Potassium Dichromate

Potassium Dichromate:

Potassium dichromate (K2Cr2O7) is a solid crystalline ionic with a bright, reddish-orange colour. It has a wide range of applications as an oxidising agent in laboratories and industries. It is very hazardous and chronic to health as all hexavalent chromium compounds. It is a strong oxidising agent with corrosive properties, soluble and denser than water. Potassium dichromate can be dangerous to human health if exposed for a long time; therefore, it should be handled with care and attention.

Molecular formula and structure:

The chemical formula of potassium dichromate is K2Cr2O7, while its molecular weight is 294.185 g / mol. It consists of two potassium ions (K+) and a badly charged dichromate ion (Cr2O7–).

In the dichromate ion, two chromium ions have a hexagonal structure, with a +6-oxidation state and are bound together by three oxygen atoms and an oxygen atom blocking the bridge.

Production of K2Cr2O7:

Potassium dichromate is usually regulated by the reaction of potassium chloride to sodium dichromate. Alternatively, it can also be found in potassium chromate by the process of roasting chromite iron with potassium hydroxide. It dissolves in water, and in its dissolution ionise:

K2Cr2O7 → 2 K+ + Cr2O72−

Cr2O72− + H2O ⇌ 2 CrO42− + 2 H+

Reactions with potassium dichromate:

Potassium dichromate has oxidising properties in organic chemistry and is milder in nature than potassium permanganate. It is used to oxidise alcohol. It converts essential alcohols into aldehyde and, under due stress, into carboxylic acids. In contrast, potassium permanganate tends to provide carboxylic acid as a byproduct. The second alcohol is converted into ketones.

In an aqueous solution, the indicated colour change can be used to test the separation of aldehyde and ketones. Aldehyde lowers dichromate from +6 to +3 oxidation state, changing colour from orange to green. This colour change arises because the aldehyde can be oxidised to the corresponding carboxylic acid. The ketone will not show such a change because it cannot be oxidised continuously, so the solution will always be orange.

1. It gives rise to oxygen when heated strong:

4 K2Cr2O7 → 4 K2CrO4 + 2 Cr2O3 + 3 O2

2. From potash, potassium chromate is produced industrially:

K2Cr2O7 + K2CO3 → 2 K2CrO4 + CO2

3. On reaction with concentrated acids, oxygen is released:

2 K2Cr2O7 + 8 H2SO4 → 2 K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3 O2

Physical properties:

1.  Potassium dichromate exists as a solid substance at room temperature and has a reddish-orange crystal.

2. Potassium dichromate has no natural odour.

3. It does not burn naturally and is highly corrosive.

4. The boiling point of potassium dichromate is 500°C, and the melting point is 398°C. When it boils, potassium dichromate decomposes.

5. The solubility of potassium dichromate varies at different temperatures, which means that it is very soluble in water at high temperatures. However, potassium dichromate does not dissolve in alcohol and acetone.

6. The refractive index of potassium dichromate is 1.738.

Chemical properties:

1. Heat action on potassium dichromate:

When heated with potassium dichromate, chromate is obtained along with oxygen gas.

4K2Cr2O7 ⟶ 4K2CrO4 + 2Cr2O3 + 3O2

2. Potassium dichromate action on alkalis:

When reacting with alkalis, potassium dichromate loses its reddish-orange colour and turns yellow.

 K2Cr2O7 + 2KOH ⟶ 2K2CrO4 + H2O (Yellow-orange)

3. Oxidising properties of potassium dichromate:

Potassium dichromate acts as an excellent oxidising agent.

i.  In reaction with dilute sulfuric acid, one potassium dichromate molecule produces three moles of oxygen gas.

 K2Cr2O7 + 4H2SO4 ⟶ K2SO4 + Cr2 (SO4)3+ 4H2O + 3O

ii. When potassium dichromate reacts with KI, iodine gas is released.

K2Cr2O7 + 7H2SO4 + 6KI⟶ 4K2SO4 + Cr2 (SO4)3+ 7H2O + 3I2

4. Chromyl Chloride Testing:

Potassium dichromate combines with concentrated sulfuric acid to give off a reddish-brown vapour chromyl chloride. The reaction is also called the Chromyl Chloride Test.

K2Cr2O7 + 4KCl + 6H2SO4 ⟶ 2CrO2Cl2 + 6KHSO4 + 3H2O

Uses:

1. Treatment of woods.

2. Cleaning

3. Construction

4. Photography

5. Printing

Conclusion

Potassium dichromate is less oxidising than KMnO4. However, its solutions display long-lasting stability in acid and light, in many organisms and in chloride ions. It is often used in acid solutions. The main disadvantage is that both the Cr2O72− reactor and the product Cr3+ have orange and green colours, respectively.