3 Na2S2IIO3 + 4 K2Cr2VIO7 + 13 H2SVIO4 3 Na2SVIO4 + 4 Cr2III(SVIO4)3 + 4 K2SVIO4 + 13 H2O

This is an oxidation-reduction (redox) reaction:

6 SII - 24 e- 6 SVI (oxidation)

8 CrVI + 24 e- 8 CrIII (reduction)

Change the oxidation states