3 PbO2 + 16 NaOH + Cr2(SO4)3 → 3 Na2PbO2 + 3 Na2SO4 + 2 Na2CrO4 + 8 H2O
This is an oxidation-reduction (redox) reaction:
3 PbIV + 6 e- → 3 PbII (reduction)
2 CrIII - 6 e- → 2 CrVI (oxidation)
PbO2 is an oxidizing agent, Cr2(SO4)3 is a reducing agent.