Question

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

Answer 1

Potassium dichromate is prepared from chromite ore (FeCr2O4) in the following steps
Step (1) : Preparation of sodium chromate
4FeCr2O4+16NaOH+7O2 $\rightarrow$ 8Na2CrO4+2Fe2O3+8H2o
Step (2) : Conversion of sodium chromate into sodium dichromate
2Na2CrO4+conc.H2SO4 $\rightarrow$ Na2Cr2O7+Na2SO4+H2o
Step(3) : Conversion of sodium dichromate to potassium dichromate
Na2Cr2O7+2KCl $\rightarrow$ K2Cr2O7+NaCl
Potassium dichromate being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.
The dichromate ion (Cr2O72-) exists in equilibrium with chromate CrO2-4 ion at pH 4. However, by changing the pH, they can be interconverted.
2CrO2-4-ââ€'-