Question

Describe the preparation of potassium dichromate.

Answer 1

Preparation of potassium dichromate:

1. Potassium dichromate is prepared from chromite ore. The ore FeO.Cr₂O₃ is concentrated by the gravity separation process.

2. The concentrated ore is mixed with excess sodium carbonate and lime and roasted in a reverberatory furnace.

$$4\mathrm{FeCr}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_4+8\mathrm{Na}{\phantom{A}}_2\mathrm{CO}{\phantom{A}}_3+7\mathrm{O}{\phantom{A}}_2\stackrel{900-1000°\mathrm{C}}{\to }8\mathrm{Na}{\phantom{A}}_2\mathrm{CrO}{\phantom{A}}_4+2\mathrm{Fe}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_3+8\mathrm{CO}{\phantom{A}}_2\uparrow$$

3. The roasted mass is treated with water to separate soluble sodium chromate form insoluble iron oxide. The yellow solution of sodium chromate is treated with concentrated sulphuric acid which converts sodium chromate into sodium dichromate.

$$\underset{\text{[Sodium chromate (yellow)]}}{2\mathrm{Na}{\phantom{A}}_2\mathrm{CrO}{\phantom{A}}_4}+\mathrm{H}{\phantom{A}}_2\mathrm{SO}{\phantom{A}}_4⟶\underset{\text{[Sodium dichromate (orange red)]}}{\mathrm{Na}{\phantom{A}}_2\mathrm{Cr}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_7}+\mathrm{Na}{\phantom{A}}_2\mathrm{SO}{\phantom{A}}_4+\mathrm{H}{\phantom{A}}_2\mathrm{O}$$

4. The above solution is concentrated to remove less soluble sodium sulphate. The resulting solution is filtered and concentrated. It is cooled to get the crystals of Na₂SO₂.2H₂O.

5. The saturated solution of sodium dichromate in water is mixed with KCl and then concentrated to get crystals of NaCl. It is filtered while hot and the filtrate is cooled to obtain K₂Cr₂O₇ crystals.

$$\underset{\text{[Sodium dichromate (orange red)]}}{\mathrm{Na}{\phantom{A}}_2\mathrm{Cr}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_7}+2\mathrm{KCl}⟶\underset{\text{[Potassium dichromate (orange red)]}}{\mathrm{K}{\phantom{A}}_2\mathrm{Cr}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_7}+2\mathrm{NaCl}$$