Potassium dichromate, in acidic medium acts as a strong oxidising agent by gaining 6e’ and forms Cr3+.
Cr2O72- + 14H+ + 6e– → 2Cr3+ + 7H2O ..(1)
(i) iodides
6I– → 3I2 + 6e– ..(2)
(1) + (2)
Cr2O72- + 14 H+ + 6I– → 2Cr3+ + 3I2 + 7H2O
(ii) iron (II) solution
6Fe2+ → 6Fe3+ + 6e– ..(3)
(1) + (3)
Cr2O72- +14H++ 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O
(iii) H2S
3H2S → 6H+ + 3S + 6e– ..(4)
(1) + (4)
Cr2O72- +14H+ + 3H2S → 6H+ + 3S + 2Cr3+ + 7H2O