Magnesium Arsenites

Description

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.