length of the tube is from thirty-five to forty centimeters. It is
filled first to a depth of two centimeters with calcium oxalate;
afterwards to a depth of five centimeters with soda-lime in small
fragments; afterwards with the mixture to be analyzed; _viz._, of ten
grams of the sample of soil, or twenty grams if poor in nitrogen and
organic matters, with soda-lime reduced to a coarse powder. This mixture
should occupy a length of about twenty centimeters in the tube. The soil
and soda-lime are mixed in a mortar. Afterwards the mortar is rubbed
with small quantities of soda-lime, and this, together with the copper
boat which has been used in introducing the mixture, is thoroughly
washed with the soda-lime, which is poured into the tube until it is
filled to within four centimeters of its open extremity. The open end of
the tube is then closed with a wad of asbestos packed sufficiently tight
to prevent the carrying off of the soda-lime by the gas which may be
generated during the combustion. The combustion should be commenced by
heating the tube near the open extremity until it is red and carrying
the heat progressively towards the part containing the soil mixed with
the soda-lime. An ordinary gas combustion furnace should be used and the
heat graduated in such a way that the bubbles of gas pass off regularly
and not too rapidly. The gas is conducted into a bulb tube containing a
decinormal standard sulfuric acid colored with litmus. The combustion is
continued until the whole of the organic material is decomposed, care
being taken not to raise the combustion tube above a low redness in
order to avoid its softening. At the end, however, the temperature of
the combustion tube should be raised to a bright red, and the part
containing the calcium oxalate should be heated little by little for the
purpose of evolving hydrogen, which is used to drive out the last traces
of ammonia. After the combustion is completed, and the last traces of
ammonia driven out, the standard acid which has received the evolved
ammonia is removed, the tube leading to it washed, the wash-water
collected with the rest of the liquid and titrated with a standard
solution of lime-water, the strength of which has previously been
determined against standard sulfuric acid.
=402. Preparation of the Standard Sulfuric Acid.=—The sulfuric acid to
be used in making the standard solutions should be previously boiled for
half an hour in a platinum dish and allowed to cool in a desiccator. It
should contain 61.25 grams of sulfuric acid in one liter. It is
recommended that the flask which holds the sulfuric acid should be one
which has been used for a long time for holding concentrated sulfuric
acid, in order to avoid any action of the alkali in the glass upon the
acid after its strength has been determined. The solution before
described is of such strength as to have each cubic centimeter
equivalent to one milligram of nitrogen.
For the estimation of the nitrogen in the soil a tenth normal solution
should be used, which is prepared by taking 100 cubic centimeters of the
normal solution, described above, and diluting to one liter.
_Preparation of the Lime-Water._—From 200 to 300 grams of slaked lime
are placed in a closed flask of about five liters capacity. This is
filled with water and shaken frequently, and left to deposit the matter
in suspension. The water which contains the saline particles which may
have been present in the lime is then poured off. Fresh water is then
poured on and the flask shaken from time to time. To use this lime-water
the clear part of it is decanted into a flask, avoiding, as much as
possible, access to the air. The flask is closed with a cork carrying
two tubes drawn out and bent at a right angle. One of these serves for
pouring off the water and the other serves for the entrance of the air.
These two tubes are themselves closed by means of a rubber tube carrying
a pinch-cock. The strength of the lime-water is fixed by titration with
the decinormal standard sulfuric acid.
_Preparation of the Soda-Lime._—Six hundred grams of slaked lime in fine
powder are saturated with 300 grams of caustic soda dissolved in 300
cubic centimeters of water. The whole is rubbed into a paste and
introduced into a crucible which is heated to redness. The contents of
the crucible, still hot, are poured out, and rapidly reduced to
fragments in a copper mortar in such a manner as to have the pieces
about the size of a pea, and without having too much finely powdered
soda-lime mixed with it. While the matter is still hot it is placed in a
flask and well-stoppered. In order that this reagent should contain no
nitrogen it is indispensable to use in its preparation materials which
contain no trace of nitrates.
_Preparation of the Calcium Oxalate._—In a small copper vessel place 100
grams of oxalic acid and add gradually, bringing it to boiling, enough
water to dissolve it. Afterwards place in the solution small portions of
slaked lime in a state of powder, constantly testing it until turmeric
paper indicates that there is a little lime in excess. It is then
evaporated, stirring vigorously on the open fire, and the evaporation is
finally finished on a steam-bath. The dried material is placed in a
flask and well-stoppered. The oxalic acid which is used in this
preparation should be free from every trace of nitrogen.
_Preparation of the Litmus Solution._—Five grams of litmus are placed in
a flask with a flat bottom. Afterwards a few cubic centimeters of
ammonia are added, twenty five grams of crystallized sodium carbonate,
and ten cubic centimeters of water. This mixture is left to digest for
sometime, with frequent stirring, at a temperature of from 60°–80°. The
digestion is finished in about four or five days, during which time, at
intervals, a few drops of ammonia are added, sufficient to maintain
always the ammoniacal odor. At the end of this time 200 cubic
centimeters of water are added and the digestion allowed to continue
several days more, still maintaining the solution alkaline with ammonia.
A slight excess of hydrochloric acid is added, and the matter which is
precipitated is received upon a filter where it is washed several times
with cold water and allowed to dry at a low temperature.
For use, from one to two grams of this dry precipitate are dissolved in
100 cubic centimeters of alcohol, and there is thus obtained a litmus
solution of extreme sensibility.
=403. Treatment of Soil Containing Nitrates.=—Nitrates exist in small
quantities in all arable soils. When treated for nitrogen by the
soda-lime method above described, a part of the nitric nitrogen is
changed to the state of ammonia, while another part escapes estimation
altogether, causing an error which it is important to point out. When
the soils contain only small quantities of nitrates this error is
insignificant and does not affect sensibly the results, but in the case
of earths rich in nitrates it is necessary first to eliminate them
before the determination of the nitrogen by the soda-lime method. The
operation is carried on as follows:
Twenty grams of the soil are washed on a small funnel, furnished with a
plug of asbestos, with small quantities of pure water, in such a way as
to cause thirty to forty cubic centimeters of water to pass through. The
whole of the nitrate is thus removed. The soil is now dried and
submitted to analysis by the soda-lime method as just described. There
are removed with the nitrate only small traces of organic nitrogen, too
small to influence the results of the analysis. If, however, it is
desired to remove altogether this slight cause of error, evaporate the
wash-waters, above described, to two or three cubic centimeters; add a
few drops of a concentrated solution of ferrous chlorid and as much
hydrochloric acid, and boil some minutes in order to drive off, in the
state of nitrogen dioxid, all the nitric acid. The residue is evaporated
to dryness and contains the traces of organic nitrogen. This is added to
the soil which is to be treated by the soda-lime method.
=404. Müller’s Method.=—The determination of nitrogen in the soil by
soda-lime is carried on as follows by Müller:[269]
Fifteen grams of fine earth, dried and mixed with a little sugar, are
mixed with thirty grams of soda-lime in powder. The bottom of the
combustion tube contains a little moist soda-lime, which is heated at
the end of the operation at the same time that a current of pure
hydrogen is made to pass through it, and the temperature of the tube is
raised, little by little, to a distinct redness. The contents of the
receiving bulbs are distilled, after the addition of water and soda, in
the same apparatus which served in the estimation of nitrogen, by the
kjeldahl method; the determinations and titrations are made also under
the same conditions.
Blank determinations are also made under the same conditions to
determine the amount of correction to be made by the two methods.
Soda-lime, heated with pure sugar, gave 0.0002 gram of nitrogen for a
total weight of fifty-five grams of the soda-lime contained in the tube.
The fuming sulfuric acid gave 0.0011 cubic centimeters of ammoniacal
nitrogen for the volume of thirty cubic centimeters.
The numbers obtained by the kjeldahl method in general, are lower than
those obtained by the soda-lime method when no stearic or benzoic acid
is used. The numbers obtained when stearic acid alone was used were
sometimes inferior to those obtained by the soda-lime method. The
numbers obtained when benzoic acid is used are, in general, about the
same as those obtained by the soda-lime method.
It would seem that the double distillation, outlined above, for the
kjeldahl method, would not be necessary if due care were exercised in
the first distillation. This variation, therefore, seems to be
unnecessary.
In the soda-lime method, time would be saved by the reception of the
ammonia in standard acid, and its titration in the usual way, unless a
further purification of the nitrogenous products of the combustion by
the final distillation be desired.
=405. Volumetric Determination of the Nitrogen.=—Instead of separating
the nitrates, the total nitrogen in the soil can be determined directly
by the classic method of Dumas, which consists in bringing the whole of
the nitrogen into a gaseous state and afterwards measuring its volume.
The following method illustrates the general principles of the
determination:
A glass combustion tube closed at one end, about one meter in length, is
selected. In the bottom of this tube is placed some potassium
bicarbonate in a crystalline form, in small pieces, filling the tube to
a distance of about twenty centimeters. Afterwards copper oxid is placed
to the depth of ten centimeters and finally a mixture of from twenty to
thirty grams of the earth with thirty to forty grams of copper oxid in a
fine state of subdivision, and about ten grams of metallic copper
obtained by reducing the copper oxid by hydrogen. Next the tube is
filled with copper oxid to a depth of from twenty to twenty-five
centimeters, and afterwards with reduced copper to the depth of at least
twenty-five centimeters, and after this another layer of copper oxid of
about five centimeters, and finally a plug of asbestos. The combustion
tube is closed with a stopper carrying a glass tube of about ninety
centimeters in length, of which the extremity, bent into the form of a
=ᥩ=, extends to a mercury trough. The glass combustion tube is
surrounded with brass gauze, except that part which contains the
potassium bicarbonate. The beginning of the operation consists in
heating the tube to decompose a part of the potassium bicarbonate, until
the whole of the apparatus is filled with carbon dioxid. In order to
determine that the whole of the air has been expelled and that the
apparatus is entirely filled with carbon dioxid, a part of the gas which
is disengaged, is received into a jar filled with mercury, in which a
little potash-lye has been placed. If the gas is entirely absorbed by
the potash, so that there remain only unappreciable particles, the tube
can be regarded as completely free of air. When assurance is given that
the air is all out of the apparatus, a jar of about 300 cubic
centimeters capacity, filled with mercury and containing from thirty to
forty cubic centimeters of a solution of potash of a density of 42°
baumé, is placed over the outlet tube. The combustion is commenced by
heating the anterior part of the tube, avoiding the heating of the part
containing the earth. When the first part of the tube has reached the
red stage the part containing the earth is gradually heated in order to
obtain a gentle evolution of gas. The temperature of the tube is carried
to redness and the heating gradually carried back toward the closed
extremity, but avoiding raising the temperature of the part containing
the potassium bicarbonate. The red heat is continued as long as bubbles
of gas are discharged into the reservoir. When the evolution of gas has
ceased the apparatus is again filled with carbon dioxid for the purpose
of driving out the last traces of nitrogen, by heating again the part of
the tube containing the potassium bicarbonate. The evolution of the
carbon dioxid should be maintained for about fifteen minutes. At the end
of this time all the nitrogen will be found in the receiving jar.
Sometimes a small quantity of nitrogen dioxid is formed incidentally in
the operation. After waiting for a quarter of an hour, in order to
permit all the carbon dioxid which may have escaped into the reservoir
to be completely absorbed, the receiving jar is carried to a water-basin
and the mercury allowed gradually to escape; its place being taken by
the water. The gas is then transferred into an azotometer where its
volume and temperature are read in the usual way.
In order to absorb any nitrogen dioxid which may be admixed with the
nitrogen itself, a little crystal of ferrous sulfate is introduced. The
reservoir containing the nitrogen is carried to the mercury trough, and
the water which it contains is nearly all run out in such a way as to be
replaced with mercury, great care being exercised to avoid any escape of
gas. Afterwards there is introduced over the mercury a crystal of
ferrous sulfate and the azotometer is shaken until this crystal is
dissolved by the water which it still contains. It is then allowed to
remain for twenty hours. At the end of this time the nitrogen dioxid is
absorbed and the volume of the gas is again read as before. One-half
only of the total loss should be subtracted, since the volume of the
nitrogen dioxid is twice the volume of the nitrogen itself. For the
practice of this method, in connection with the use of a mercury pump,
the directions which will be given under fertilizers may be consulted.
=406. Estimation of Ammonia.=—Ammonia exists ordinarily only in very
small quantities in the soil, since it is incessantly transformed into
nitrate or diffused in the air. Nevertheless, it is sometimes
interesting to determine its quantity.
The method of determining the ammonia in soils is one of extreme
delicacy on account of the small proportion therein, and the difficulty
of expelling it without at the same time converting some of the organic
nitrogen into ammoniacal compounds. The various methods employed for
this purpose may be classified as follows:
1. Treatment of the soil with soda-lye in the cold, and the absorption
of the ammonia given off by standard sulfuric acid.
2. The method of Boussingault, which consists in replacing the soda-lye
with magnesia and distilling the ammonia at a boiling temperature,
absorbing the distillate in a standard acid.
3. A modification of the above method, due to Schloesing, which consists
first in extracting the ammonia by hydrochloric acid and subjecting the
extract to distillation with magnesia.
4. The method of Knop consists in treating the soil in a closed cylinder
with soda-lye containing bromin. The ammonia set free by the lye is
decomposed in the presence of bromin into free nitrogen and hydrochloric
acid. The nitrogen is collected and measured in an azotometer. The
brom-soda-lye is prepared by dissolving 100 grams of sodium hydroxid in
1,200 cubic centimeters of water and adding twenty-five cubic
centimeters of bromin.
5. The process described under 4, as shown by Baumann,[270] does not
give accurate results and it has been modified by him as follows: Two
hundred grams of soil are treated with 100 cubic centimeters of dilute
hydrochloric acid (one part acid and four of water) free of ammonia; 300
cubic centimeters of ammonia-free distilled water are added and the
whole digested for two hours with frequent stirring. If a soil contain
much calcium carbonate larger quantities of acid must be used. Two
hundred cubic centimeters of the filtrate are placed in an evolution
flask, connected with an azotometer, with five grams of freshly burned
magnesia. The mixture is then oxidized as follows: Ozone is generated by
adding three parts by weight of sulfuric acid to one part of dry and
powdered potassium permanganate. A stream of air is drawn through the
ozone generator by an aspirator, and the ozone is conducted into a flask
containing the hydrochloric acid extract of the soil and magnesia. The
oxidation is completed in about ten minutes. The mixture is then brought
into the evolution flask of the azotometer and the nitrogen set free and
measured in the usual way.
It has been shown that if asparagin or glutamin be present in the soil
they are decomposed by the soda-lye and the results obtained are too
high. It has been further proved that soils which contain a notable
quantity of humus give, with soda-lye in the cold, a practically
continuous evolution of ammonia. Moreover, soils which are rich in humus
and which have been treated by distillation with magnesia give, on
subsequent treatment with soda-lye, considerable additional quantities
of ammonia.
_Comparison of Methods of Estimating Ammonia._—Baumann has determined
the ammonia-nitrogen in various soils by the soda-lime method;
distillation of the hydrochloric acid extract with magnesia, and the
azotometric method modified as indicated above. These methods will be
designated as 1, 2, 3, respectively in the following table.
METHOD.
1. 2. 3.
Ammonia-nitrogen in one
kilogram of soil.
—————— —————— ——————
No. of sample. Gram. Gram. Gram.
1 0.0448 0.02227 0.02781
2 0.0168 0.01105 0.01326
3 0.0336 0.01771 0.02214
4 0.0056 0.00443 0.00443
5 0.0280 0.02337 0.02894
6 0.0196 0.01243 0.01672
From the above figures it is seen that the method usually attributed to
Schloesing gives uniformly higher numbers than either of the other
processes, while the third gives slightly higher values than the second.
=407. The Magnesia Distillation Process.=—If a sample of soil be
distilled directly with magnesia and water, there is danger on the one
side of not extracting all the ammonia, by reason of the absorbing power
of these bodies, and on the other, of transforming into ammonia the
nitrogen of the organic matters. It is therefore preferable to separate
the ammonia from the soil in the form of chlorid, and to subject this
extract to distillation.
In fifty grams of the soil the humidity is determined by drying at 100°
until there is no further loss of weight. The quantity of moisture being
known, 200 grams of soil are taken and moistened with water, and then
there is added, in small portions, some dilute hydrochloric acid,
shaking frequently until the whole of the calcium carbonate present is
decomposed. The liquor should remain acid at the end of the operation,
but without containing a notable excess of acidity. Knowing beforehand
the quantity of moisture contained in the 200 grams, water is added
until the total quantity shall be equal to 500 cubic centimeters. The
whole is then shaken and allowed to repose, and filtered rapidly,
covering the funnel with a glass vessel and receiving the liquid which
runs through in a flask with a narrow opening. Two hundred and fifty
cubic centimeters of this liquor, or mixture, represent 100 grams of
earth of known humidity. This quantity is introduced into a flask for
determining the ammonia and five grams of calcined magnesia added.
Before commencing the distillation, assurance should be had that the
magnesia has completely saturated the acid in excess, and that the
liquor is alkaline.
If, by chance, the liquor should be still acid it would be necessary to
add sufficient magnesia in order that the reaction should be manifestly
alkaline. Afterwards the distillation is begun and the ammonia is
received in an appropriate vessel containing one-tenth normal sulfuric
acid and titrated in the usual way, or nesslerized.
Inasmuch as the quantities of ammonia contained in the earth are
generally very small it is necessary to be very particular in order to
avoid errors. The distilled water which is employed should be deprived
of all traces of ammonia by prolonged ebullition, and the hydrochloric
acid should be distilled in the presence of a little sulfuric acid. The
treatment with hydrochloric acid is for the purpose of destroying the
absorbing properties of the soil for ammonia, and to permit this last to
enter into solution as chlorid. When there is need of very great
precision it is convenient to make a blank operation with the
hydrochloric acid and water which are employed, in order to make a
correction for the traces of ammonia which these reagents may contain.
=408. Estimation of Ammoniacal and Amid Nitrogen by the Method of
Berthelot and André.=[271]—Heat one hundred grams of earth for thirty
hours on a steam-bath with about .500 cubic centimeters of dilute
hydrochloric acid (fifteen grams of hydrochloric acid to 500 cubic
centimeters of water). At the end of this time throw the contents of the
flask on a filter and wash with hot water until acid reaction has
ceased. Determine both the ammoniacal and amid nitrogen in the soluble,
and the total nitrogen in the insoluble portion, the ammoniacal by
distillation with magnesia, and the amid and total with soda-lime.
_Example._ A soil contained 0.1669 per cent total nitrogen. Of this
there were obtained:
As ammoniacal nitrogen 13.7 per cent
In the soluble part as amid nitrogen 56.2 „ „
In the insoluble part, total nitrogen 29.7 „ „
————
Sum 99.6 „ „
_Treatment of the Insoluble Portion._—Treat the part insoluble in
hydrochloric acid with a three per cent solution of potash on a
steam-bath for thirty hours. Estimate the nitrogen remaining insoluble,
from which the part dissolved can be determined by difference. The
potash will dissolve usually about two-thirds of the remaining nitrogen.
About ninety per cent of the total nitrogen present in an arable soil
will be rendered soluble by successive treatment with acid and alkali.
The reverse treatment will give practically the same result. It is
therefore immaterial, from an analytical standpoint, whether the acid or
alkali be used first.
=409. Estimation of Volatile Nitrogenous Compounds Emitted by Arable
Soil.=—The following method, due to Berthelot and André,[272] may be
practiced:
Porcelain pots, containing one kilogram of soil, are placed under
bell-jars of fifty liters capacity adjusted to glass dishes designed to
receive the waters of condensation.
During the first period the pots are to be sprinkled from time to time,
during the duration of the experiment, through the upper tubulature, so
as to prevent the soil from becoming dry. The water is partly condensed
on the sides of the bell-jar. It is removed each week through the
inferior tubulature, treated with a little dilute sulfuric acid, and
preserved for further study. A small vessel containing dilute sulfuric
acid is placed near the porcelain pot for the purpose of collecting, as
far as possible, the evolved ammonia.
During the second period the pots are not sprinkled, the soil becomes
dry and there is no longer any condensation of water on the walls of the
bell-jar. The two periods should include about five months, from May to
October.
At the end of the second period the following determinations are to be
made:
1. The ammonia absorbed by the dilute sulfuric acid.
2. The ammonia set free by distillation with magnesia, such as may have
accumulated in the condensed water.
3. The organic nitrogen contained in the latter after elimination of the
ammonia. This is determined by adding a slight excess of acid,
evaporation to dryness, and combustion with soda-lime, or by moist
combustion with sulfuric acid.
Example: