Magnesium Arsenates

Magnesium Orthoarsenate may be obtained in various hydrated forms. When solutions containing equivalent quantities of magnesium sulphate and normal sodium orthoarsenate are mixed, an amorphous precipitate forms which, on prolonged keeping in contact with the solution, becomes crystalline, and on drying has the composition Mg₃(AsO₄)₂.22H₂O. The crystals are monoclinic and isomorphous with the corresponding phosphate. The density at 15° C. is 1.788, and the indices of refraction along the three axes α = 1.548 to 1.563, β = 1.556 to 1.571 and γ = 1.574 to 1.596. The octahydrate, Mg₃(AsO₄)₂.8H₂O, is obtained by mixing aqueous solutions of magnesium sulphate, sodium monohydrogen arsenate and sodium bicarbonate and keeping the amorphous precipitate in contact with the solution for 24 hours, when it becomes crystalline. This hydrate is found in Nature as the mineral haernesite, the crystals of which are monoclinic and probably isomorphous with vivianite. The density of the mineral is 2.474; that of the artificially prepared product is 2.609. The heptahydrate, Mg₃(AsO₄)₂.7H₂O, is obtained if sodium dihydrogen arsenate is substituted for the monohydrogen salt in the above preparation. The decahydrate, Mg₃(AsO₄)₂.10H₂O, is formed when potassium or ammonium magnesium orthoarsenate is digested with water for several hours. On dehydration, hydrates containing 6, 5, 3, 2 and 1H₂O are obtained. The anhydrous salt decomposes when heated above 1100° C. in a vacuum, magnesium and arsenious oxides and oxygen being formed.

A light form of magnesium arsenate may be obtained by heating a slurry of magnesium hydroxide with the theoretical quantity of arsenic acid and 15 per cent, of the equivalent amount of sodium hydroxide in an autoclave at 180° C. until the filtrate contains less than 0.2 per cent, of arsenic pentoxide.

Magnesium Monohydrogen Orthoarsenate, MgHAsO₄.7H₂O, may be prepared by adding the calculated amount of magnesium sulphate to a solution of sodium monohydrogen arsenate neutralised with acetic acid, and keeping the amorphous precipitate in contact with the solution for some days, during which it becomes crystalline. It loses water when heated. This hydrate occurs in Nature as roesslerite, a fibrous mineral consisting of monoclinic crystals with a:b:c = 0.4473:1:0.2598 and β = 94°26'. A hemihydrate, 2MgHAsO₄.H₂O, is obtained in the form of prismatic crystals by heating a solution of magnesium carbonate in excess of arsenic acid at 225° C. in a sealed tube.

Magnesium Dihydrogen Orthoarsenate, Mg(H₂AsO₄)₂, has been described as a very deliquescent crystalline mass obtained by dissolving magnesium oxide in aqueous arsenic acid. It is soluble in water.

Magnesium Pyroarsenate, Mg₂As₂O₇, results from ignition of the monohydrogen arsenate or the double arsenate of magnesium and ammonium. It is a white powder of density 3.75 which, on ignition, melts but does not decompose. When heated above 800° C. in a vacuum it yields the normal orthoarsenate, arsenious oxide and oxygen.

Several double arsenates of magnesium and the alkali metals, ammonium and calcium are known. The most important of these is magnesium ammonium orthoarsenate, MgNH₄AsO₄.6H₂O, which is obtained as a crystalline precipitate when ammonium orthoarsenate is added to a solution of a magnesium salt; it may be dried over sulphuric acid. It is also formed when an alkali arsenate is added to a solution of a magnesium salt in the presence of an ammonium salt and ammonia. When heated above 40° C. it loses ammonia and water and it is doubtful if any lower hydrate is formed, although such are described in the literature. On strong ignition it forms the pyroarsenate, as stated above.

The solubility in water of magnesium ammonium arsenate is as follows (expressed in grams of MgNH₄AsO₄.6H₂O per 100 grams of solution):

Temp.,°C.	0	20	40	50	60	70	80
Solubility	0.03388	0.02066	0.02746	0.02261	0.02103	0.01564	0.02364

It is less soluble in the presence of ammonia, 1 part of the anhydrous salt dissolving in about 15,800 parts of water containing 2.46 per cent. NH₃ (1 part of 0.96 ammonia to 3 parts H₂O).

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

Sodium Magnesium Arsenate, NaMgAsO₄, has been obtained by dissolving magnesia in molten sodium dihydrogen arsenate, the alkali being removed from the residue by washing with very dilute nitric acid. By precipitation from magnesium sulphate with excess of sodium orthoarsenate, the enneahydrate, NaMgAsO₄.9H₂O, is obtained. This loses water at 110° C. to give the octahydrate. In the presence of water the salt yields normal magnesium orthoarsenate. The corresponding potassium salt has been prepared, and also the more complex salts Na₄Mg₄As₆O₂₁, K₄Mg₄As₆O₂₁; Mg₂KH(AsO₄)₂.xH₂O (x = 2, 4, 5 and 15); Mg₂KNa(AsO₄)₂.14H₂O.

Certain magnesium calcium arsenates, sometimes associated with other metals, are found in Nature and are enumerated in the table of minerals.