9. Preparation of Potassium dichromate( K 2 Cr 2 O 7): It is prepared by fusion of chromate ore( FeCr 2 O 4) with sodium carbonate in excess of air.
10. Effect of pH on chromate and dichromate ions:
The chromates and dichromate are interconvertible in aqueous solution depending upon pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.
2 CrO 4
2-
+ 2H + Cr 2 O 7
2-
+ H 2 O
Cr 2 O 7
2-
+ 2 OH-
2 CrO 4
2-
+ H 2 O
11. Potassium dichromate acts as a strong oxidizing agent in acidic medium.
Cr 2 O 7
2-
+ 14 H + + 6e-
2 Cr 3 + + 7 H 2 O
12. Preparation of Potassium permanganate( KMnO 4):
a. Potassium permanganate is prepared by fusion of MnO 4 with alkali metal
hydroxide( KOH) in presence of O 2 or oxidising agent like KNO 3.
It produces
dark green K 2 MnO 4 which undergoes oxidation as well as reduction in neutral
or acidic solution to give permanganate.