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Disodium hydrogen arsenate
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<p>Disodium hydrogen arsenate is the <a href="/facts/Inorganic_compound/TfFfzxSz">inorganic compound</a> with the <a href="/facts/Chemical_formula/hghUtthl">formula</a> Na2HAsO4.7H2O. The compound consists of a salt and seven molecules of <a href="/facts/Water_of_crystallization/sbaKJNJX">water of crystallization</a> although for simplicity the formula usually omits the water component. The other sodium arsenates are NaH2AsO4 and Na3AsO4, the latter being called <a href="/facts/Sodium_arsenate/swF6S8aP">sodium arsenate</a>. Disodium hydrogen arsenate is highly toxic. The salt is the conjugate base of <a href="/facts/Arsenic_acid/NURfJaGw">arsenic acid</a>. It is a white, water-soluble solid. </p><p>Being a <a href="/facts/Diprotic/FySU18n1">diprotic</a> acid, its acid-base properties is described by two equilibria: </p> H2AsO−4 + H2O ⇌ HAsO2−4 + H3O+ (p<i>K</i>a2 = 6.94) HAsO2−4 + H2O ⇌ AsO3−4 + H3O+ (p<i>K</i>a3 = 11.5)