c. for each 0.1 gram of phosphoric oxide.
Allow to stand for one hour. The white crystalline precipitate contains
the phosphorus as ammonium-magnesium phosphate.
Phosphate of lead is decomposed by sulphuric acid; the lead is converted
into the insoluble lead sulphate, and the phosphoric acid is dissolved.
Phosphate of copper and phosphate of iron may be treated with
sulphuretted hydrogen; the former in an acid, and the latter in an
alkaline, solution. Phosphate of alumina is generally weighed without
separation of the alumina, since this requires a fusion. In all cases
the aim is to get the phosphoric oxide either free, or combined with
some metal whose phosphate is soluble in ammonia.
Joulie's method of separation is as follows:--One to ten grams of the
sample are treated with hydrochloric acid, and evaporated to dryness
with the addition (if any pyrites is present) of a little nitric acid.
The residue is taken up with hydrochloric acid, cooled, transferred to a
graduated flask, and diluted to the mark. It is then shaken up, filtered
through a dry filter, and a measured portion (containing about 0.05 gram
of phosphoric acid) transferred to a small beaker. Ten c.c. of a
citric-acid solution of magnesia[110] is added, and then an excess of
ammonia. If an immediate precipitate is formed, a fresh portion must be
measured out and treated with 20 c.
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