Qualitative Analysis of Arsenic Ion

On passing H₂S in the solution of salt, yellow precipitate is obtained. It is soluble in yellow ammonium sulphide to form ammonium thioarsenate.

As₂O₃ + 3(NH₄)₂S_x → 2(NH₄)₂AsS₄

Confirmatory Test

On addition of dil HCl in the solution, yellow precipitate is formed.

(NH₄)₂AsS₄ + 6HCl → As₂S₃ (Yellow ppt) + 6NH₄Cl + H₂S

Dissolve this precipitate in conc HNO₃ and then add ammonium molyblate, canary yellow precipitate indicates the presence of As³⁺ ion.

As₂O₃ + 12HNO₃ → 2H₃AsO₄

H₃AsO₄ + 12(NH₄)₂MoO₄ + 21HNO₃ → (NH₄)AsS₄.12MoO₃ + 2NH₄NO₃ + 12H₂O