6 NH4I-I + (NH4)2Cr2VIO7 + 7 H2SO4 3 I20 + Cr2III(SO4)3 + 4 (NH4)2SO4 + 7 H2O

This is an oxidation-reduction (redox) reaction:

6 I-I - 6 e- 6 I0 (oxidation)

2 CrVI + 6 e- 2 CrIII (reduction)

Change the oxidation states