2 K2Cr2O7 + 18 HI + 10 HClO4 → 14 H2O + 9 I2 + 4 KClO4 + 2 Cr2(ClO4)3
This is an oxidation-reduction (redox) reaction:
18 I-I - 18 e- → 18 I0 (oxidation)
6 ClVII + 48 e- → 6 Cl-I (reduction)
4 CrVI - 30 e- → 4 Cr135/10 (oxidation)
HI is a reducing agent, HClO4 is an oxidizing agent, K2Cr2O7 is a reducing agent.