92

QUANTITATIVE AGRICULTURAL ANALYSIS

Use' 2.5 gm of sample, weighed accurately to milligrams. Dissolve as
directed for the gravimetric determination of phosphorus in rock phos-
phate, page 90, and dilute the solution to 500 cc in a volumetric flask. Mix
well and pour into a dry filter. Reject the first 25 cc and collect the rest
in a dry flask. Pipette 25-cc portions of this solution into 250-cc flasks.
Add ammonium hydroxide until a slight precipitate persists and clear with a
few drops of nitric acid. Place a thermometer in the flask, immerse in the
water bath and heat to 65°, then add 35 cc of freshly filtered molybdate
solution. Mix and leave the flask in the water bath for 15 minutes, then
filter at once. Wash twice with water by decantation, using 25 cc each
time, pouring the washings into the filter. Transfer the precipitate to the
filter as thoroughly as can be done without the use of a policeman and wash
the flask and filter with cold water until the filtrate from two fillings of the
filter yields a pink color upon the addition of phenolphthalein and one drop
of standard base. (Test the wash water in this manner before using.)
Return the filter and precipitate to the flask in which precipitation was
made and add 50 cc of cold water. Add half-normal base from a burette
until all of the precipitate is dissolved, mixing by gentle rotation. Imme-
diately add a drop of phenolphthalein and titrate with standard acid. From
the total Volume of base used, deduct that equivalent to the acid required
and from^e remainder-calculate -the per- cent of phosphorus and of phos-
phorus pentoxide in the sample.