278 QUANTITATIVE AGRICULTURAL ANALYSIS
In the case of fertilizers containing less than 5 per cent of phosphorus
pentoxide, use an aliquot corresponding to 0.4 grn of substance. If the
percentage is between 5 and 20 use an aliquot corresponding to 0.1 gin of
substance.
Add 5 to 10 cc of concentrated nitric acid, the amount depending upon
whether this acid has been used in making the solution; or add ammonium
nitrate equivalent to this amount of nitric acid. Nearly neutralize with
ammonium hydroxide, precipitation of hydroxide of iron or aluminum
serving as indicator. Clear with a drop of nitric acid, dilute to about 100 cc
and heat by immersing in water at 60° to 65°. For phosphorus pentoxide
per cents below 5 add 25 cc of freshly filtered molybdate solution; for
percentages between 5 and 20 add 35 cc of molybdate solution. For
percentages greater than 20 add sufficient molybdate solution to insure
complete precipitation of the phosphorus. Stir, allow to stand in the bath
for 15 minutes and filter at once. Wash twice with water by deeantation,
using 25 to 30 cc each time and agitating and settling each time before
decanting. Transfer the precipitate to the filter as thoroughly as can' be
done without the use of a policeman and wash the flask, paper and precipi-
tate with cold, recently boiled water until the nitrate from two fillings of the
filter yields a pink color upon the addition of phenolphthalein and one drop
of the standard base. Remember that a trace of acid left in any of these
materials will vitiate the results of the titration.
Return the filter paper and precipitate to the flask in which precipitation
was made. Add a measured, small excess of the standard base to dissolve
the yellow precipitate, then add a few drops of phenolphthalein and titrate
the unused excess of base with standard acid. Calculate the per cent of
phosphorus pentoxide in the sample.
The following changes in the method just described are made optional:
(a) Heat the solution to only 45° to 50° and allow to stand in the bath,
after the addition of the molybdate solution, for 30 minutes.
(6) Cool to room temperature before adding the molybdate solution.
Add the latter at the rate of 75 cc for each decigram of phosphorus pentoxide
present, place the stoppered flask containing the solution in a mixing appa-
ratus (Fig. 51) and mix for 30 minutes at room temperature. Filter at once
and proceed as already directed.
Determination of Water-soluble Phosphorus: Gravimetric Method.—
Place an accurately weighed 2-gm sample on a filter and wash with small
portions of cold water until about 250 cc of washings has been obtained.
Allow each portion of water to run through before adding another. Keep
the residue for the determination of citrate-insoluble phosphorus. Dilute
the filtrate to exactly 500 cc and mix.
Place 50-cc aliquots in flasks, add 10 cc of concentrated nitric acid
and then ammonium hydroxide until a slight permanent precipitate is
formed. Clear with a few drops of nitric acid, dilute to about 100 cc
and determine the water-soluble phosphorus gravimetrically as in the
case of total phosphorus. Report as phosphorus pentoxide of water-
soluble compounds.