The reaction you provided is a combination of an oxidation-reduction (redox) reaction and an acid-base reaction. The oxidation-reduction component of the reaction is the conversion of As(III) to As(V) and the conversion of NO3- (nitrate) to NO (nitric oxide). The acid-base component of the reaction is the conversion of H+ (hydrogen ions) to H2O (water).
Here is the step-by-step oxidation-reduction reaction:
As(III) (arsenite) is oxidized to As(V) (arsenate) by losing electrons:
As(III) → As(V) + 3e-
NO3- (nitrate) is reduced to NO (nitric oxide) by gaining electrons:
NO3- + 4H+ + 4e- → NO + 2H2O
The electrons lost by As(III) are gained by NO3-:
As(III) + NO3- → As(V) + NO + 2H2O
The acid-base component of the reaction is the conversion of H+ (hydrogen ions) to H2O (water) by reacting with the water generated by the oxidation-reduction reaction:
2H+ + 2H2O → 4H+
The overall balanced equation of this redox reaction is:
As₂O3 + H+ + NO3- → H₂ASO4 + NO
Please note that As₂O₃ is Arsenic trioxide and H₂ASO₄ is Arsenic Acid and NO is Nitric oxide.