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Chemistry · Ch 8 — Transition and Inner Transition Elements

K₂Cr₂O₇ : Preparation of potassium dichromate

8.7.4

K₂Cr₂O₇ : Preparation of potassium dichromate

In the industrial production, finely powdered chromite ore (FeOCr2O3\mathrm{FeOCr_2O_3}) is heated with anhydrous sodium carbonate (Na2CO3\mathrm{Na_2CO_3}) and a flux of lime in air in a reverbatory furnace.

4(FeO.Cr2O3)+8Na2CO3+7O2→Δ8Na2CrO4+2Fe2O3+8CO2\mathrm{4(FeO.Cr_2O_3) + 8Na_2CO_3 + 7O_2 \xrightarrow{\Delta} 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2}

Sodium chromate (Na2CrO4\mathrm{Na_2CrO_4}) formed in this reaction is then extracted with water and treated with concentrated sulphuric acid to get sodium dichromate and hydrated sodium sulphate :

2Na2CrO4+H2SO4⟶Na2Cr2O7+Na2SO4+H2O\mathrm{2Na_2CrO_4 + H_2SO_4 \longrightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O}

Note

The book prints this second equation as "2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + 2NaCl + Na₂Cr₂O₇.H₂O" — its own misprint: there is no chloride in the reaction, and the book's own sentence above the equation names the products as sodium dichromate and hydrated sodium sulphate. The balanced equation consistent with that sentence is the one shown here (equivalently written with the hydrate as Na2SO4.H2O\mathrm{Na_2SO_4.H_2O}). …

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