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 .