Preparation of Potassium Dichromate, K2Cr2O7
Potassium dichromate is an important chemical used in industries as an oxidizing agent and for the preparation of many other compounds. Dichromates are usually prepared from chromates and this is obtained by the combination of chromite ore with sodium/potassium carbonate in the presence of air.
The reaction can be given as:
4FeCr2O4 + 8Na2CO3 + 7O2 \(\rightarrow\)
The solution of sodium chromate (Na2CrO4) is further purified with sulphuric acid to form a solution from which the crystals of orange coloured sodium dichromate (Na2Cr2O7.2H2O) can be extracted.
2Na2CrO4 + 2H+ \(\rightarrow\)
Now potassium dichromate can be obtained by reacting a solution of sodium dichromate with potassium chloride.
Na2Cr2O7 + 2KCl \(\rightarrow\)
Thus we finally obtain the orange crystals of potassium dichromate.
Preparation of Potassium Permanganate, KMnO4
We can get this (KMnO4) by reacting MnO2 with an alkali metal hydroxide and KNO3 (oxidizing agent).
This will result in the production of dark green K2MnO4 which disproportionate in an acidic or neutral medium to give permanganate.
2MnO2 + 4KOH + O2 \(\rightarrow\)
3MnO42- + 4H+ \(\rightarrow\)
Therefore, preparation of potassium permanganate involves a reaction of MnO2 with KOH to give MnO42- followed by electrolytic oxidation of manganate to give permanganate ion, MnO4–.
After this manganese ion salt gets oxidized by peroxodisulphate to permanganate ion as per the reaction given below.
2Mn2+ + 5S2O82- + 8H2O \(\rightarrow\)
Thus the dark purple coloured crystals of potassium permanganate are obtained.
Potassium Dichromate, K2Cr2O7
- On heating potassium dichromate decomposes to form potassium chromate, chromic oxide and oxygen.
- It is a powerful oxidizing agent. It oxidizes iodide to iodine.
Cr2O72- + 14H+ + 6I– \(\rightarrow\)
Potassium permanganate, KMnO4
- When potassium permanganate is heated it gives potassium manganate, manganese dioxide and oxygen.
- It is a powerful oxidizing agent in acidic or alkaline solution.
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