it as An oxide, NW, has been obtained by heating nickel chloride in a current of moist oxygen at about 400° C., or by heating the sesquioxide in hydrogen at 19o° C. The f or mer method yields greyish, metallic-looking, microscopic crystals, the latter a grey amorphous powder. A hydrated form, is obtained when the monoxide is fused with sodium peroxide at a red heat, and the fused mass extracted with water.
Nickel Salts.—Only one series of salts is known, namely those corresponding to the monoxide. In the anhydrous state they are usually of a yellow colour, whilst in the hydrated condition they are green. They may be recognized by the brownish violet colour they impart to a borax bead when heated in an oxidizing flame. The caustic alkalis added to solutions of nickel salts give a pale green precipitate of the hydroxide, insoluble in excess of the pre cipitant. This latter reaction is hindered by the presence of many organic acids (tartaric acid, citric acid, etc.). Potassium cyanide gives a greenish yellow precipitate of nickel cyanide, soluble in excess of potassium cyanide, forming a double salt, Ni which remains unaltered when boiled with excess of potassium cyanide in presence of air (cf. COBALT). Ammonium sulphide precipitates black nickel sulphide, which is somewhat soluble in excess of the precipitate (especially if yellow ammo nium sulphide be used), forming a dark-coloured solution. Ammo nium hydroxide gives a green precipitate of the hydroxide, soluble in excess of ammonia, forming a blue solution. Numerous methods have been devised for the separation of nickel and cobalt, the more important of which are :—the cobaltinitrite method in which the cobalt is precipitated in the presence of acetic acid by means of potassium nitrite (the alkaline earth metals must not be pres ent) ; the cyanide method, in which the two metals are precipi tated by excess of potassium cyanide in alkaline solution, bromine being afterwards added and the solution warmed, when the nickel is precipitated ; the dimethyl glyoxime method, whereby nickel is precipitated as a scarlet flocculent compound sta ble at i 20° C. Similar separations are based on the insoluble cobalt compound of nitroso-f3-naphthol, and on the in soluble compound formed by nickel with dicyanodi-amidine.