Potassium Dichromate, K₂Cr₂O₇

   

It is prepared from the ore called chromate or ferrochrome or chrome iron, FeO.Cr₂O₃. The various steps involved are

        4FeO.Cr₂O₃ + O₂ ———→ Fe₂O₃ + 4Cr₂O₃

        4Na₂CO₃ + 2Cr₂O₃ + 3O₂ ———→ 4Na₂CrO₄ + 4CO₂

(b) Conversion of sodium chromate into sodium dichromate.

        2Na₂CrO₄ + H₂SO₄ ———→ Na₂Cr₂O₇ + Na₂SO₄ + H₂O

(c) Conversion of sodium dichromate into potassium dichromate.

        Na₂Cr2O₇ + 2KCl ———→ K₂Cr₂O₇ + 2NaCl

It forms orange red crystals. It is moderately soluble in cold water but freely soluble in hot water.

    When heated, it decomposed to its chromate

    With alkalis it is converted into chromate which on acidifying gives back dichromate.

        K₂Cr₂O₇ + 2KOH ———→ 2K₂CrO₄ + H₂O

        2K₂Cr₂O₇ + H₂SO₄ ———→ K₂Cr₂O₇ + K₂SO₄ + H₂O

   In dichromate solution the Cr₂O₇^2– ions are in equilibrium with Cr₂O₇^2–  ions at pH = 4.

    (a) In cold conditions

             K₂Cr₂O₇ + 2H₂SO₄ ———→ 2CrO₃ + 2KHSO₄ + H₂O

    (b) In hot conditions

             2K₂Cr₂O₇ + 8H₂SO₄ ———→ 2K₂SO₄ + 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

     It is a powerful oxidising agent. In the presence of dil. H₂SO₄ it furnishes 3 atoms of available oxygen.

         K₂Cr₂O₇ + 4H₂SO₄ ———→ K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

         K₂Cr₂O₇ + 7H₂SO₄ + 6Kl ———→ 4K₂SO₄ + Cr₂(SO₄)₃ + 3l₂ + 7H₂O

 

(b) It oxidises ferrous salts to ferric salts

         K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ ———→ K₂SO₄ + Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + 2H₂O

 

         K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S ———→ K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S

(d) It oxidises nitrites to nitrates

         K₂Cr₂O₇ + 4H₂SO₄ + 3NaNO₂ ———→ K₂SO₄ + Cr₂(SO₄)₃ + 3NaNO₃ + 4H₂O

(e) It oxidises SO₂ to SO₄^2–

         K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ———→ K₂SO₄ + Cr₂(SO₄)₃ + 3H₂O

(f) It oxidises ethyl alcohol to acetaldehyde and acetic acid.

When heated with conc. HCl or with a chloride in the presence of sulphuric acid, reddish brown vapours of chromyl chloride are obtained.

         K₂Cr₂O₇ + 4KCl + 6H₂SO₄ ———→ 2CrO₂Cl₂ + 6KHSO₄ + 3H₂O

Thus reaction is used in the detection of chloride ions in qualitative analysis.

1. In volumetric analysis for the estimation of Fe²⁺ and I^-.

3. In photography and in hardening gelatin film.