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QUANTITATIVE AGRICULTURAL ANALYSIS

concentrated sulphuric acid, with or without the addition of a
catalyzer. From the solution produced by any of these methods
phosphorus is precipitated with a molybdate solution as am-
monium phosphomolybdate. The precipitate is either dissolved
in standard base and the excess of the latter titrated, or it is
dissolved in ammonium hydroxide and the phosphorus precipi-
tated as magnesium ammonium phosphate, ignited to magnesium
pyrophosphate and this weighed. The principles of these meth-
ods are discussed in Part I, pages 88 and 91.
Determination of Phosphorus.—Use one of the following methods for
obtaining the phosphate solution:
(a) Place 10 gm of sodium peroxide in an iron crucible, add 5 gm of the
soil and mix thoroughly by means of a glass rod. If the soil contains only a
small proportion of organic matter add 0.5 gm of starch and mix as before.
The starch will hasten the action. Heat the mixture by applying the flame
of a burner directly upon the surface of the charge and the sides of the
crucible until the action starts. Cover the crucible until the action is
over and continue heating at a temperature of dull redness for 15 minutes.
The residue in the crucible should not be fused. Transfer the charge to a
250-ce beaker with about 150 cc of water; add hydrochloric acid until acid
to methyl red and boil. Cool, rinse into a 250-cc volumetric flask, dilute
to the mark and mix. If the action has taken place properly there should be
no particles of undecomposed soil in the bottom of the flask, although the
solution will usually be turbid from silicic acid.
(6) Place 5 gm of soil in a 50-cc porcelain crucible and moisten with 5 cc
of 50-per cent magnesium nitrate solution. • Evaporate to dryness on the
steam bath and ignite at dull redness. Let the crucible cool and add 5 cc of
water and 10 cc of concentrated hydrochloric acid, then cover and heat on
the steam bath for two hours. Stir several times while digesting. Transfer
the contents of the crucible to a 250-cc volumetric flask, cool to room
temperature, dilute to the mark and mix well.
(c) Place 5 gm of soil in a 500-cc Kjeldahl flask and digest with 30 cc of
concentrated sulphuric acid and 0.5 gm of mercuric oxide until the carbona-
ceous matter has been oxidized. Cool to room temperature (do not place
the flask in cold water until it has cooled somewhat in air), then add 100 cc
of water, 5 cc of concentrated hydrochloric acid and 2 cc of concentrated
nitric acid. Boil for 5 minutes, cool, dilute to 250 cc in a volumetric flask
and mix well.
Filter the phosphate solution through a dry folded filter until the filtrate
is no longer turbid. By means of a pipette or volumetric flask measure
100 cc of the clear solution and deliver into a 10-cm porcelain dish. Evapo-
rate on the steam bath to dryness, take up with 5 cc of hydrochloric acid and
an equal amount of water, filter to remove silica and wash. From this point
proceed as directed in Part I, page 90, beginning with "Add ammonium