® 6H + + 3S + 6e – ; Fe + ® 6Fe + + e The full ionic equation may be obtained by adding the half-reaction for potassium dichromate to the half-reaction for the reducing agent, for e.g., Cr O – + H + + Fe + ® Cr + + Fe + + H O Potassium permanganate KMnO Potassium permanganate is prepared by fusion of MnO with an alkali metal hydroxide and an oxidising agent like KNO . This produces the dark green K MnO which disproportionates in a neutral or acidic solution to give permanganate. 2MnO + 4KOH + O ® 2K MnO + 2H O 3MnO – + 4H + ® 2MnO – + MnO + 2H O Commercially it is prepared by the alkaline oxidative fusion of MnO followed by the electrolytic oxidation of manganate (Vl). F d used with KOH, oxidise with air or KNO MnO MnO ; manganate ion − → Electrolytic oxidation in alkaline solution MnO MnO manganate permanganate ion In the laboratory, a manganese (II) ion salt is oxidised by peroxodisulphate to permanganate.
2Mn + + 5S O – + 8H O ® 2MnO – + 10SO – + 16H + Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of KClO . The salt is not very soluble in water ( . g/ g of water at K), but when heated it decomposes at K. 2KMnO ® K MnO + MnO + O It has two physical properties of considerable interest: its intense colour and its diamagnetism along with temperature-dependent weak paramagnetism.
These can be explained by the use of molecular orbital theory which is beyond the present scope. The manganate and permanganate ions are tetrahedral; the p - bonding takes place by overlap of p orbitals of oxygen with d orbitals of manganese. The