Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution 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

effect of increasing the ph of solution
The chromates and dichromates can be inter convertible.

2CrO4 2- + 2H+ ---------> 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+ <-_-_-_-_> 2HCrO4-


2HCrO4- <-_-_-_-_> Cr2O7 2- + H2O