139.

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

The following steps are involved in preparation of K₂Cr₂O₇ from iron chromite (FeCr₂O₄) ore:
(i) Preparation of sodium chromate: The chromite ore (FeO.Cr₂O₃) is finely powdered and mixed with sodium carbonate and quick lime and then heated to redness in a reverberatory furnace with free supply of air.
$$\begin{gathered} 4\mathrm{FeO}.{\mathrm{Cr}}_2{\mathrm{O}}_3 +{\mathrm{O}}_2\to 2{\mathrm{Fe}}_2{\mathrm{O}}_3 +4{\mathrm{Cr}}_2{\mathrm{O}}_3 \\ [4{\mathrm{Na}}_2{\mathrm{CO}}_3 +2{\mathrm{Cr}}_2{\mathrm{O}}_3 +3{\mathrm{O}}_2 \to 4{\mathrm{Na}}_2{\mathrm{CrO}}_4 +4{\mathrm{CO}}_2]\times 2 \\ \mathrm{thus} \mathrm{overall} \mathrm{reaction} \\ 4\mathrm{FeO}.{\mathrm{Cr}}_2{\mathrm{O}}_3 +8{\mathrm{Na}}_2{\mathrm{CO}}_3 + 7{\mathrm{O}}_2 \to 8{\mathrm{Na}}_2{\mathrm{CrO}}_4 +2{\mathrm{Fe}}_2{\mathrm{O}}_3 +8{\mathrm{CO}}_2 \end{gathered}$$

The mass is then extracted with water, when sodium chromate is completely dissolved while Fe₂O₃ is left behind.
(ii)    Conversion of sodium chromate into sodium dichromate (NaCr₂O₇) : The sodium chromate extracted with water in previous step is acidified.
3Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄), + H₂O
On cooling Na₂SO₄ separates out as Na₂SO₄. 10H₂O and Na₂Cr₂O₇, is remains in solution.
(iii)    Conversion of Na₂Cr₂O₇ into K₂Cr₂O₇: The solution containing Na₂Cr₂O₇ is treated with KCl
Na₂Cr₂O₇ + KCl → K₂Cr₂O₇ + 2NaCl
Sodium chloride (NaCl) being less soluble separates out on cooling. On crystallising the remaining solution, orange coloured crystals of K₂Cr₂O₇ separate out.
Effect of Change of pH: When pH of solution of K₂Cr₂O₇ is increased slowly the medium changes from acidic to basic. The chromates and dichromates are interconvertible in aqueous solution depending upon pH of solution.
$\underset{\underset{(\mathrm{orange} \mathrm{in} \mathrm{acidic} \mathrm{medium})}{\mathrm{dichromate} \mathrm{ion} \mathrm{at} \mathrm{low} \mathrm{pH}}}{{\mathrm{Cr}}_2{{\mathrm{O}}_{7 }}^{2-} + {\mathrm{H}}_2\mathrm{O}} \underset{\mathrm{acid}}{\overset{\mathrm{alkali}}{\rightleftharpoons }} \underset{\underset{(\mathrm{yellow} \mathrm{in} \mathrm{alkaline} \mathrm{medium})}{\mathrm{chromate} \mathrm{ion} \mathrm{at} \mathrm{high} \mathrm{pH}}}{2{{\mathrm{CrO}}_4}^{2-}+ 2{\mathrm{H}}^{+}}$

At low pH (acidic medium), K₂Cr₂O₇ solution is oranged coloured while at higher pH (alkaline medium) it changes to yellow due to formation of chromate ions.