Preparation and properties of Potassium dichromate (K2Cr2O7)

Preparation of Potassium dichromate (K₂Cr₂O₇)

Potassium dichromate (K₂Cr₂O₇) is prepared from chromite ore FeCr₂O₄. The chromite ore is fused with sodium or potassium carbonate in free access of air.

4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ -------> 8Na₂CrO₄ + 2FeO₃ + 8CO₂

The yellow solution of sodium chromate is filtered and acidified with sulfuric acid to give a solution from which orange sodium dichromate , Na2Cr2O7 2H2O can be crystallized.

2Na₂CrO₄ + 2H⁺ ---------> Na₂Cr₂O₇ + 2Na⁺ + H₂O

Sodium dichromate is more soluble than potassium dichromate. Hence sodium dichromate when fused with KCl forms orange crystals of potassium dichromate.

Na₂Cr₂O₇ + 2KCl --------> K₂Cr₂O₇ + 2NaCl

The chromates and dichromates can be inter convertible.

2CrO₄ ^2- + 2H²⁺ ---------> Cr₂O₇ ^2- + H₂O

Cr₂O₇ ^2- + 2OH- ---------> 2CrO₄ ^2- + H₂O

The dichromate ion and chromate ion exist in equilibrium with each other at a pH of 4. Yellow chromate changes into orange dichromate in acid medium and dichromate changes into chromate in basic medium.


2CrO₄ ^2- + 2H⁺ <-_-_^Acid_Alkali-_-_> 2HCrO₄^- (Hydrogen chromate)


2HCrO4^-<-_-_^Acid_Alkali-_-_> Cr₂O₇ ^2- + H₂O Dichromate (orange)

The chromate ion is tetrahedral and the dichromate ion consists of two tetrahedral sharing at one corner, with Cr-O-Cr bond angle 126 degree.

Properties Potassium dichromate (K₂Cr₂O₇)

Action of heat

Potassium dichromate decomposes on heating to from potassium chromate, chromic oxide and oxygen.

4K₂Cr₂O₇ ------> 4K₂CrO₄ + 2CrO₃ + 3O₂

Oxidizing properties

Potassium dichromate is a powerful oxidizing-agent in acidic medium.

Cr₂O₇ ^2- + 14H⁺ + 6 electron -------> 2Cr³⁺ + 7H₂O

A few examples for this oxidizing character in acidic medium are

1. It oxidize iodides to iodine.

Cr₂O7 ^2- + 14H⁺ + 6I- ------> 2Cr ³⁺ + 7H₂O + 3I₂

2. It oxidizes ferrous salts to ferric salts.

Cr₂O₇ ^2- + 14H⁺ + 6 Fe2+ -------> 2Cr ³⁺+ 7H₂O + 6Fe³⁺

3. It oxidizes stannous salts to stannic salts.

Cr₂O₇ ^2- + 14H⁺ + 3Sn ²⁺ --------> 2Cr ³⁺ + 7H₂O + 3Sn ⁴⁺

4. It oxidizes H2S to sulphur

Cr₂O₇ ^2- + 8H⁺ + 3H₂S -------> 2Cr ³⁺ + 7H₂O + 3S

Uses of potassium dichromate

(K₂Cr₂O₇)

Sodium and potassium dichromate are strong oxidizing agents. Potassium dichromate is used as a primary standard in volumetric analysis. It is also used in chrome plating.


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