9 KNO3 + Cr4(SO4)3 + 7 Na2CO3 → 9 KNO2 + 4 Na2CrO4 + 3 Na2SO4 + 7 CO2
This is an oxidation-reduction (redox) reaction:
3 SIV - 6 e- → 3 SVI (oxidation)
9 NV + 18 e- → 9 NIII (reduction)
4 CrIII - 12 e- → 4 CrVI (oxidation)
Cr4(SO4)3 is a reducing agent, KNO3 is an oxidizing agent, Cr4(SO4)3 is a reducing agent.