These ions have a strong tendency to form complexes. Six CN ions will join with Fe+2 to make the ferrocyanide ion, Fe(CN)e; six of these cyanide ions will also join with Fe+3 to form the ferricyanide ion, Fe(CN)e3. These six CN groups are arranged octahedrally around the central iron atom; that is, the iron can be thought of as surrounded by a cube with a CN group on each face. The ion SCN will combine with the ferric Fe+3 ion to form the red complex FeSCN+2. Another interesting complex ion, Fe(C2O4)3, is formed when Fe+3 is combined with the oxalate ion, C2O72. One can think of the oxalate ion as consisting of the group "OOCCOO" This is a U-shaped ion, and it can form a bridge between two faces of the imaginary cube surrounding the Fe; so that, with each U-shaped ion bridging two faces, it takes only three of these bridges to cover the six faces of the cube. Oxalate ions have such a strong tendency to combine with iron in this complex that solutions of oxalic acid are useful for removing rust stains.
Many of the complex ions of iron are strongly colored. The potassium salts of ferrocyanide are yellow; those of ferricyanide are red. If ferric ion is added to a solution of potassium ferrocyanide, a blue pigment (Prussian blue) precipitates; if ferrous ion is added to potassium ferricyanide a different blue pigment (Turnbull's blue) precipitates. Both of these pigments, which are different, have the same approximate composition, KFeFe(CN)e-H2O. If instead of combining iron atoms with different oxidation states we combine those with the same oxidation states, the result is less colorful. Ferrous ion and ferrocyanide ion form a white precipitate; ferric ion and ferricyanide form a greenish brown solution.
Metallic iron has a tendency to rust. Rain water always contains a little carbon dioxide and, in industrial areas, can also contain traces of many other substances that make it acidic. Under these conditions, moist iron rusts badly, combining with oxygen in the air to form ferric oxide which has no structural strength. Once the rust process starts, it is autocatalytic in nature and proceeds rapidly. For this reason, it is important to use protective coatings to prevent the initial formation of rust.
Cobalt. Cobalt is much less abundant than iron, though not really a rare element, constituting about 0.002% of the earth's crust. The name may come from a German word, kobold, meaning devil, and probably is associated with the difficulty encountered in smelting ores containing Ni and Co. It is found combined with elements like sulfur and arsenic, and the poisonous properties of the fumes complicate purification. Cobalt has a melting point of 1490° С and a boiling point of 2900° С Its oxidation potential for the metal to form the ion in the +2 oxidation state is +0.28 volt. It dissolves slowly in acids, liberating hydrogen; it will not dissolve in nitric acid because CQ of the formation of an unreactive coating. As contrasted with ferrous ion, the cobaltous ion is quite stable. The cobaltous solutions are, in general, pink; and, indeed, this is the color of all crystals that contain the hydrated ion, Co(H2O)Ј2. In. basic solutions, the dark-blue insoluble hydroxide is precipitated, and this can be dehydrated to give the greenish yellow cobaltous oxide, CoO. This will oxidize in air, when heated, to give CO3O4, which is similar in structure to Fe3O4. The cobaltic ion is shown also in Table 6.
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