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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Potassium dichromate: Preparation, Properties and 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.

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.


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