phosphate is not a wholly insoluble
substance, even under the most favorable analytical conditions. It
is least soluble in a liquid containing one fourth of its volume of
concentrated aqueous ammonia (sp. gr. 0.90) and this proportion should
be carefully maintained as prescribed in the procedure. On account of
this slight solubility the volume of solutions should be kept as small
as possible and the amount of wash-water limited to that absolutely
required.

A large excess of the magnesium solution tends both to throw out
magnesium hydroxide (shown by a persistently flocculent precipitate)
and to cause the phosphate to carry down molybdic acid. The tendency
of the magnesium precipitate to carry down molybdic acid is also
increased if the solution is too concentrated. The volume should not
be less than 90 cc., nor more than 125 cc., at the time of the first
precipitation with the magnesia mixture.]

[Note 2: The magnesium ammonium phosphate should be perfectly
crystalline, and will be so if the directions are followed. The slow
addition of the reagent is essential, and the stirring not less so.
Stirring promotes the separation of the precipitate and the formation
of larger crystals, and may therefore be substituted for digestion in
the cold. The stirring-rod must not be allowed to scratch the glass,
as the crystals adhere to such scratches and are removed with
difficulty.]

REPRECIPITATION AND IGNITION OF MAGNESIUM AMMONIUM PHOSPHATE

A single precipitation of the magnesium compound in the presence of
molybdenum compounds rarely yields a pure product. The molybdenum can
be removed by solution of the precipitate in acid and precipitation
of the molybdenum by sulphureted hydrogen, after which the magnesium
precipitate may be again thrown down. It is usually more satisfactory
to dissolve the magnesium precipitate and reprecipitate the phosphate
as magnesium ammonium phosphate as described below.

PROCEDURE.--Dissolve the precipitate from the filter in a little
dilute hydrochloric acid (sp. gr. 1.12), allowing the acid solution to
run into the beaker in which the original precipitation was made (Note
1). Wash the filter with water until the wash-water shows no test for
chlorides, but avoid an unnecessary amount of wash-water. Add to
the solution 2 cc. (not more) of magnesia mixture, and then dilute
ammonium hydroxide solution (sp. gr. 0.96), drop by drop, with
constant stirring, until the liquid smells distinctly of