Preparation of K2Cr2O7
Potassium dichromate, (K2Cr2O7) is an orange-ish inorganic chemical reagent. In different laboratory or industry it is basically used as an oxidizing agent usually for alcohols.
It can be prepared through the following process:
- At first the fusion of chromite ore FeCr2O4 with sodium or potassium carbonate in the presence of access of air.
4FeCr2O4 + 8Na2CO3 + 7O2 --> 8Na2CrO4 + 2FeO3 + 8CO2
- Solution of sodium chromate is first filtered and then acidified with a solution of sulfuric acid which results in an orange sodium dichromate solution Na2Cr2O7 2H2O can be crystallized.
2Na2CrO4 + 2H+ --> Na2Cr2O7 + 2Na+ + H2O
- Sodium dichromate is more soluble than potassium dichromate and therefore it is fused with KCl that leads to the formation of orange crystals of potassium dichromate.
Na2Cr2O7 + 2KCl --> K2Cr2O7 + 2NaCl
- At pH equal to 4 the dichromates and chromates exists in equilibrium and can be inter convertible.
2CrO4 2- + 2H2+ --> Cr2O72- + H2O
Cr2O7 2- + 2OH- --> 2CrO42- + H2O
- The yellow colour of chromate changes to orange coloured dichromate in the presence of acidic medium whereas the dichromate changes into chromate in the presence of basic medium.
2CrO42- + 2H+ --> 2HCrO4- (Hydrogen chromate)
2HCrO4- --> Cr2O72- + 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.