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
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.
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
on acidifying the yellow colour of K2CrO4 again changes to orange red due to
reformation of K2Cr2O7.
2K2CrO4 + H2SO4 ® K2Cr2O7 + K2SO4 + H2O
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
This reaction is used in the detection of chloride ions in qualitative analysis.
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.