Oxidimetry

Oxidimetry, a volumetric analysis method based on oxidation-reduction reactions. The most common oxidizing agent in O. is potassium permanganate (permanganate, chameleon), KMnO₄; if oxalic acid is acted upon by KMnO₄, the latter loses its color due to reduction, while the oxalic acid is oxidized to CO₂ and H₂O. Depending on the medium, the reduction of KMnO₄ proceeds differently. In an acidic medium, KMnO₄ gives manganous oxide and releases 5 equivalents of oxygen. This can be schematically represented by the following equations: 2 KMnO₄ + 3H₂SO₄ = K₂SO₄ + 2MnSO₄ + 3H₂O + 5O. In a neutral or alkaline medium, manganese dioxide is formed and 3 equivalents of oxygen are released: 2KMnO₄ + H₂O = 2KOH + 2MnO₂ + 3O. If the titer of oxalic acid is known, the titer of KMnO₄ can also be determined from the titration results. It should be borne in mind that the normal weight of KMnO₄ in an acidic medium is 1/5 of its molecular weight, and in an alkaline medium is 1/3 (see Normal solutions). Most often, titration in an acidic medium is required, but if there is a need to switch to titration in an alkaline medium, the same solution can still be used, making the appropriate conversion afterward, based on the ratio of normal weights of KMnO₄ in acidic and alkaline media (3:5). No special indicator is required for titration, since KMnO₄ itself is an indicator. In the case of titration in an acidic medium, diluted H₂SO₄ is added; hydrochloric acid and generally Cl are harmful, as they are also oxidized by KMnO₄. Burettes for oxidimetric titration must have a glass stopcock (since rubber is oxidized by KMnO₄). To establish the titer of KMnO₄, oxalic acid can be used. A solution of oxalic acid can be prepared, for example, N/10, containing 6.3023 g of crystalline acid per 1 liter. The KMnO₄ solution is prepared with approximately the same normality (N/10) by dissolving (for use in an acidic medium) 3.16 g of the salt in 1 liter (molecular weight of KMnO₄). The solution is left to stand for several days. During this time, various impurities in the water capable of oxidation will be oxidized by KMnO₄; the MnO₂ present in the salt itself and formed will settle. After this, the titer of the KMnO₄ solution is established by titrating oxalic acid in the presence of diluted (1:10) H₂SO₄ at 70°C; the first drops of KMnO₄ are not decolorized immediately, since the interaction of KMnO₄ with oxalic acid occurs only in the presence of even a minimal amount of manganous salt formed. Titration ends when the faint pink coloration that appears in the liquid from the addition of one drop of KMnO₄ does not disappear.--Suppose that 10 cm³ of oxalic acid with a factor of 1.016 required 11 cm³ of KMnO₄; then the factor of KMnO₄ = 10/11 × 1.016 = 0.9236. The titer of KMnO₄ is also established by ammonium oxalate C₂(NH₄)₂O₄ + H₂O or even better by sodium oxalate, which does not contain crystallization water; 1 liter of N/10 solution of the first salt contains 71.0475 g of substance, and of the second salt - 66.997 g. The titer can also be established by Mohr's salt: FeSO₄·(NH₄)₂SO₄ + 6H₂O (1 liter of N/10 solution contains 39.2132 g of substance). Various analytical problems are solved by methods of oxidimetry. This method easily determines ferrous oxide, hydrogen peroxide, nitric acid, nitrous acid, oxidizability of water. Ferric oxide can also be determined after preliminary reduction to ferrous oxide, which is then titrated with chameleon. If both ferrous and ferric oxides are present in the solution, the ferrous oxide is titrated first; then in a separate sample, the ferric oxide is reduced to ferrous oxide and titrated again; the difference determines the ferric oxide. As an example of titration in an alkaline medium (in the presence of caustic alkali or soda), the determination of formic acid salts can be mentioned. The KMnO₄ titration method is often called manganometry; another method, also based on oxidation-reduction reactions, is iodometry (see).

The titration method with KMnO₄ is often called manganometry; another method, also based on oxidation-reduction reactions, is iodometry (see).