88 QUANTITATIVE AGRICULTURAL ANALYSIS

extent if ammonium salts are present in immodera*f.o
quantities, decomposes at high temperatures into magnesium
metaphosphate :

Mg[(NH4)2P04]2 -> Mg(P03)2 4- 4NH3 + 2H2O. (5)

Also, ^n'magnesium phosphate, Mg3(P04)2; may be precipitated
if the solution contains too much ammonia and this will remain.
unchanged by ignition.

Insoluble Phosphates. — Phosphates of other than alkali metals
are usually soluble in acids but a direct precipitation of magne-
sium ammonium phosphate cannot be made because phosphates
of the original metals reprecipitate as soon as ammonium hydrox-
ide is added to neutralize the acid. For example, tricalcium
phosphate, Ca3(P04)2, furnishes much of the phosphorus of
fertilizers as the mineral apatite. This is soluble in acids but if"
an attempt were made to determine the phosphorus by SL
magnesium precipitation, the precipitate would be a mixture of
phosphates of calcium and magnesium.

In order to prepare such a phosphate for a determination of
phosphorus a preliminary separation of the phosphate radical
is made by the addition of ammonium molybdate to the solution
in nitric acid. This results in the formation of a yellow pre-
cipitate of ammonium phosphomolybdate :

(NH4)3P04 + 12(NH4)2Mo04 + 24HN03 ->

(NH4)3P04.12Mo03+24NH4N03 + 12H20. . (1)

This is filtered out and washed free from the alkaline earth
metals. The precipitate is then dissolved in ammonium hydrox-
ide, reforming ammonium molybdate and ammonium phos-
phate, both of which are soluble. This reaction is represented
as follows :

(NH4)3P04-12Mo03 + 24NH4OH -» (NH4)3P04 +

. 12(NH4)2Mo04 + 24NH4N03 + 12H20. (2)

The magnesium salt is now added and the precipitation and sub-
sequent treatment are carried out as described for soluble
phosphates.

Determination of Phosphorus in Soluble Phosphates. — Prepare a solution
of " magnesia mixture11 as follows: Dissolve 55 gm of crystallized magnesium

•:bL