HAsO32

HAsO3^2−, commonly referred to as hydrogen arsenite, is an oxoanion of arsenic in the +3 oxidation state. It represents the doubly deprotonated form of arsenous acid (H3AsO3) and is part of the arsenite/arsenous acid equilibria in aqueous solution. In practice, its presence is pH-dependent, with HAsO3^2− becoming more prevalent as solutions become more basic.

Chemically, HAsO3^2− consists of an arsenic center bonded to three oxygens, with one oxygen bearing a hydrogen in the parent acid form. The geometry around arsenic in arsenite species is typically described as a pyramidal arrangement due to the lone pair on the arsenic atom. The dianion carries a −2 charge, and in solution it can form salts with alkali metals, for example Na2HAsO3 or K2HAsO3, depending on stoichiometry and pH.

Occurrence and preparation are largely linked to arsenic-containing systems and environmental chemistry. HAsO3^2− forms under alkaline conditions from deprotonation of arsenous acid species in water. It can also be generated or stabilized in analytical or synthetic contexts where arsenites are treated with bases. In natural waters, the speciation between HAsO3^2− and related arsenite forms is influenced by redox state, pH, and competing ions.

Safety and handling are important due to the toxic and potentially carcinogenic nature of arsenic compounds. Arsenite species, including hydrogen arsenite, can be toxic to humans and ecological systems through exposure routes such as ingestion and inhalation. Appropriate safety measures and regulatory guidelines are essential when working with arsenic-containing species.