:Disodium hydrogen arsenate

{{Chembox

| ImageFile = Na2HAsO4.svg

| ImageSize = 132px

| ImageAlt =

| IUPACName = Disodium hydrogen arsorate

| OtherNames = Sodium arsenate dibasic

|Section1={{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 7778-43-0

| ChemSpiderID = 22908

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = ANU034TUJ4

| EINECS = 231-902-4

| PubChem = 24500

| ChEBI_Ref = {{ebicite|correct|EBI}}

| ChEBI = 91256

| UNNumber = 1685

| StdInChI=1S/AsH3O4.2Na/c2-1(3,4)5;;/h(H3,2,3,4,5);;/q;2*+1/p-2

| StdInChIKey = IHZDYHDJAVUIBH-UHFFFAOYSA-L

| SMILES = O[As](=O)([O-])[O-].[Na+].[Na+]

}}

|Section2={{Chembox Properties

| Formula = H₁₅Na₂AsO₁₁ (heptahydrate)

| MolarMass = 312.01 g/mol (heptahydrate)

| Appearance = white solid

| Density =

| MeltingPt =

| BoilingPt =

| Solubility = good}}

|Section3={{Chembox Hazards

| GHSPictograms = {{GHS06}}{{GHS08}}{{GHS09}}

| GHSSignalWord = Danger

| HPhrases = {{H-phrases|301|331|350|410}}

| PPhrases = {{P-phrases|201|202|261|264|270|271|273|281|301+310|304+340|308+313|311|321|330|391|403+233|405|501}}

| MainHazards = poison

| FlashPt =

| AutoignitionPt = }}

}}

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.{{Ullmann | author = Grund, S. C. | author2 = Hanusch, K. | author3 = Wolf, H. U. | title = Arsenic and Arsenic Compounds | doi = 10.1002/14356007.a03_113.pub2 }}

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

:{{chem|H|2|AsO|4|-}} + H₂O {{eqm}} {{chem|HAsO|4|2-}} + H₃O⁺ {{pad|2em}}(pK_a2 = 6.94)

:{{chem|HAsO|4|2-}} + H₂O {{eqm}} {{chem|AsO|4|3-}} + H₃O⁺ {{pad|2em}}(pK_a3 = 11.5)