276 QUANTITATIVE AGRICULTURAL ANALYSIS
Preparation of Solution.—Treat 2.5 gin of the sample by one of the
following methods:
(a) Ignite in a crucible until organic matter is removed (the residue will
not necessarily be white), then dissolve in hydrochloric acid.
(6) Evaporate with 5 cc of magnesium nitrate solution, made as follows:
Dissolve 320 gni of calcined magnesium oxide in nitric acid, avoiding an
excess of the latter; add a little calcined magnesium oxide in excess, boil,
filter from the residue and dilute to 2000 cc.
After evaporating the fertilizer and magnesium nitrate solution, ignite
until organic matter is removed and dissolve in hydrochloric acid.
(c) Boil with 20 or 30 cc of concentrated sulphuric acid in a Kjeldahl
flask, adding 2 to 4 gm of sodium nitrate at the beginning of the digestion
and a small quantity after the solution has become nearly colorless, or adding
the nitrate in small portions from time to time during the digestion. After
the solution is colorless add 150 cc of water and boil for a few minutes.
(d) Digest in a Kjeidahl flask with concentrated sulphuric acid and such
other reagents as are used in either the plain or modified Kjeldahl or Gunning
method for the determination of nitrogen (page 152). Do not add any
potassium permanganate but, after the solution has become colorless, add
about 100 cc of water and boil for a few minutes.
(e) Dissolve in 30 cc of concentrated nitric acid and 5 cc of concentrated
hydrochloric acid and boil until organic matter is destroyed.
(/) Add 30 cc of concentrated hydrochloric acid, heat and add cautiously,
in small quantities at a time, about 0.5 gm of finely pulverized potassium
or sodium chlorate to destroy organic matter.
(g) Dissolve in 15 to 30 cc of concentrated hydrochloric acid and 3 to
10 ce of concentrated nitric acid. This method is recommended for fertil-
izers containing much ferric or aluminium phosphate.
After the sample of fertilizer has been brought into solution by any of the
methods described above, cool, dilute to 250 cc, mix and pour into a dry
filter, discarding the first 10 cc of the filtrate and allowing the remainder to
run into a dry flask which can be stoppered.
Gravimetric Determination.—Prepare solutions of ''magnesia mixture"
and ammonium niolybdate as directed on pages 88 and 89, Part I. Prepare
also:
"(a) Ammonium Hydroxide.—Dilute the concentrated solution ten times.
(5) A nmonium Nitrate.—A 10-per cent solution.
Measure 25, 50 or 100 cc of the fertilizer solution, according to the
probable per cent of phosphorus, using a pipette or volumetric flask. Trans-
fer to a 250-cc flask of resistance glass, neutralize with ammonium hydroxide
and clear with a few drops of nitric acid, thus dissolving the small amount
of precipitated hydroxides of iron and aluminium. In case hydrochloric
or sulphuric acid has been used as a solvent for the fertilizer material add
also 15 gm of dry ammonium nitrate.
To the hot solution add ammonium molybdate solution, about 70 cc for
each decigram of phosphorus pentoxide thought to be present. Immerse in
water and digest at 65° for an hour and determine whether the phosphorus