3 As2IIIO3 + 4 HNVO3 + 7 H2O 6 H3AsVO4 + 4 NIIO

This is an oxidation-reduction (redox) reaction:

6 AsIII - 12 e- 6 AsV (oxidation)

4 NV + 12 e- 4 NII (reduction)

Change the oxidation states