Chemical reactions

Reaction of arsenic with air

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)

Reaction of arsenic with other metals/metal ions

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]

Reaction of arsenic with sulfide

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)

Reaction of hydrogen with the water

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

Qualitative analysis

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