[Note 5:  The precipitation of the phosphomolybdate takes place more promptly in warm than in cold solutions, but the temperature should not exceed 60 deg.C. during precipitation; a higher temperature tends to separate molybdic acid from the solution.  This acid is nearly white, and its deposition in the filtrate on long standing should not be mistaken for a second precipitation of the yellow precipitate.  The addition of 75 cc. of ammonium molybdate solution insures the presence of a liberal excess of the reagent, but the filtrate should be tested as in all quantitative procedures.

The precipitation is probably complete in many cases in less than twelve hours; but it is better, when practicable, to allow the solution to stand for this length of time.  Vigorous shaking or stirring promotes the separation of the precipitate.]

[Note 6:  The composition of 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.)