Magnesia differs from the other alkaline earths by the solubility of its
sulphate in water.
Magnesia is dissolved by boiling with moderately strong acids; the
insoluble compounds are fused with "fusion mixture," and treated as
described under _Silicates_.
~Separation.~--It is separated by evaporating the acid solution to
dryness to render silica insoluble, and by taking up with dilute
hydrochloric acid. The solution is freed from the second group of metals
by means of sulphuretted hydrogen, and the iron, alumina, &c., are
removed with ammonic chloride, ammonia, and ammonic sulphide. The
somewhat diluted filtrate is treated, first, with ammonia, and then with
carbonate of ammonia in slight excess. It is allowed to stand for an
hour in a warm place, and then filtered. The magnesia is precipitated
from the filtrate by the addition of an excess of sodium phosphate and
ammonia. It is allowed to stand overnight, filtered, and washed with
dilute ammonia. The precipitate contains the magnesia as
ammonic-magnesic phosphate.
In cases where it is not desirable to introduce sodium salts or
phosphoric acid into the assay solution, the following method is
adopted. The solution (freed from the other alkaline earths by ammonium
carbonate) is evaporated in a small porcelain dish with nitric acid. The
residue (after removing the ammonic salts by ignition) is taken up with
a little water and a few crystals of oxalic acid, transferred to a
platinum dish, evaporated to dryness, and ignited.
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