(i) Potassium dichromate (K₂Cr₂O₇) is prepared from chromite ore (FeCr₂O₄) in the following steps.

Step (1): Preparation of sodium chromate

FeCr₂O₄ + 16NaOH + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8H₂O

Step (2): Conversion of sodium chromate into sodium dichromate

2Na₂CrO₄ + conc.H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step(3): Conversion of sodium dichromate to potassium dichromate

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.

The dichromate ion (Cr₂O₇^2-) exists in equilibrium with chromate (CrO₄^2-) ion at pH 4. However, by changing the pH, they can be interconverted.

CrO₄^2- \(\underset{Acid}{\stackrel{Alkali}{\longrightarrow}}\) 2HCrO₄^-\(\underset{Acid}{\stackrel{Alkali}{\longrightarrow}}\)Cr₂O₇^2-

(ii) Potassium permanganate (KMnO₄) can be prepared from pyrolusite (MnO₂). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO₃ or KClO₄, to give K₂MnO₄.

2MnO₂ + 4KOH + O₂ [/latex]\overset{heat}\longrightarrow[/latex] 2K₂MnO₄ + 2H₂O

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution.

Electrolytic oxidation

K₂MnO₄ ↔️ 2K⁺ + MnO₄^2-

H₂O ↔️ H⁺ + OH^-

At anode, manganate ions are oxidized to permanganate ions.

MnO₄^2- ↔️ MnO₄^- + e^-

Green       Purple

Oxidation by chlorine

2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl

2MnO₄^2- + Cl₂ → 2MnO₄^2-+ 2Cl^-

Oxidation by ozone

2K₂MnO₄ + O₃ + H₂O → 2K₂MnO₄ + 2KOH + O₂

2MnO₄^2- + O₃ + H₂O → 2MnO₄^2- + 2OH^- + O₂