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Question

Indicate the steps in the preparation of K2Cr2O7 from chromite ore.

Solution
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Potassium dichromate: (K2Cr2O7) is prepared from chromite ore (FeCl2O4) in the following steps:

(1) Preparation of sodium chromate

4FeCr2O4+16NaOH+7O28Na2CrO4+2Fe2O3+8H2O

(2) Conversion of sodium chromate into sodium dichromate

2Na2CrO4+conc.H2SO4Na2Cr2O7+Na2SO4+H2O

(3) Conversion of sodium dichromate to potassium dichromate

Na2Cr2O7+2KCl2NaCl+K2Cr2O7

Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filteration.

The dichromate ion (Cr2O27) exists in equilibrium with chromate (CrO24) ion at pH.4. However, by changing the pH they can be interconverted.

2CrO24chromate(yellow)acidalkali2HCrO4HydrogenchromateacidalkaliCr2O27Dichromate(orange)

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