Arsenic acid

Structural formula
General
Surname	Arsenic acid
other names	Arsenic (V) acid
Molecular formula	H 3 AsO 4
Brief description	transparent to white, odorless solid
External identifiers / databases
CAS number 7778-39-4 EC number 231-901-9 ECHA InfoCard 100.029.001 PubChem 234 Wikidata Q413334
properties
Molar mass	141.94 g mol −1
Physical state	firmly
density	2.50 g cm −3
Melting point	35 ° C
boiling point	120 ° C
Vapor pressure	55 h Pa (50 ° C)
pK s value	p K s 1 : 02.26 (25 ° C) p K s 2 : 06.76 (25 ° C) p K s 3 : 11.29 (25 ° C)
solubility	easily soluble in water
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary danger H and P phrases H: 301-331-350-410 P: 202-273-280-285-304 + 340-312
Authorization procedure under REACH	of particular concern : carcinogenic ( CMR ); subject to approval
MAK	DFG : not specified because it is carcinogenic Switzerland: 0.1 mg m −3 (based on arsenic, measured as inhalable dust )
Toxicological data	48 mg kg −1 ( LD 50 ,  rat ,  oral )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Arsenic acid (arsenic (V) acid called) is of Diarsenic pentaoxide derived acid with the molecular formula H ₃ AsO ₄ . It is a three-proton, medium-strength acid and about as strong as phosphoric acid . The salts of arsenic acid are called arsenates .

Extraction and presentation

Arsenic acid is formed by dissolving arsenic pentoxide in water :

${\displaystyle \mathrm {As_{2}O_{5}+3\ H_{2}O\rightleftharpoons 2\ H_{3}AsO_{4}} }$

The equilibrium of this chemical reaction can be shifted to the left. By removing water, for example with phosphorus pentoxide , arsenic acid is converted back to arsenic pentoxide.

Another reaction for the preparation of arsenic acid is the oxidation of arsenic or arsenic (III) oxide with the help of concentrated nitric acid . The hemihydrate is formed:

${\displaystyle \mathrm {As+5\ HNO_{3}\longrightarrow 5\ NO_{2}+H_{2}O+H_{3}AsO_{4}} }$

${\displaystyle \mathrm {As_{2}O_{3}+2\ HNO_{3}+3\ H_{2}O\longrightarrow } }$ ${\displaystyle \mathrm {N_{2}O_{3}+2\ H_{3}AsO_{4}\cdot {\tfrac {1}{2}}H_{2}O} }$

The dihydrate (H ₃ AsO ₄ · 2H ₂ O) is only accessible via crystallization for several days at −30 ° C.

properties

Solid arsenic acid removes water very quickly from the air and forms hydrates : H ₃ AsO ₄ · ½H ₂ O or H ₃ AsO ₄ · 2H ₂ O at −30 ° C. When heated to 100 ° C, As ₂ O ₅ · 1.66H ₂ O is formed, above 300 ° C all water is released.

A saturated aqueous solution is approximately 80%.

Individual evidence

1. ↑ ^{a b c d e f g h i j} Entry for CAS no. 7778-39-4 in the GESTIS substance database of the IFA , accessed on February 8, 2013(JavaScript required) .
2. ↑ ^{a b c} David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Dissociation Constants of Inorganic Acids and Bases, pp. 8-40.
3. ↑ Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the specified labeling it falls under the group entry arsenic acid and its salts, unless specifically listed in this appendix in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA) , accessed on January 9, 2017. Manufacturers or distributors can expand the harmonized classification and labeling .
4. ↑ Entry in the SVHC list of the European Chemicals Agency , accessed on July 17, 2014.
5. ↑ Entry in the register of substances subject to authorization of the European Chemicals Agency , accessed on July 17, 2014.
6. ↑ Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 7778-39-4 or arsenic acid ), accessed on October 24, 2016.
7. ↑ G. Brauer (Ed.), Handbook of Preparative Inorganic Chemistry 2nd ed., Vol. 1, Academic Press 1963, p. 601.