Potassium dichromate and permanganate

First-row oxides form when the metals are heated in oxygen. Every metal except Sc forms an ionic MO oxide. As the oxidation number rises, ionic character falls and acidic character rises: Mn₂O₇ is a covalent green oil and gives HMnO₄; CrO₃ gives H₂CrO₄ and H₂Cr₂O₇. Vanadium runs from basic V₂O₃ to less basic V₂O₄ to amphoteric V₂O₅. CrO is basic and Cr₂O₃ amphoteric.

Disproportionation: when one oxidation state is less stable than the states on either side, it converts into both. In acid, manganate(VI) does this: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O.

K₂Cr₂O₇ from chromite ore: fuse FeCr₂O₄ with sodium carbonate in air, 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂. Acidify the yellow chromate solution to get orange dichromate, 2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O, then add KCl: Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl. The potassium salt, being less soluble, crystallises.

Chromate and dichromate interconvert with pH, and chromium is +6 in both: 2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O, and Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O. CrO₄²⁻ is tetrahedral; Cr₂O₇²⁻ is two tetrahedra sharing one corner, with a Cr-O-Cr angle of 126°.

Acidified dichromate is a strong oxidant: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, E° = 1.33 V. It oxidises I⁻ to I₂, H₂S to S, Sn²⁺ to Sn⁴⁺ and Fe²⁺ to Fe³⁺: Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O. K₂Cr₂O₇ is a primary standard in volumetric analysis and is used in the leather industry; the more soluble sodium salt is used in organic chemistry.

KMnO₄ from pyrolusite: fuse MnO₂ with KOH and an oxidant such as KNO₃ (or air) to get green K₂MnO₄, 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O, which disproportionates in neutral or acid solution to permanganate. Commercially, manganate(VI) is oxidised electrolytically in alkali. In the laboratory, peroxodisulphate oxidises Mn²⁺: 2Mn²⁺ + 5S₂O₈²⁻ + 8H₂O → 2MnO₄⁻ + 10SO₄²⁻ + 16H⁺.

KMnO₄ forms dark purple, almost black crystals (isostructural with KClO₄), dissolves only 6.4 g per 100 g of water at 293 K, and decomposes at 513 K: 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂. Manganate and permanganate are tetrahedral, with π bonds from oxygen p to manganese d orbitals; green manganate has one unpaired electron (paramagnetic) and permanganate none (diamagnetic).

Electrons taken by MnO₄⁻ depend on the medium: to MnO₄²⁻, 1 e⁻ (E° = +0.56 V); to MnO₂ in acid, 3 e⁻ (+1.69 V); to Mn²⁺ in acid, 5 e⁻ (+1.52 V): MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

In acid: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺ (green to yellow); 10I⁻ + 2MnO₄⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 5I₂; oxalate at 333 K, 5C₂O₄²⁻ + 2MnO₄⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 10CO₂; also H₂S to S, sulphite to sulphate and nitrite to nitrate. HCl is not used to acidify, because permanganate oxidises it to chlorine.

In neutral or faintly alkaline solution MnO₄⁻ goes to MnO₂: iodide becomes iodate (2MnO₄⁻ + H₂O + I⁻ → 2MnO₂ + 2OH⁻ + IO₃⁻), thiosulphate becomes sulphate, and Mn²⁺ becomes MnO₂ (catalysed by ZnSO₄ or ZnO). Permanganate is also used to bleach wool, cotton and silk and to decolourise oils.