Preparation and properties of Potassium dichromate (K2Cr2O7)

Preparation of Potassium dichromate (

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+ <-_-_AcidAlkali-_-_> 2HCrO4- (Hydrogen chromate)

2HCrO4-<-_-_AcidAlkali-_-_> 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

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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