QUANTITATIVE DETERMINATIONS

91

(2) on page 88. If the base is added as a standard solution in
measured excess and the unused portion is titrated by a standard
acid the phosphorus (or the corresponding phosphoric anhy-
dride) may be calculated.

Variation in Composition.—It has been found that the ratio
of molybdic anhydride to ammonium phosphate in the precipitate
is somewhat variable unless the conditions of precipitation are
standardized and kept constant. This is probably due to
the coprecipitation of some free molybdic acid. Such varia-
tions must occasion an error in the volumetric determination,
since it is the molybdic anhydride or acid that furnishes the acid
properties of the compound. If the method is followed as
outlined the composition of the precipitate will be fairly accu-
rately .represented by the formula already given.

Titration.—Sodium hydroxide reacts as follows:

(NH4)3P04.12Mo03 + 24NaOH

>(NH4)3P04 +
12Na2Mo03

12H20. (1)

Upon addition of acid any excess of base is neutralized and if
the acid is added to a color change with phenolphthalein, tri-
ammonium phosphate will have been changed to diammonium
phosphate, since the normal salt is basic to this indicator. The
net result is therefore to be expressed by the following equation:
2(NH4)3P04.12Mo03 + 46NaOH -* 2(NH4)2HPO4 +
23Na2Mo03 + (NH4)2Mo04 + 22H20. (2)
It should be noted that it is necessary to have the solution
cold when the excess of base is added and to have present suffi-
cient water to prevent the escape of ammonia, which would
be produced by reaction of ammonium phosphate with sodium
hydroxide.
Determination of Phosphorus in Rock Phosphate : Volumetric Method. —
The molybdate solution that was used for the gravimetric determination
may be used here also, first adding 5 cc of concentrated nitric acid to each
100 cc of solution. This additional acid serves to prevent the precipitation
of molybdic acid.
Prepare a half -normal solution of hydrochloric acid, standardizing
against .sodium carbonate as directed on page 83, modifying the weights
according to the different normality of this solution. Prepare a half-
normal solution of sodium hydroxide in boiled and cooled water, standard-
izing against the acid, using phenolphthalein.