Dichromate are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free access of air.
The reaction with sodium carbonate occurs as follows:
4FeCr2O4 + 8Na2CO3 + 7O2→ 8Na2CrO4 + 2Fe2O3 + 8CO2
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7.2H2O can be crystallised.
2Na2CrO4 + 2H+ → Na2Cr2O7 + 2Na+ + H2O
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