Acid-base reactions Arsenic trioxide is an
amphoteric oxide, and its aqueous solutions are weakly acidic. Thus, it dissolves readily in alkaline solutions to give
arsenites: : Arsenic trioxide is less soluble in acids, although it will dissolve in
hydrochloric acid. When treated with anhydrous HF and HCl, arsenic trioxide converts to the corresponding trihalide. The tribromide and triiodide are made using concentrated
hydrobromic acid and
hydroiodic acid, respectively: : (X = F, Cl, Br, I)
Redox reactions Only with strong oxidizing agents such as ozone, hydrogen peroxide, and nitric acid does it yield
arsenic pentoxide, or its corresponding acid:
Precursor to organoarsenic compounds Arsenic trioxide has played a special role as entry to
organoarsenic chemistry. In the 18th century it was found that combining arsenic trioxide and four equivalents of
potassium acetate () gives a product called "
Cadet's fuming liquid", which is often considered the first
organometallic compound. Cadet's fuming liquid is a derivative of
cacodylic acid, and
cacodyl, . Arsenic trioxide reacts with
phenyl magnesium bromide as described by the following idealized equation: It reacts with aqueous copper(II) acetate to give , known as
Paris green.
Structure In the gas phase below , arsenic trioxide has the formula and is isostructural with . Above dissociation into molecular , with the same structure as , becomes significant. Three crystalline forms (
polymorphs) are known: a high temperature (over ) cubic form, containing molecular , and two related polymeric forms. The polymers, which both crystallize as monoclinic crystals, feature sheets of pyramidal units that share O atoms. One of the polymeric forms (presumably I, as II was not known at the time) is apparently the most stable form. The liquid state is agreed to be polymeric, and can form a glass; the liquid and glass have bonding of the same general type as the polymeric crystalline forms. ==Safety==