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]

Quick Facts Names, Identifiers ...

Disodium hydrogen arsenate

Names
IUPAC name Disodium hydrogen arsorate
Other names Sodium arsenate dibasic
Identifiers
CAS Number	7778-43-0 Y
ChEBI	CHEBI:91256 Y
ECHA InfoCard	100.029.002
EC Number	231-902-4
PubChem CID	24500
UNII	ANU034TUJ4 Y
CompTox Dashboard (EPA)	DTXSID8032047
Properties
Chemical formula	H15Na2AsO11 (heptahydrate)
Molar mass	312.01 g/mol (heptahydrate)
Appearance	white solid
Solubility in water	good
Hazards
Occupational safety and health (OHS/OSH):
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

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Being a diprotic 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)

• Monopotassium arsenate, KH₂AsO₄