Preparation of Potassium Dichromate and Potassium Permanganate

What is Potassium Dichromate?

Potassium dichromate or anhydrochromate is prepared by adding to the neutral yellow chromate of potassium in solution, a moderate quantity of one of the stronger acids.

Potassium permanganate is commercially prepared by mixing a solution of potassium hydroxide and powdered manganese oxide with oxidizing agents like potassium chlorate.

Table of Contents

• Recommended Videos
• Preparation of Potassium Dichromate
• Structure of Potassium Dichromate Molecules
• Preparation of Potassium Permanganate
• Properties of K2Cr2O7 & KMnO4
• Applications of Potassium Dichromate
• Frequently Asked Questions – FAQs

Recommended Videos

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.

Potassium Dichromate

The reaction can be given as:

4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

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⁺  →  Na₂Cr₂O₇ + 2Na⁺ + H₂O

Now potassium dichromate can be obtained by reacting a solution of sodium dichromate with potassium chloride.

Na₂Cr₂O₇ + 2KCl →  K₂Cr₂O₇ + 2NaCl

Thus, we finally obtain the orange crystals of potassium dichromate.

Structure of Potassium Dichromate Molecules

Potassium dichromate is an ionic compound consisting of two potassium cations and one dichromate anion. The coordination geometries around the chromium atoms are tetrahedral. The structure of a potassium dichromate molecule is illustrated below.

It can be noted that in the potassium dichromate molecule, potassium exhibits an oxidation state of +1, oxygen exhibits an oxidation state of -2 and chromium exhibits an oxidation state of +6. It can be noted that crystals of potassium dichromate have a triclinic structure.

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 is disproportionate in an acidic or neutral medium to give permanganate.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

3MnO₄^2- + 4H⁺ → 2MnO₄^– + MnO₂ + 2H₂O

Therefore, the preparation of potassium permanganate involves a reaction of MnO₂ with KOH to give MnO₄^2- 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⁺ + 5S₂O₈^2- + 8H₂O → 2MnO₄^–  +10SO₄^2- + 16H⁺

Thus the dark purple coloured crystals of potassium permanganate are obtained.

2KMnO₄ →  K₂MnO₄ + MnO₂ + O₂

Properties of K₂Cr₂O₇ & KMnO₄

1. Properties of Potassium Dichromate, K₂Cr₂O₇

• On heating, potassium dichromate decomposes to form potassium chromate, chromic oxide and oxygen.

4K₂Cr₂O₇ →  4K₂CrO₄ + 2CrO₃ + 3O₂

• It is a powerful oxidizing agent. It oxidizes iodide to iodine.

Cr₂O₇^2- + 14H⁺ + 6I^– →  2Cr³⁺ + 7H₂O + 3I₂

• The compound has bright red crystals and is used for dyeing, staining, tanning etc.
• For a medical purposes, it can be an astringent, antiseptic and caustic. It emits toxic chromium fumes when heated
• It is highly corrosive and is a strong oxidizing agent for which, it is used in wood preservatives, pigments manufacture and photochemical processes.

2. Properties of Potassium permanganate, KMnO₄

• When potassium permanganate is heated it gives potassium manganate, manganese dioxide and oxygen.

2KMnO₄ →  K₂MnO₄ + MnO₂ + O₂

• It is a powerful oxidizing agent in acidic or alkaline solutions.
• It is in the form of purple crystals and gets soluble in hot water
• It is noncombustible but works as a catalyst in the burning of combustible materials.
• It will start a fire when mixed with glycerine.
• It can become toxic in high concentration

Applications of Potassium Dichromate

The primary application of K2Cr2O7 is in the preparation of potassium chrome alum, a compound which is used extensively in the tanning of leather. Chromic acid can also be prepared from this compound. Potassium dichromate is known to be used in the production of cement since it improves the texture and the density of the cement mixture. 

Another important application of potassium dichromate is in the photography industry, where it is used in combination with a powerful mineral acid as an oxidizing agent for photographic screen printing. Since it is non-hygroscopic in nature, this compound is also employed for several wet tests in the field of analytical chemistry.

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