K2Cr2O7O7 + 20 NaI + 14 H2SO4 → 10 Na2SO4 + 10 I2 + Cr2(SO4)3 + K2SO4 + 14 H2O
This is an oxidation-reduction (redox) reaction:
20 I-I - 20 e- → 20 I0 (oxidation)
2 CrXIII + 20 e- → 2 CrIII (reduction)
NaI is a reducing agent, K2Cr2O7O7 is an oxidizing agent.