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CARBON AND CARBRO 453
John Arthur Wilson called "Modern Practice in Leather Manufacture," published by the Reinhold Publishing Corporation.
Any discussion of chrome tanning must be preceded by a resume of the chemistry of the trivalent chromium salts. This is indeed complex. But the effect of salts like oxalates, sulphites, etc., upon tanning, can only be understood when the effect upon the chromium ion is understood. Leather is tanned mainly by treating the hide with chrome liquors that have been made "basic." This has a peculiar technical meaning. The salts CrCl3 • 6H2O, CrCla • 5H2O, and CrCls • 4H2O, according to the empirical chemical formula, should not differ from each other when dissolved in water. In each case we would expect that treatment with three equivalents of silver nitrate would bring down a precipitate of silver chloride that would correspond to three mols of chloride. This is true only for the first body, hexaquo chromi chloride. The second body will precipitate only two of the three chloride ions, while the third body will precipitate only one halogen, the other halogens existing in non-ionic or non-dissociable form. Compounds such as CrCls • 5H2O and CrCla • 4H2O, show all three types of chemical bonds, ionic, co-valent, and co-ordinate. The union between the un-ionized halogens and the chromium is by means of a shared electron pair to which the chromium and the halogen each contribute one electron, hence this bond is co-valent. The union between the water and the chromium is also by means of an electron pair, but now the pair is contributed solely by the oxygen atom in the water molecule, hence this is a coordinate bond. The complex ion composed of the chromium to which are attached six other groups by co-valent or co-ordinate bonds is held to the ionized or dissociable chloride ions, by means of electrovalences or ionic bonds.
Before the advent of the quantum theory, Werner proposed to explain the chemistry of those complexes by the statement that chromium has a coordinate number of 6, meaning that the trivalent chromium ion could attach six units that are either neutral or negative ions. The new complex so formed would act as a new "atom" whose charge would be the algebraic sum of the charges on the chromium and the attached groups. Today we explain the existence of co-ordinate number by means of the quantum theory. The trivalent chromium ion can add a shell of twelve electrons to achieve a stable electronic structure. These electrons can be contributed by the adsorption of electron-saturated bodies which have at least one pair of electrons that are not utilized in a chemical bond. Water is such a body, the oxygen atom containing four electrons that are not utilized. This substance can attach itself to the chromium ion, the point of attachment being one of the electron pairs, which in that way forms a co-ordinate bond between the oxygen and the chromium. Six molecules of water attached to chromium satiate its desire for electrons, forming, at the same time, the new unit
H20->Cr<-OH2 NOH2J