Basic magnesium arsenite cannot be prepared by
reacting the oxides together either in a solid-state reaction
in an inert atmosphere or in an aqueous reaction. An
investigation of the system MgO–As2O3–H2O at 25°C
gave no evidence of chemical combination, but considerable
adsorption of arsenious oxide by the magnesium
hydroxide occurred. The key to forming this compound
lies in controlling the pH around 7.5–8.0. Combination
of the two oxides occurs at higher solution temperatures
where some of the MgO dissolved to form the hydroxide
and some basic metarsenite was formed.
However, the composition was mostly a basic arsenite
of composition, MgHAsO3·3H2O.
Crystals are formed when the filtered solution is
evaporated. At low temperature, i.e. <15°C, the
18-hydrate is obtained. At higher temperatures, the
13-hydrate and the 3-hydrate are also known, each
a product of the crystallization temperature used. Like
most of the magnesium arsenites, this salt has found
little usage in industry but is offered for sale by a small
number of manufacturers and suppliers, notably
Chinese. It has no CAS number. There are few to no
scientific papers which deal with this compound. Its
physical properties like melting point have not been
documented. This salt was described in the early work
in the 1800s concerning inorganic salts. Since that time,
the orthoarsenite of magnesium has received little attention
for industry or academia.
