c. of the citrate of magnesia
solution and with ammonia as before. The beaker is put aside for from
two to twelve hours. The precipitate is then filtered off and washed
with weak ammonia; it contains the phosphorus as ammonium-magnesium
phosphate.
GRAVIMETRIC DETERMINATION.
If the phosphate is not already in the form of ammonic-magnesic
phosphate, it is converted into this by the addition to its solution of
an excess of ammonia and "magnesia mixture." In order to get the
precipitate pure, the "magnesia mixture" is run in gradually (by drops)
from a burette, with constant stirring. A white crystalline precipitate
at once falls, if much phosphorus is present; but, if there is only a
small quantity, it may be an hour or two before it shows itself. The
solution is best allowed to rest for twelve or fifteen hours (overnight)
before filtering. The presence of tartaric acid should be avoided; and
the appearance of the precipitate should be crystalline. The solution is
decanted through a filter, and the precipitate washed with dilute
ammonia, using as little as may be necessary. The precipitate is dried,
transferred to a weighed Berlin or platinum crucible; the filter-paper
is carefully burnt, and its ash added to the precipitate, which is then
ignited, at first gently over a Bunsen burner, and then more strongly
over the blowpipe or in the muffle.
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