Arsenic

Arsenic does not react with air at room temperature without the presense of moisture. With moisture, arsenic reacts slowly forming arsenic(V) oxide. When heated, arsenic ignites, also forming arsenic(V) oxide.

4 As(s) + 5 O₂(g) 2 As₂O₅(s)

When heated in a pure oxygen atmosphere, arsenic(III) oxide is formed instead

4 As(s) + 3 O₂(g) As₄O₆(s)

As(III) as arsenate is precipitated by silver ions under neutral and alkaline conditions.

3 Ag⁺(aq) + [As(OH)₂O]⁻(aq) Ag₃AsO₃(s) [yellow] + 2 H⁺(aq)

The precipitate is easily dissolved in acetic acid and ammonia

Ag₃AsO₃(s) + 3 H⁺(aq) As(OH)₃(aq) + 3 Ag⁺(aq)
Ag₃AsO₃(s) + 6 NH₃(aq) AsO₃³⁻(aq) + 3 [Ag(NH₃)₂]⁺(aq)

As(V) is precipitated by silver ions in neutral and alkaline solution

3 Ag⁺(aq) + HAsO₄²⁻(aq) Ag₃AsO₄(s) [reddish brown] + H⁺(aq)

The precipitate is easily dissolved in nitric acid and ammonia, but not in acetic acid

Ag₃AsO₄(s) + 3 H⁺(aq) H₃AsO₄(aq) + 3 Ag⁺(aq)
Ag₃AsO₄(s) + 6 NH₃(aq) AsO₄³⁻(aq) + 3 [Ag(NH₃)₂]⁺(aq)

As(V) is precipitated by magnesium ions in the presence of ammonia

Mg²⁺(aq) + AsO₄³⁻(aq) + NH₄⁺(aq) MgNH₄AsO₄(s) [white]

As(III) is precipitated by hydrogen sulfide in hydrochloric acid.

2 As³⁺(aq) + 3 S²⁻(aq) As₂S₃(s) [yellow]

The precipitate cannot be dissolved in strong hydrochloric acid, but can be dissolved in NaOH, (NH₄)₂CO₃ and Na₂S

As₂S₃(s) + 6 OH⁻(aq) [AsO₃]³⁻(aq) + [AsS₃]³⁻(aq) + H₂O(l)
As₂S₃(s) + 3 S²⁻(aq) 2 [AsS₃]³⁻(aq)

Thioarsenate(III) ions are oxidized by disulfide ions to thioarsenate(V) ions

[AsS₃]³⁻(aq) + S₂²⁻(aq) [AsS₄]³⁻(aq) + S²⁻(aq)

As(V) is precipitated by sulfide in either strong hydrochloric acid (6-12 M) or 0.4 M hydrochloric acid, heat, and pressure

14 H⁺(aq) + 5 S²⁻(aq) + 2 HAsO₄²⁻(aq) As₂S₅(s) [yellow] + 8 H₂O(l)

The precipitate is insoluble in boiling concentrated hydrochloric acid, but soluble in hot nitric acid and bases like (NH₄)₂CO₃ and Na₂S

4 As₂S₅(s) + 24 OH⁻(aq) 3 [AsO₄]³⁻(aq) + 5 [AsS₄]³⁻(aq) + 12 H₂O(l)
As₂S₅(s) + 3 S²⁻(aq) 2 [AsS₄]³⁻(aq)

Arsenic does not react with water in the absence of air, under normal conditions.

Method 3500-As D Inductively Coupled Plasma Method [2]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.005 mg/L
Method limit of detection in soil = 2.00 mg/kg