Arsenic

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Other names/abbreviations:

As

Element no:	33
Element group:	15 (IUPAC) V A (American labelling) V B (European labelling) Pnictogens
Element type:	Nonmetals
CAS-Number:	7440-38-2
EINECS-Number:	231-148-6

Chemical properties

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) AsO₃^3- (aq) + 3 [Ag(NH₃)₂]⁺ (aq)

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

3 Ag⁺ (aq) + HAsO₄^2- (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) AsO₄^3- (aq) + 3 [Ag(NH₃)₂]⁺ (aq)

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

Mg²⁺ (aq) + AsO₄^3- (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 S^2- (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 S^2- (aq) 2 [AsS₃]^3- (aq)

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

[AsS₃]^3- (aq) + S₂^2- (aq) [AsS₄]^3- (aq) + S^2- (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 S^2- (aq) + 2 HAsO₄^2- (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 S^2- (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.


Solubility
Water: Insoluble (20 °C) [3]
Nitric acid: Soluble (20 °C) [3]

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

Safety

Symbol: T
R-phrases: 23/25
S-phrases: 20/21-28-45

Physical properties

Appearance
	Physical state @ 20°C:	Solid [4]
	Color:	Steel grey [4]. Tin white as mineral [5]. Yellow as As4 [4].
	Luster:	Metallic [5]
	Transparancy:	Opaque [5]
	Odor:	Garlic [4]

Bulk properties
	Molecular weight (g/mol):	74.92160(2) [1]
	Melting point (°C):	816 (38.6 atm, alpha form) [1]
	Boiling point (°C):	615 (Sublimation, alpha form) [1]
	Density (g/cm3):	1.97 (Yellow) [1] 5.778 (Grey/alpha form) [1]
	Molar volume (cm3):	12.95

Thermodynamic properties
	ΔfH°gas (kJ/mol):	302.5 (As) [4] 144 (As4) [6]
	ΔfH°solid (kJ/mol):	0.0 (As) [4]
	ΔH°fusion (kJ/mol):	24.44 [4]
	S°gas (J/K·mol):	174.1 (As) [6] 314 (As4) [6]
	S°solid (J/K·mol):	35.1 (As) [4]
	ΔfG°gas (kJ/mol):	261.0 (As) [4] 92.5 (As4) [6]
	ΔfG°solid (kJ/mol):	0 (As) [6]
	Cp (gas) (J/K·mol):	20.8 (25 °C) [4]
	Cp (solid) (J/K·mol):	24.64 (25 °C) [4]
	Critical temperature (°C):	1400 [4]

Electronic properties
	Electron configuration:	1s2-2s2-2p6-3s2-3p6-3d10-4s2-4p3
	Ionic radius (Å):	As(III): 0.58 [1] As(V): 0.46 [1]

Crystal structure
	Crystal type	Trigonal, rhombohedral [5]
	Lattice constant (Å):	4.13
	Cleavage:	Perfect [5]
	Fracture:	Uneven [5]

Conductivity
	Electrical resisitvity (μΩ·cm):	33.3 (20 °C, alpha form) [1]

History

Discovery
It is believed that Albertus Magnus obtained the element in 1250 A.D. Schroeder published two methods for preparation of the element in 1649 [4].

Origin of name
From Greek arsenikon and latin arsenicum meaning yellow orpiment. Also arsenicos, male as it was believed that metals were different sexes. The name also occurs in the arabic Az-zernikh, the orpiment from Persian zerni-zar, gold [4].

References

1: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

2: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

3: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5

4: CRC Handbook of Chemistry and Physics
75th edition (1994) Edited by Lide,D.R. , CRC Press Inc. Boca Raton, USA

5: Korbel,P., Novák,M. The Complete Encyclopedia of Minerals
1st edition (1999) pp1-296, Chartwell Books, Inc. New Jersey, USA

6: General chemistry
7th edition (1984) Edited by Holtzclaw, Jr.,H.F., Robinson,W.R. & Nebergall,W.H. pp. 1-932. D.C. Heath and Company. Lexington

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© Michael Pilgaard
Created: April 29, 2006
Last update: October 7, 2008