6 K4[Fe(CN)6] + K2Cr2O7 + 7 H2SO4 → 6 K3[Fe(CN)6] + Cr2(SO4)3 + 4 K2SO4 + 7 H2O
This is an oxidation-reduction (redox) reaction:
6 FeII - 6 e- → 6 FeIII (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
K4[Fe(CN)6] is a reducing agent, K2Cr2O7 is an oxidizing agent.