(see A. Recoura, Annales de Chimie et de Physique, 1895 (7), 4, p. 505.)
Chromic sulphate combines with the sulphates of the alkali metals to form double sulphates, which correspond to the alums. Chrome alum, K2SO4·Cr2(SO4)3·24H2O, is best prepared by passing sulphur dioxide through a solution of potassium bichromate containing the calculated quantity of sulphuric acid,
K2Cr2O7 + 3SO2 + H2SO4 = H2O + K2SO4 + Cr2(SO4)3.
On evaporating the solution dark purple octahedra of the alum are obtained. It is easily soluble in warm water, the solution being of a dull blue tint, and is used in calico-printing, dyeing and tanning. Chromium ammonium sulphate, (NH4)2SO4·Cr2(SO4)3·24H2O, results on mixing equivalent quantities of chromic sulphate and ammonium sulphate in aqueous solution and allowing the mixture to crystallize. It forms red octahedra and is less soluble in water than the corresponding potassium compound. The salt CrClSO4·8H2O has been described. By passing ammonia over heated chromic chloride, the nitride, CrN, is formed as a brownish powder. By the action of concentrated sulphuric acid it is transformed into chromium ammonium sulphate.
The nitrate, Cr(NO3)3·9H2O, crystallizes in purple prisms and results on dissolving the hydroxide in nitric acid, its solution turns green on boiling. A phosphide, PCr, is known; it burns in oxygen forming the phosphate. By adding sodium phosphate to an excess of chrome alum the violet phosphate, CrPO4·6H2O, is precipitated; on heating to 100° C. it loses water and turns green. A green precipitate, perhaps CrPO4·3H2O, is obtained on adding an excess of sodium phosphate to chromic chloride solution.
Carbides of chromium are known; when the metal is heated in an electric furnace with excess of carbon, crystalline, C2Cr3, is formed; this scratches quartz and topaz, and the crystals are very resistant to the action of acids; CCr4 has also been described (H. Moissan, Comptes rendus, 1894, 119, p. 185).
Cyanogen compounds of chromium, analogous to those of iron, have been prepared; thus potassium chromocyanide, K4Cr(CN)6·2H2O, is formed from potassium cyanide and chromous acetate; on exposure to air it is converted into the chromicyanide, K3Cr(CN)6, which can also be prepared by adding chromic acetate solution to boiling potassium cyanide solution. Chromic thiocyanate, Cr(SCN)3, an amorphous deliquescent mass, is formed by dissolving the hydroxide in thiocyanic acid and drying over sulphuric acid. The double thiocyanate, Cr(SCN)3·3KCNS·4H2O, is also known.
Chromium salts readily combine with ammonia to form complex salts in which the ammonia molecule is in direct combination with the chromium atom. In many of these salts one finds that the elements of water are frequently found in combination with the metal, and further, that the ammonia molecule may be replaced by such other molecular groups as -NO2, &c. Of the types studied the following may be mentioned: the diammine chromium thiocyanates, M[Cr(NH3)2·(SCN)4], the chloraquotetrammine chromic salts, R¹2[Cr(NH3)4·H2O·Cl], the aquopentammine or roseo-chromium salts, R¹3[Cr(NH3)5·H2O], the chlorpentammine or purpureo-chromium salts, R¹2[Cr(NH3)5·Cl], the nitrito pentammine or xanthochromium salts, R¹2[NO2·(NH3)5·Cr], the luteo or hexammine chromium salts, R¹3[(NH3)6·Cr], and the rhodochromium salts: where R¹ = a monovalent acid radical and M = a monovalent basic radical. For the preparation and properties of these salts and a discussion on their constitution the papers of S.F. Jörgensen and of A. Werner in the Zeitschrift für anorganische Chemie from 1892 onwards should be consulted.
P. Pfeiffer (Berichte, 1904, 37, p. 4255) has shown that chromium salts of the type [Cr{C2H4(NH2)2}2X2]X exist in two stereo-isomeric forms, namely, the cis- and trans- forms, the dithiocyan-diethylene-diamine-chromium salts being the trans- salts. Their configuration was determined by their relationship to their oxalo-derivatives; the cis-dichloro chloride, [CrC2H4(NH2)2Cl2]Cl·H2O, compound with potassium oxalate gave a carmine red crystalline complex salt, [Cr{C2H4(NH2)2}C2O4][CrC2H4(NH2)2·(C2O4)2]1½H2O, while from the trans-chloride a red complex salt is obtained containing the unaltered trans-dichloro group [CrC2H4(NH2)2·Cl2].