Potassium dichromate
is used as a primary standard in volumetric analysis In acidic solution,
its oxidising action can be represented as follows:
Cr2O7
2– + 14H
+ + 6e
– ® 2Cr
3+ + 7H2O (E
o = 1 33V)
Thus, acidified potassium dichromate will oxidise iodides to iodine,
sulphides to sulphur, tin(II) to tin(IV) and iron(II) salts to iron(III) The
half-reactions are noted below:
6 I
– ® 3I2 + 6 e
– ;
3 Sn
2+ ® 3Sn
4+ + 6 e
–
3 H2S ® 6H
+ + 3S + 6e
– ;
6 Fe
2+ ® 6Fe
3+ + 6 e
–
The full ionic equation may be obtained by adding the half-reaction for
potassium dichromate to the half-reaction for the reducing agent, for e