=153. Classification of Methods.=—In general there are three direct
methods of determining the nitrogen content of fertilizers. First the
nitrogen may be secured in a gaseous form and the volume thereof, under
standard conditions, measured and the weight of nitrogen computed. This
process is commonly known as the absolute method. Practically it has
passed out of use in fertilizer work, or is practiced only as a check
against new and untried methods, or on certain nitrogenous compounds
which do not readily yield all their nitrogen by the other methods.
The process, first perfected by Dumas, who has also given it his name,
consists in the combustion of the nitrogenous body in an environment
of copper oxid by which the nitrogen, by reason of its inertness, is
left in a gaseous state after the oxidation of the other constituents;
_viz._, carbon and hydrogen, originally present.
In the second class of methods the nitrogen is converted into ammonia
which is absorbed by an excess of standard acid, the residue of which
is determined by subsequent titration with a standard alkali. There are
two distinct processes belonging to this class, in one of which ammonia
is directly produced by dry combustion of an organic nitrogenous
compound with an alkali, and in the other ammonium sulfate is produced
by moist combustion with sulfuric acid, and the salt thus formed is
subsequently distilled with an alkali, and the free ammonia thus formed
estimated as above described. Nitric nitrogen may also be reduced to
ammonia by nascent hydrogen either in an acid or alkaline solution as
described in volume first.
In the third class of determinations is included the estimation of
nitric nitrogen by colorimetric methods as described in the first
volume. These processes have little practical value in connection with
the analyses of commercial fertilizers, but find their chief use in the
detection and estimation of extremely minute quantities of nitrites
and nitrates. In the following paragraphs will be given the standard
methods for the determination of nitrogen in practical work with
fertilizing materials and fertilizers.
=154. Official Methods.=—The methods adopted by the Association of
Official Agricultural Chemists have been developed by more than ten
years of co-operative work on the part of the leading agricultural
chemists of the United States. These methods should be strictly
followed in all essential points by all analysts in cases where
comparison with other data are concerned. Future experience will
doubtless improve the processes both in respect of accuracy and
simplicity, but it must be granted that, as at present practiced, they
give essentially accurate results.
=155. Volumetric Estimation by Combustion with Copper Oxid.=—This
classical method of analysis is based on the supposition that by the
combustion of a substance containing nitrogen in copper oxid and
conducting the products of the oxidation over red-hot copper oxid and
metallic copper, all of the nitrogen present in whatever form will be
obtained in a free state and can subsequently be measured as a gas.
The air originally present in all parts of the apparatus must first
be removed either by a mercury pump or by carbon dioxid or by both
together, the residual carbon dioxid being absorbed by a solution of
caustic alkali. Great delicacy of manipulation is necessary to secure
a perfect vacuum and as a rule a small quantity of gas may be measured
other than nitrogen so that the results of the analyses are often
a trifle too high. The presence of another element associated with
nitrogen, or the possible allotropic existence of that element, may
also prove to be a disturbing factor in this long-practiced analytical
process. For instance, if nitrogen be contaminated with another
element, _e. g._, argon, of a greater density the commonly accepted
weight of a liter of nitrogen is too great and tables of calculation
based on that weight would give results too high.
First will be given the official method for this process, followed by a
few simple variations thereof, as practiced in this laboratory.
=156. The Official Volumetric Method.=—This process may be used for
nitrogen in any form of combination.[133]
The apparatus and reagents needed are as follows:
_Combustion tube_ of best hard Bohemian glass, about sixty-six
centimeters long and 12.7 millimeters internal diameter.
_Azotometer_ of at least 100 cubic centimeters capacity, accurately
calibrated.
_Sprengel mercury air-pump._
_Small paper scoop_, easily made from stiff writing paper.
_Coarse cupric oxid._—To be ignited and cooled before using.
_Fine cupric oxid._—Prepared by pounding ordinary cupric oxid in a
mortar.
_Metallic copper._—Granulated copper, or fine copper gauze, reduced and
cooled in a current of hydrogen.
_Sodium bicarbonate._—Free from organic matter.
_Caustic potash solution._—Make a supersaturated solution of caustic
potash in hot water. When absorption of carbon dioxid, during the
combustion, ceases to be prompt, the solution must be discarded.
_Filling the tube._—Of ordinary commercial fertilizers take from
one to two grams for analysis. In the case of highly nitrogenized
substances the amount to be taken must be regulated by the amount of
nitrogen estimated to be present. Fill the tube as follows: (1) About
five centimeters of coarse cupric oxid: (2) Place on the small paper
scoop enough of the fine cupric oxid to fill, after having been mixed
with the substance to be analyzed, about ten centimeters of the tube;
pour on this the substance, rinsing the watch-glass with a little of
the fine oxid, and mix thoroughly with a spatula; pour into the tube,
rinsing the scoop with a little fine oxid: (3) About thirty centimeters
of coarse cupric oxid: (4) About seven centimeters of metallic copper:
(5) About six centimeters of coarse cupric oxid (anterior layer):
(6) A small plug of asbestos: (7) From eight-tenths to one gram of
sodium bicarbonate: (8) A large, loose plug of asbestos; place the
tube in the furnace, leaving about two and five-tenths centimeters of
it projecting; connect with the pump by a rubber stopper smeared with
glycerol, taking care to make the connection perfectly tight.
_Operation._—Exhaust the air from the tube by means of the pump. When a
vacuum has been obtained allow the flow of mercury to continue; light
the gas under that part of the tube containing the metallic copper,
the anterior layer of cupric oxid (see (5) above), and the sodium
bicarbonate. As soon as the vacuum is destroyed and the apparatus
filled with carbon dioxid, shut off the flow of mercury and at once
introduce the delivery-tube of the pump into the receiving arm of the
azotometer just below the surface of the mercury seal, so that the
escaping bubbles will pass into the air and not into the tube, thus
avoiding the useless saturation of the caustic potash solution.
Set the pump in motion and when the flow of carbon dioxid has very
nearly or completely ceased, pass the delivery-tube down into the
receiving arm, so that the bubbles will escape into the azotometer.
Light the gas under the thirty centimeter layer of oxid, heat gently
for a few moments to drive out any moisture that may be present, and
bring to a red heat. Heat gradually the mixture of substance and oxid,
lighting one jet at a time. Avoid a too rapid evolution of bubbles,
which should be allowed to escape at the rate of about one per second
or a little faster.
When the jets under the mixture have all been turned on, light the gas
under the layer of oxid at the end of the tube. When the evolution
of gas has ceased, turn out all the lights except those under the
metallic copper and anterior layer of oxid, and allow to cool for a
few moments. Exhaust with the pump and remove the azotometer before
the flow of mercury is stopped. Break the connection of the tube with
the pump, stop the flow of mercury, and extinguish the lights. Allow
the azotometer to stand for at least an hour, or cool with a stream of
water until a permanent volume and temperature have been reached.
Adjust accurately the level of the potash solution in the bulb to that
in the azotometer; note the volume of gas, temperature, and height of
barometer; make calculation as usual, or read results from tables.
=156. Note on Official Volumetric Method.=—The determination of
nitrogen in its gaseous state by combustion with copper oxid, has
practically gone out of use as an analytical method. The official
chemists rarely use it even for control work on samples sent out for
comparative analysis. The method recommended differs considerably from
the process of Jenkins and Johnson, on which it is based. The only
source of oxygen in the official method is in the copper oxid. Hence it
is necessary that the oxid in immediate contact with the organic matter
be in a sufficiently fine state of subdivision, and that the substance
itself be very finely powdered and intimately mixed with the oxidizing
material. Failure to attend to these precautions will be followed by
an incomplete combustion and a consequent deficit in the volume of
nitrogen obtained.
The copper oxid before using is ignited, and is best filled into the
tube while still warm by means of a long pointed metal scoop, or other
convenient method. The copper spiral, after use, is reduced at a red
heat in a current of hydrogen, and may thus be used many times.
=157. The Pump.=—Any form of mercury pump which will secure a complete
vacuum may be used. A most excellent one can be arranged in any
laboratory at a very small expense. The pump used in this laboratory
for many years answers every purpose, and costs practically nothing,
being made out of old material not very valuable for other use.
The construction of the pump and its use in connection with the
combustion tube will be clearly understood from the following
description:
[Illustration: FIGURE 10.