8 HClO4 + 22 HI + K2Cr2O7 → 2 Cr(ClO4)3 + 11 I2 + 2 KCl + 15 H2O
This is an oxidation-reduction (redox) reaction:
2 ClVII + 16 e- → 2 Cl-I (reduction)
22 I-I - 22 e- → 22 I0 (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
HClO4 is an oxidizing agent, HI is a reducing agent, K2Cr2O7 is an oxidizing agent.