ammonia. Stir
for a few moments and then add a volume of strong ammonia (sp. gr.
0.90), equal to one third of the volume of the solution. Allow the
solution to stand for some hours, and then filter off the magnesium
ammonium phosphate, which should be distinctly crystalline in
character. Wash the precipitate with dilute ammonia water, as
prescribed above, until, finally, 3 cc. of the washings, after
acidifying with nitric acid, show no evidence of chlorides. Test both
filtrates for complete precipitation by adding a few cubic centimeters
of magnesia mixture and allowing them to stand for some time.

Transfer the moist precipitate to a weighed porcelain or platinum
crucible and ignite, using great care to raise the temperature slowly
while drying the filter in the crucible, and to insure the ready
access of oxygen during the combustion of the filter paper, thus
guarding against a possible reduction of the phosphate, which would
result in disastrous consequences both to the crucible, if of
platinum, and the analysis. Do not raise the temperature above
moderate redness until the precipitate is white. (Keep this precaution
well in mind.) Ignite finally at the highest temperature of the
Tirrill burner, and repeat the heating until the weight is constant.
If the ignited precipitate is persistently discolored by particles of
unburned carbon, moisten the mass with a drop or two of concentrated
nitric acid and heat cautiously, finally igniting strongly. The
acid will dissolve magnesium pyrophosphate from the surface of the
particles of carbon, which will then burn away. Nitric acid also aids
as an oxidizing agent in supplying oxygen for the combustion of the
carbon.

From the weight of magnesium pyrophosphate (Mg_{2}P_{2}O_{7})
obtained, calculate the phosphoric anhydride (P_{2}O_{5}) in the
sample of apatite.

[Note 1: The ionic change involved in the precipitation of the
magnesium compound is

PO_{4}^{---} + NH_{4}^{+} + Mg^{++} --> [MgNH_{4}PO_{4}].

The magnesium ammonium phosphate is readily dissolved by acids, even
those which are no stronger than acetic acid. This is accounted for
by the fact that two of the ions into which phosphoric acid may
dissociate, the HPO_{4}^{--} or H_{2}PO_{4}^{-} ions, exhibit the
characteristics of very weak acids, in that they show almost no
tendency to dissociate further into H^{+} and PO_{4}^{--} ions.
Consequently the ionic changes which occur when the magnesium ammonium
phosphate