Magnesium Arsenates

Crystallised Mg₃(AsO₄)₂.22H₂O (density 1.788) and Mg₃(AsO₄)₂.8H₂O (corresponding to hoernesite, of density 2.609) were prepared analogously to the corresponding phosphates. Prismatic crystals of 2MgHAsO₄.H₂O are obtained by heating a solution of magnesium carbonate in excess of arsenic acid at 225° C. in sealed tubes. The pyroarsenate, Mg₂As₂O₇, results from the ignition of the double arsenate of ammonium and magnesium.

These arsenates are insoluble in water but soluble in acids. Mg(H₂AsO₄)₂ is deliquescent, and soluble in water.

Ammonium Magnesium Arsenate resembles the corresponding phosphate, is formed under analogous conditions, and ignites to the pyroarsenate. The hexahydrate, NH₄MgAsO₄.6H₂O, precipitates under ordinary conditions, and, when heated between 40° C. and 235° C., loses ammonia and water. It is therefore doubtful whether either the monohydrate or NH₄MgAsO₄.½H₂O is produced at 100° C.

One part of NH₄MgAsO₄ dissolves in 2788 parts of water at 15° C., and in 15,786 parts of ammonia solution containing 1 part of 0.96 ammonia to 3 parts of water.

Magnesium can be estimated by precipitation as this double arsenate, which is then ignited or titrated.