The residue is
extracted with small quantities of boiling water and filtered off; while
the insoluble magnesia is washed. The filtrate contains the alkalies.
The residue is ignited, and weighed as magnesia. It is MgO.
GRAVIMETRIC DETERMINATION.
The solution containing the magnesia is mixed with chloride of ammonium
and ammonia in excess. If a precipitate should form, more ammonic
chloride is required. Add sodium phosphate solution in excess, stir and
allow to stand overnight. Filter and wash the precipitate with dilute
ammonia. Dry, transfer to a platinum or porcelain crucible, and ignite
(finally at intense redness); cool, and weigh. The substance is magnesic
pyrophosphate (Mg_{2}P_{2}O_{7}), and contains 36.04 per cent. of
magnesia.
VOLUMETRIC METHOD.
The magnesia having been precipitated as ammonic-magnesic phosphate,
which is the usual separation, its weight can be determined
volumetrically by the method of titration described under _Phosphates_.
The same standard solution of uranium acetate is used. Its standard for
magnesia is got by multiplying the standard for phosphoric oxide by
0.5493. For example, if one hundred c.c. are equivalent to 0.5 gram of
phosphoric oxide, they will be equivalent to (0.5 ? .5493) 0.2746 gram
of magnesia. The method of working and the conditions of the titration
are the same as for the phosphate titration.
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