IROX GROUT.] C H EM I S T II Y 535 Cr i (X0 3 )(C 2 H 3 2 ) 5 , have been obtained by dissolving the hydroxides in mixtures of two acids in various proportions. Like aluminic sulphate, chromic, ferric, and manganic sulphates form double salts with the sulphates of the alkali metals, which correspond both in composition and crystalline form with the alums. The chrome alums are almost as stable as the true alums, but the iron alums are much less stable, and the manganese alums are so unstable that water decom poses them into their component salts. Ferrous, manganous, cobaltous, and nickelous sulphates form double sulphates with the sulphates of the alkali metals, corresponding in composition and isomorphous with double sulphates such as K 2 Zn(SO 4 ) 2 , GH 2 O and K 2 Cu(S0 4 ) 2 , 6H 2 0. The relation of chromium to sulphur and the allied elements is indicated by the existence of the trioxide CrO 3 ; this oxide, however, is not merely analogois in composition to sulphuric anhydride, but also resembles it in properties. Thus, it dissolves readily in water, forming a strongly acid, yellow solution of chromic acid, H 2 CrO 4 ; and the salts of this acid are not only analogous in composition to the cor responding sulphates, but are also isomorphous with them, and are similar to them in many respects, the chromates of metals which form insoluble or difficultly soluble sulphates being as a rule also insoluble or difficultly soluble. But although the chromates are stable salts, chromic acid is not known except in solution, chromic anhydride, CrO 3 , being obtained when the solution is evaporated; moreover, chromic acid very readily parts with oxygen, and on this account is a most valuable oxidizing agent. Acid salts of chromic acid corresponding to the acid sulphates apparently cannot exist, for when a solution of a chromate is rendered acid the colour changes from pale yellow to orange-yellow, the chromate being converted into an anhydrochromate or di- chromate, which is formed from two molecules of the acid chromate by the withdrawal of the elements of a molecule of water ; thus Cr0 2 (OK)., Cr0 8 (OK) a Potassium chromate. Cr0 2 .OK Cr0 2 .OK Potassium anhydrochromate. The anhydrochromates are mostly orange-red, ths cor responding chromates being yellow ; they are very stable, and unlike the anhydiosulphates (p. 507), to which they correspond in composition, dissolve in water unchanged. The relation of chromium to sulphur is also indicated by the existence of an oxychloride, CrO 2 Cl 2 , corresponding to sulphuric chloride (p. 505), SO 2 C1 2 ; it is a blood-red coloured liquid, which is violently decomposed by water and boils at 118 C. The determination of the vapour density of this compound shows that it is correctly repre sented by the formula CrO,,Cl. 2 . On fusing an oxide of manganese with potassium nitrate, a deep green-coloured mass is formed, from which potassium manganate, K 9 Mu0 4 , may be separated in crystals isomorphous with potassium sulphate. A concentrated solution of this salt is decomposed merely by dilution, and the solution cannot be boiled ; on adding an acid the colour changes from green to purple-red, the manganate being converted into permanganate : 4K 2 Mn0 4 Potassium manganate. 2 + 20H 2 = 2K 2 Mn 2 8 Potassium permanganate. 4KOH The permanganates are much more stable than the man- ganates, and tbeir solutions may be boiled without their decomposing ; but they readily part with oxygen, and are therefore most powerful oxidizing agents As the manganates and sulphates are isomorphous it is probable that the two classes of compounds are analogous in constitution; and from the manner in which the permangan ates are formed from the manganates it is probable that the former contain at least two atoms of manganese in the mole cule, and that potassium permanganate, for example, is re- . O.MuOJOK) presented bv the formula . , _ ,, . But potassium O.Mn0 2 (OK) permanganate is isomorphous with potassium perchlorate and periodate ; it therefore appears probable that, as already pointed out (p. 497), the formula K 2 C1 2 O 8 for the former salt is preferable to the simpler formula KC1O 4 , by which it is usually represented. The isomorphism of potassium permanganate with potassium perchlorate and periodate is, however, chiefly of interest as an indication that manganese is related to the halogens. Iron is also capable of yielding a compound analogous to potassium manganate, but much less stable, termed potassium ferrate, K 2 Fe0 4 ; it is produced by fusing an oxide of iron with nitre. The solution has a cherry-red colour, and on the addition of acids is at once decomposed with evolution of oxygen. The most characteristic com pounds of iron, however, are the so-called ferrocyan ides and ferricyanides. Potassium ferrocyanide, K 8 Fe 2 (CX) 12 , is produced on adding potassium cyanide to a solution of a ferrous salt in sufficient amount to redissolve the precipi tate which at first forms ; it crystallizes from the concen trated solution in beautiful yellow plates. The tendency to form this salt is so great that metallic iron is rapidly dissolved when heated with an aqueous solution of potas sium cyanide : 12KCX + 2Fe + 4H.O = K 8 Fe 2 (CN) 12 + 4KHO + 2H 2 . Not only is the presence of iron in this compound not discoverable by the ordinary tests, but on treatment with hydrochloric acid it furnishes hydroferrocyanic acid, H s Fe 2 (CN) 12 , and is not, like most double cyanides, decom posed with evolution of hydrocyanic acid. Potassium ferricyanide, K 6 Fe 2 (ON) 12 , is produced by the action of chlorine on the ferrocyani-de : K s Fe 2 (CN) 12 + C1 2 = K 6 Fe.,(CX) 12 + 2KCl ; it crystallizes in red prisms, and like the ferrocyanide is an extremely stable compound. Other ferro- and ferri-cyanides may be prepared from the potassium compounds by double decomposition. Cobalticyanides, such as K Co 2 (CX) 12 , isomorphous with the ferricyanides, are readily obtained, and are equally stable ; but the cobaltocyauides are very unstable. Chromi- cyanides and mangano- and mangani-cyanides, isomorphous with the corresponding iron compounds are also known, but they are much less stable than the latter. Xickel does not furnish compounds of this kind, but like copper forms double cyanides, such as Xi(CX) 2 , 2KCX, which are readily decomposed by acids with evolution of hydrocyanic acid. Xickel, although of the same atomic weight as cobalt, it will be manifest, is possessed of very different properties, being in many respects much more nearly related to copper than to cobalt, iron, <tc. ; it is noteworthy that nickel and copper are the only metals whose compounds form blue solutions with ammonia. The relation of chromium, iron, manganese, and cobalt to one another, and of the first two especially to aluminium, will be sufficiently evident with out further remark ; the relation of iron, cobalt, and nickel to the platinum metals will be referred to later on. Manganese and iron are the only elements of this group which have been submitted to thermochemical investiga tion. Many of the results are of considerable interest ; thus, it will be noticed, on reference to the following tables, that much more heat is developed in the formation of r.ian gaums chloride, MnCl., or Mn 2 Cl 4 , than in that of the corresponding chloride of iron ; this is of interest in connection with and may perhaps serve to explain, the
great instability of the higher chlorides of manganese.