Disodium hydrogen arsenate is the inorganic compound with the formula Na<sub>2</sub>HAsO<sub>4</sub><sup>.</sup>7H<sub>2</sub>O. 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 NaH<sub>2</sub>AsO<sub>4</sub> and Na<sub>3</sub>AsO<sub>4</sub>, 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.
Being a diprotic acid, its acid-base properties is described by two equilibria: