2 As-IIIH3 + 3 Fe2III(SO4)3 2 As0 + 6 FeIISO4 + 3 H2SO4

This is an oxidation-reduction (redox) reaction:

2 As-III - 6 e- 2 As0 (oxidation)

6 FeIII + 6 e- 6 FeII (reduction)

Change the oxidation states