Potassium dichromate: Preparation, Properties and Uses

Potassium dichromate (K2Cr2O7)

Preparation of potassium dichromate from chrome iron ore

The preparation of K2Cr2O7 from its ore involves the following steps

i)     Conversion of chrome iron ore to sodium chromate

The powdered ore is mixed with Na2CO3 and quick lime, then roasted in a reverberatory furnace with free expose to air.

4FeO.Cr2O3 + 8Na2CO3 + 7O2 ® 8Na2CrO4 + 2Fe2O3 + 8CO2­

ii)    Conversion of Na2CrO4 to Na2Cr2O7

Sodium chromate solution so obtained is filtered and treated with con.H2SO4, when sodium chromate is converted to sodium dichromate.

2Na2CrO4 + H2SO4 ® Na2Cr2O7 + Na2SO4 + H2O

iii) Conversion of sodium dichromate into potassium dichromate

Hot concentrated solution of sodium dichromate is treated with KCl, when potassium dichromate, being much less soluble than sodium salt, crystallizes out on cooling.

Na2Cr2O7 + 2KCl         ® K2Cr2O7 + 2NaCl

Properties

Physical properties

1.    It forms orange red crystals which melt at 396 o C.

2.    It is moderately soluble in cold water but freely soluble in hot water.

Chemical properties

1.    Action of Heat

On heating, K2Cr2O7 decomposes to give potassium chromate with evolution of O2 gas.

4K2Cr2O7 ® 4K2CrO4 + 2Cr2O3 + 3 O2

2.    With Alkalies

When an alkali is added to an orange-red solution of K2Cr2O7, a yellow solution results due to formation of potassium chromate.

K2Cr2O7 + 2KOH ® 2K2CrO4 + H2O

Orange-red             Yellow

on acidifying the yellow colour of K2CrO4 again changes to orange red due to

reformation of K2Cr2O7.

2K2CrO4 + H2SO4 ® K2Cr2O7 + K2SO4 + H2O

Orange red

The inter conversion is explained on the basis of the fact that in K2Cr2O7 solution, orange red Cr2O72- ions are in equilibrium with yellow CrO42- ions.

Cr2O72- + H2O 2CrO42- + 2H+

Orange red              Yellow

3.    Oxidising properties

K2Cr2O7 is a powerful oxidising agent. In presence of dil.H2SO4, one mole of this compound gives three atoms of oxygen as shown below.

K2Cr2O7 + 4 dil.H2SO4 ® K2SO4 + Cr2 (SO4)3 + 4H2O + 3(O)

Some other examples of oxidising property of K2Cr2O7 are given below.

It liberates I2 from KI K2Cr

2O7 + 7H2SO4 + 6KI ® 4 K2SO4 + Cr2 (SO4)3 + 3 I2 + 7H2O

It oxidises ferrous to ferric salt

K2Cr2O7 + 7H2SO4 + 6 FeSO4 ®

K2SO4 + Cr2 (SO4)3 + 3 Fe2 (SO4)3+ 2H2O

Iii )It oxidises H2S to sulphur

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

4.    Chromyl chloride test

When salt containing chloride is treated with K2Cr2O7 and con. H2SO4 reddish brown vapours of chromyl chloride are obtained.

K2Cr2O7 + 4KCl + 6H2SO4 ® 2CrO2Cl2 + 6 KHSO4 + 3H2O

Chromylchloride

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

Uses

i)     It is used in volumetric analysis

ii)    In chrome tanning in leather industry

iii)    In calico printing and dyeing

iv)       In photography and in hardening gelatin film.