100 cc. in volume.  Add aqueous ammonia (sp. gr. 0.96) until the precipitate of calcium phosphate first produced just fails to redissolve, and then add a few drops of nitric acid until this is again brought into solution (Note 4).  Warm the solution until it cannot be comfortably held in the hand (about 60 deg.C.) and, after removal of the burner, add 75 cc. of ammonium molybdate solution which has been !gently! warmed, but which must be perfectly clear.  Allow the mixture to stand at a temperature of about 50 or 60 deg.C. for twelve hours (Notes 5 and 6).  Filter off the yellow precipitate on a 9 cm. filter, and wash by decantation with a solution of ammonium nitrate made acid with nitric acid.[1] Allow the precipitate to remain in the beaker as far as possible.  Test the washings for calcium with ammonia and ammonium oxalate (Note 3).

[Footnote 1:  This solution is prepared as follows:  Mix 100 cc. of ammonia solution (sp. gr. 0.96) with 325 cc. of nitric acid (sp. gr. 1.2) and dilute with 100 cc. of water.]

Add 10 cc. of molybdate solution to the nitrate, and leave it for a few hours.  It should then be carefully examined for a !yellow! precipitate; a white precipitate may be neglected.

[Note 1:  Magnesium ammonium phosphate, as noted below, is slightly soluble under the conditions of operation.  Consequently the unavoidable errors of analysis are greater in this determination than in those which have preceded it, and some divergence may be expected in duplicate analyses.  It is obvious that the larger the amount of substance taken for analysis the less will be the relative loss or gain due to unavoidable experimental errors; but, in this instance, a check is placed upon the amount of material which may be taken both by the bulk of the resulting precipitate of ammonium phosphomolybdate and by the excessive amount of ammonium molybdate required to effect complete separation of the phosphoric acid, since a liberal excess above the theoretical quantity is demanded.  Molybdic acid is one of the more expensive reagents.]

[Note 2:  Soluble silicic acid would, if present, partially separate with the phosphomolybdate, although not in combination with molybdenum.  Its previous removal by dehydration is therefore necessary.]

[Note 3:  When washing the siliceous residue the filtrate may be tested for calcium by adding ammonia, since that reagent neutralizes the acid which holds the calcium phosphate in solution and causes precipitation; but after the removal of the phosphoric acid in combination with the molybdenum, the addition of an oxalate is required to show the presence of calcium.]

[Note 4:  An excess of nitric acid exerts a slight solvent action, while ammonium nitrate lessens the solubility; hence the neutralization of the former by ammonia.]