Potassium dichromate is prepared from chromate, which in turn isobtained by the fusion of chromite ore (FeCr2O4) with sodium or potassiumcarbonate in free access of air. The reaction with sodium carbonate occursas follows:
The yellow solution of sodium chromate is filtered and acidified withsulphuric acid to give a solution from which orange sodium dichromate,Na2Cr2O7⋅2H2O can be crystallised.
Sodium dichromate is more soluble than potassium dichromate. The latteris therefore, prepared by treating the solution of sodium dichromate withpotassium chloride.
Na2Cr2O7+2KCl→K2Cr2O7+2NaCl
Orange crystals of potassium dichromate crystallise out. The chromatesand dichromates are interconvertible in aqueous solution dependingupon pH of the solution. If pH of potassium dichromate is increased it isconverted to yellow potassium chromate.
2CrO42–+2H+→Cr2O72–+H2O
Cr2O72–+2OH–→2CrO42–+H2O