12 NaI + K2Cr2O7 + 10 H2SO4 → K2SO + 6 Na2SO4 + 6 I2 + Cr2(SO4)3 + 10 H2O
This is an oxidation-reduction (redox) reaction:
SVI + 6 e- → S0 (reduction)
12 I-I - 12 e- → 12 I0 (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
H2SO4 is an oxidizing agent, NaI is a reducing agent, K2Cr2O7 is an oxidizing agent.