8 (H2O)+ + 7 (SO3)2- + (Cr2O7)2- → 2 (Cr)3+ + 7 (SO4)2- + 8 H2O
This is an oxidation-reduction (redox) reaction:
7 SIV - 14 e- → 7 SVI (oxidation)
8 O-I + 8 e- → 8 O-II (reduction)
2 CrVI + 6 e- → 2 CrIII (reduction)
SO32- is a reducing agent, H2O+ is an oxidizing agent, Cr2O72- is an oxidizing agent.