Disodium hydrogen arsenate

Disodium hydrogen arsenate
Names
	Other names Sodium arsenate dibasic
Identifiers
CAS Number	7778-43-0 ;
ChEBI	CHEBI:91256 ;
ECHA InfoCard	100.029.002
EC Number	231-902-4;
PubChem CID	24500;
CompTox Dashboard (EPA)	DTXSID8032047 ;
Properties
Chemical formula	H15Na2AsO11 (heptahydrate)
Molar mass	312.01 g/mol (heptahydrate)
Appearance	white solid
Solubility in water	good
Hazards
Main hazards	poison
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H331, H350, H410
Precautionary statements	P201, P202, P261, P264, P270, P271, P273, P281, P301+P310, P304+P340, P308+P313, P311, P321, P330, P391, P403+P233, P405, P501
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
	Infobox references

Disodium hydrogen arsenate is the inorganic compound with the formula Na₂HAsO₄^.7H₂O. The compound consists of a salt and seven molecules of water of crystallization although for simplicity the formula usually omits the water component. The other sodium arsenates are NaH₂AsO₄ and Na₃AsO₄, the latter being called sodium arsenate. Disodium hydrogen arsenate is highly toxic. The salt is the conjugate base of arsenic acid. It is a white, water-soluble solid.^[1]

Being a monoprotic acid, its acid-base properties is described by two equilibria:

H
₂AsO⁻
₄ + H₂O ⇌ HAsO²⁻
₄ + H₃O⁺ (pK_a2 = 6.94)

HAsO²⁻
₄ + H₂O ⇌ AsO³⁻
₄ + H₃O⁺ (pK_a3 = 11.5)

Related compounds[]

Monopotassium arsenate, KH₂AsO₄

References[]

^ Grund, S. C.; Hanusch, K.; Wolf, H. U. "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_113.pub2.

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[Ullmann-1] Grund, S. C.; Hanusch, K.; Wolf, H. U. "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_113.pub2.

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