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The question was asked Arrhenius as to how it is possible to have in an aqueous solution of common salt either free sodium or free chlorine, since the former is violently attacked by water and the latter is a greenish-yellow gas, forming a greenish-yellow solution with water. The solution shows none of the properties of sodium or chlorine. The answer is that neither metallic sodium nor chlorine gas is assumed to be present in the solution, but only sodium ions and chloride ions. These differ from the free elements by possessing large electric charges. It has already been emphasised that ferrous and ferric salts behave like salts of two different elements, and they certainly show none of the properties of metallic iron, except in being slightly magnetic. But these substances must, on the present theory, be considered as giving two different ions in solution, viz., the ferrous ion, Fe2+ and the ferric ion, Fe3+. The addition of unit positive charge alters the properties of the ferrous ion, and it is reasonable to suppose that the properties of sodium and chlorine atoms are also changed by the assumption of charges by the elements. Metallic sodium, and iron, may be regarded as discharged ions, possessing zero charge, Na0, and Fe0. In converting an atom of iron into a ferrous ion, two electrons are removed, producing Fe2+. When this is converted into the ferric ion another electron is removed, producing Fe3+. This corresponds with oxidation, since increase of positive valency occurs. Increasing the valency of a cation, or its oxidation, therefore corresponds with increasing its positive charge. Reduction is equivalent to diminution of positive charge on an ion, or increase of negative charge. Ferricyanides are reduced to ferrocyanides by increasing the negative charge on the ion by one unit: Fe(CN)63- + e = Fe(CN)64-. Iron treated with chlorine water forms ferric chloride, i.e, ferric ions and chloride ions: the metallic iron has been oxidised: Fe-3e = Fe˙˙˙, whilst the free chlorine has simultaneously been reduced: 3Cl + 3e = 3Cl'.
The electrovalency of an ion is equal to its charge; +3 for the ferric ion or -4 for the ferrocyanide ion. It represents the number of electrons lost or gained by the neutral atom in forming the ion. This is an extension of the elementary idea of valency, since sodium chloride in solution is not Na-Cl, but Na+ and Cl-, two separate ions.
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