page_title: Arsenous acid
section_title: Properties
hierarchical_section_title: Arsenous acid / Properties
context_page_description: Arsenous acid is the inorganic compound with the formula H₃AsO₃. It is known to occur in aqueous solutions, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)₃.
context_section_description: As(OH)₃ is a pyramidal molecule consisting of three hydroxyl groups bonded to arsenic. The ¹H NMR spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorous acid H₃PO₃ adopts the structure HPO(OH)₂.  The structural analogue of arsenous acid (P(OH)₃) is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.
  One tautomer of arsenous acid is HAsO(OH)₂, which is called arsonic acid.  It has not been isolated or well-characterized.
caption: Phosphorous acid exists in as the dihydroxy tautomer in contrast to arsenous acid.