G. N. Khodalevich, V. G. Stolyarchuk, N. Ya. Negodina,
L. G. Sakovich, G. A. Kalenkovich

Oxidation of Ferrous Perchlorate by Molecular Oxygen

Compounds of ferrous iron, or ferro compounds (with the exception of some complexes), both in the solid state and in solutions, are prone to oxidation by atmospheric oxygen. A large number of studies published both in the domestic and in the foreign literature have been devoted to the oxidation of ferrous salts by oxygen. Particularly much attention has been given to the oxidation of ferrous sulfate. The oxidation of ferrous chloride, ferrous nitrate, and ferrous acetate has also been studied. The oxidation of a ferrous salt to a ferric salt (in aqueous solutions) is most often accompanied by precipitation of basic ferric salts. In order of increasing tendency toward oxidation, ferrous salts, as was shown by one of us [1], may be arranged in the following series:

chloride → nitrate → sulfate → acetate.

At the Department of General and Inorganic Chemistry in 1961–1962, work was carried out on developing methods for obtaining ferrous perchlorate. In connection with this work, a series of experiments was set up to study the oxidation by molecular oxygen of aqueous solutions of perchlorate, since this question has not been covered at all in the literature. The experiments were carried out with deci- and centimolar solutions of ferrous perchlorate, which were placed in special vessels (of the Drechsel bottle type). Oxygen was periodically passed through these solutions. The content of unoxidized ferrous salt was determined by titrating samples of the ferrous perchlorate solution with a potassium permanganate solution.

A series of experiments was also carried out on the oxidation of aqueous solutions of ferrous perchlorate in the presence of hydrochloric acid, and of potassium and barium perchlorates.

All experiments were conducted at room temperature.

The degree of oxidation of ferrous perchlorate \((\gamma)\) was calculated from the ratio

\[ \gamma = \frac{V_{\mathrm{н}} - V_{\mathrm{к}}}{V_{\mathrm{н}}}, \]

where \(V_{\mathrm{н}}\) is the number of ml of potassium permanganate solution consumed in titrating, for example, 5–10 ml of the prepared ferrous perchlorate solution before the start of the oxidation experiment;

\(V_{\mathrm{к}}\) is the number of ml of potassium permanganate solution consumed in titrating the same volume of solution after a definite interval of time following the start of the oxidation experiment.

The results of the experiments are presented in Tables 1 and 2.

As can be seen from the tabulated data, aqueous solutions of ferrous perchlorate are the most resistant, in comparison with other ferrous salts, to oxidation by molecular oxygen (atmospheric oxygen). The presence in a ferrous perchlorate solution of impurities of barium and potassium perchlorates does not affect the degree of oxidation of ferrous perchlorate. The presence in the solution of an excess of hydrochloric acid, however, makes the ferrous perchlorate solution practically unoxidizable by atmospheric oxygen.

Table 1

Table 2

If, moreover, one takes into account that pure crystalline ferrous perchlorate—\(\mathrm{Fe(ClO_4)_2 \cdot 6H_2O}\)—is very well preserved without noticeable signs of “weathering” or oxidation when stored in a desiccator over calcium chloride or in sealed ampoules, then ferrous perchlorate should be recognized as a more convenient substance for standardizing potassium permanganate, in comparison with Mohr’s salt, since the latter, when stored in aqueous solutions, is oxidized, albeit slowly.

The low degree of oxidation by molecular oxygen in aqueous solutions of ferrous perchlorate, as compared with aqueous solutions of other ferrous salts, confirms the assumption that the difference in the degree of oxidation of aqueous solutions of ferrous salts depends primarily on the degree of hydrolysis of these salts.

REFERENCES

1. G. N. Khodalevich. Oxidation of ferrous salts by molecular oxygen. Dissertation, Tomsk, 1951.