2 CrVIO42- + 4 H+ + 3 SIVO2 2 (CrIII)3+ + 3 SVIO42- + 2 H2O

This is an oxidation-reduction (redox) reaction:

3 SIV - 6 e- 3 SVI (oxidation)

2 CrVI + 6 e- 2 CrIII (reduction)

SO2 is a reducing agent, CrO42- is an oxidizing agent.