Preparation of potassium dichromate K2Cr2O7 from chromite ore FeO⋅Cr2O3(FeCr2O4): (i) Chromite ore is fused with NaOH or sodium carbonate in air to form sodium chromate. 4FeCr2O4+16NaOH+7O2→8Na2CrO4+2Fe2O3+8H2O
(ii) Sodium chromate solution is filtered and acidified to form sodium dichromate 2Na2CrO4+H2SO4→Na2Cr2O7+Na2SO4+H2O
The solution is cooled to remove crystallized sodium sulphate. Sodium dichromate remains soluble.
(iii) Equimolar proportions of a hot solution of sodium dichromate and KCl are mixed to form potassium dichromate. Na2Cr2O7+2KCl→K2Cr2O7+2NaCl
Sodium chloride precipitates out in hot solution and is removed by filtration. On cooling, potassium dichromate separates out from the mother liquor. At pH 4, there is an equilibrium between chromate ions and dichromate ions
Dichromate ion (orange red)Cr2O72−+H2O⇌Chromate ion (yellow)2CrO42−+2H+
By adding alkali, pH can be increased and the equilibrium reaction will shift in the forward direction to produce more chromate ions (yellow color). This is in accordance with Le chatelier's principle. By adding an acid, pH can be decreased and the equilibrium reaction will shift in the backward direction to produce more dichromate ions (orange-red color).