f the "yellow precipitate" undoubtedly
varies slightly with varying conditions at the time of its formation.
Its composition may probably fairly be represented by the formula,
(NH_{4})_{3}PO_{4}.12MoO_{3}.H_{2}O, when precipitated under the
conditions prescribed in the procedure. Whatever other variations may
occur in its composition, the ratio of 12 MoO_{3}:1 P seems to
hold, and this fact is utilized in volumetric processes for the
determination of phosphorus, in which the molybdenum is reduced to
a lower oxide and reoxidized by a standard solution of potassium
permanganate. In principle, the procedure is comparable with that
described for the determination of iron by permanganate.]

PRECIPITATION OF MAGNESIUM AMMONIUM PHOSPHATE

PROCEDURE.--Dissolve the precipitate of phosphomolybdate upon the
filter by pouring through it dilute aqueous ammonia (one volume of
dilute ammonia (sp. gr. 0.96) and three volumes of water, which
should be carefully measured), and receive the solution in the beaker
containing the bulk of the precipitate. The total volume of nitrate
and washings should not much exceed 100 cc. Acidify the solution with
dilute hydrochloric acid, and heat it nearly to boiling. Calculate the
volume of magnesium ammonium chloride solution ("magnesia mixture")
required to precipitate the phosphoric acid, assuming 40 per cent
P_{2}O_{5} in the apatite. Measure out about 5 cc. in excess of this
amount, and pour it into the acid solution. Then add slowly dilute
ammonium hydroxide (1 volume of strong ammonia (sp. gr. 0.90) and 9
volumes of water), stirring constantly until a precipitate forms. Then
add a volume of filtered, concentrated ammonia (sp. gr. 0.90) equal to
one third of the volume of liquid in the beaker (Note 1). Allow the
whole to cool. The precipitated magnesium ammonium phosphate should
then be definitely crystalline in appearance (Note 2). (If it is
desired to hasten the precipitation, the solution may be cooled, first
in cold and then in ice-water, and stirred !constantly! for half an
hour, when precipitation will usually be complete.)

Decant the clear liquid through a filter, and transfer the precipitate
to the filter, using as wash-water a mixture of one volume of
concentrated ammonia and three volumes of water. It is not necessary
to clean the beaker completely or to wash the precipitate thoroughly
at this point, as it is necessary to purify it by reprecipitation.

[Note 1: Magnesium ammonium