different element, and the direct or indirect measurement of the evolved
substance. The quantitative evolution of chlorin on treating a nitrate
with hydrochloric acid, the collection of the chlorin in potassium
iodid, and the determination of the iodin set free, form a process
belonging here.
=457. Relative Merit of Methods.=—The processes mentioned in the
classifications embraced under numbers (1) and (5) of the preceding
schedule are sufficiently described in the paragraphs devoted thereto,
under soil and fertilizers. In practice at the present time it is rare
to determine the nitrogen in nitrates by the copper oxid method. The
more rapid and equally exact processes of colorimetric comparison or
reduction by nascent hydrogen are in all respects to be preferred.
The indigo methods among the colorimetric processes are not so much in
use now as those which depend on the development of a color. Lawes and
Gilbert considered them far inferior to the Schloesing method. The
developed color methods are especially delicate and are to be preferred
in all cases where the detection of the merest traces of nitrates is
desired. Where nitrates are present in considerable quantities the
reduction method with nascent hydrogen is to be preferred over all
others. In all these cases the judgment of the analyst must be
exercised. The particular method to be employed in any given case can
not be determined save by the intelligent discrimination of the
operator.
=458. The Extraction of Nitric Acid from the Soil.=—The easy solubility
of nitric acid and of nitrates in water is taken advantage of in the
separation of these bodies from the soil. A convenient quantity, usually
about 1,000 grams of the fine soil, is taken for the extraction. Instead
of freeing the soil entirely from water, it is better to determine the
amount of water in the air-dried or prepared sample, and base the
calculation on 1,000 grams of the water-free soil.
All samples of soil, when taken for the purpose of examining for
nitrates, should be rapidly dried to prevent the process of
nitrification from continuing after the sample is taken. For this
purpose the soil should be placed in a thin layer in a warm place,
50°–60°, until air-dried. It still contains in this case a little
moisture but not enough to permit nitrification to go on.
One thousand grams of soil prepared as above are treated with 2,000
cubic centimeters of distilled water, free of nitric acid, and allowed
to stand for forty-eight hours with frequent shaking. One thousand cubic
centimeters of the extract are then filtered, corresponding to 500 grams
of the dry soil. A small quantity of pure sodium carbonate should be
added to the filtrate which is then evaporated on a water-bath to a
volume of about 100 cubic centimeters. Should a precipitate be formed
during evaporation it should be separated by filtration, the filter
washed thoroughly, and the filtrate again evaporated to a volume of 100
cubic centimeters.
In taking a soil for the determination of nitrates, it is well to extend
the sampling to a considerable depth. If the sample be taken only to the
depth of nine inches, it should be in dry weather when the nitrates are
near the surface.
The temperature at which a soil is dried has also an influence on the
quality of nitric nitrogen remaining after desiccation.
If a wet soil be dried at 100°, the nitrates present will suffer partial
decomposition. This is probably due to deoxidation by organic matter
present. On the other hand, ordinary air-drying affords opportunity for
continued nitrification, thus increasing the residuum of oxidized
nitrogen. The above method is essentially that followed by Warington at
Rothamstead.
The method of drying practiced at Rothamstead, in order to secure
results as nearly accurate as possible is the following:[293]
The soil is broken up directly after it is taken from the field, and
spread on trays in layers one inch deep. The trays are then placed in a
room at 55°. The drying is completed in twenty-four hours. After drying,
stones and roots are removed, and the soil is finely powdered and placed
in bottles.
For extracting the nitrates, a funnel is prepared by cutting off the
bottom from a bottle four and a half inches in diameter. A nicely
fitting disk of copper gauze is placed in the bottom of this funnel, and
this is covered with two filter papers, the upper one having a slightly
greater diameter than the lower. The paper is first moistened, and then
from 200 to 500 grams of the dry powdered soil introduced. The funnel is
connected with the receiving flask of a filter pump, and pure water
poured on the soil until it is thoroughly saturated. The water is then
added in small quantities. When the filtrate amounts to 100 cubic
centimeters the process may be discontinued, since all the nitrates in
the soil will be found in this part of the filtrate.
The extract obtained above is evaporated to convenient bulk for the
determination of nitric nitrogen.
THE NITRIC OXID PROCESS.
=459. Method of Schloesing.=—The processes for estimating nitrogen by
combustion with copper oxid and by moist combustion with sulfuric acid
have both been used for the determination of the quantity of nitrogen
existing in a highly oxidized state. These processes will be fully
discussed under the head of fertilizers. In the case of soil extracts,
drainage waters, etc., it will be sufficient to discuss, for the
present, only those processes adapted especially to a quick and accurate
estimation of oxidized nitrogen.
The principle of the method of Schloesing depends on the decomposition
of nitrates in the presence of a ferrous salt and a strong mineral
acid.[294] The nitrogen in the process appears as nitric oxid, the
volume of which may be directly measured, or it may be converted into
nitric acid and titrated by an alkali.
The typical reactions which take place are represented in the following
equation:
6FeCl₂ + 2KNO₃ + 8HCl = 3Fe₂Cl₆ + 2KCl + 4H₂O + 2NO.
=460. Schloesing’s Modified Method.=—The Schloesing method as now
practiced by the French chemists is conducted in the apparatus shown in
Fig. 75.[295] The carbon dioxid is generated by the action of the
hydrochloric acid in F on the fragments of marble in A. After passing
the wash-bottle the gas enters the small tubulated retort, C, which
contains the nitrate in solution. For ordinary soils 100 grams are
placed in an extraction flask, plugged with cotton, and a layer of the
same material is placed over the soil for the purpose of securing an
even distribution of the extracting liquid. This liquid is distilled
water containing in each liter one gram of calcium chlorid. The purpose
of using the calcium chlorid is to prevent the soil from becoming
compacted which would render the extraction of the nitrate difficult.
The extracting liquid is allowed to fall drop by drop from a mariotte
bottle until the filtrate amounts to 500 cubic centimeters. This volume
is concentrated on a sand-bath until it is reduced to ten or fifteen
cubic centimeters when it is transferred to a flat-bottomed dish and the
evaporation finished over steam, care being taken not to allow the
temperature to exceed 100°.
[Illustration:
FIGURE 75. SCHLOESING’S APPARATUS FOR NITRIC ACID.
]
Another and more rapid method for dissolving the nitrate, may also be
practiced. In a flask holding about one liter, place 220 grams of the
soil and 660 cubic centimeters of distilled water and shake vigorously,
or enough water to make 660 cubic centimeters together with the moisture
remaining in the air-dried sample taken. All the nitrates pass into
solution. Throw the contents of the flask into a filter and take 600
cubic centimeters of the filtrate which will contain all the nitrates in
200 grams of the sample taken. This filtrate is evaporated as described
above.
In the flat dish containing the dried nitrates, pour three or four cubic
centimeters of ferrous chlorid solution and stir with a small glass rod
until complete solution of the nitrate takes place. By means of a small
funnel the solution is poured into C, and the capsule and funnel are
well rinsed with two cubic centimeters of hydrochloric acid. The washing
is repeated three times as above described, and once with one cubic
centimeter of water, which is added cautiously so as to form a layer
over the surface of the heavier liquid. The tubulated flask is then
connected with the carbon dioxid apparatus, previously freed from air,
and the gas allowed to flow evenly until the whole of the apparatus is
completely air-free. The other details of the method are essentially the
same as those adopted by the Commission of French Agricultural Chemists
which will be given below.
=461. The French Agricultural Method.=—The Schloesing method as
practiced by the French agricultural chemists is very slightly different
from the procedure just described.[296] The process with soils is
carried on as follows:
Five hundred grams of the soil are taken and introduced into a flask of
about two liters capacity and shaken thoroughly with a liter of
distilled water. The whole of the nitrates of the soil is thus passed
into solution. The solution is filtered and 400 cubic centimeters of the
filtrate are taken, which correspond to 200 grams of the soil. This
liquid is evaporated in a flask, adding a fragment of paraffin to
prevent foaming, until its volume is reduced to fifteen or twenty cubic
centimeters. It is afterwards transferred through a filter into a
capsule with a flat bottom in which the evaporation is finished on a
steam-bath, taking care that the temperature does not exceed 100°. An
important precaution is, not to allow the contact of the water with the
soil to be too prolonged, to avoid the reduction of the nitrates which
could take place under the influence of the denitrifying organisms which
are developed with so great a rapidity in moist earth. The apparatus in
which the transformation of the nitrates into nitric oxid takes place is
essentially that already described (Fig. 75). The carbon dioxid
generator is connected by means of a rubber tube and a small wash-bottle
to the small retort in which the reaction takes place, and from which
the exit tube leads to a mercury trough. The gas which is disengaged is
received under a jar drawn out to a fine point in its upper part, which
carries about fifteen cubic centimeters of potash solution containing
two parts of water to one of potash.
The operation is conducted as follows:
Into the small capsule which contains the dried matter, three or four
cubic centimeters of ferrous chlorid are poured. By means of a stirring
rod the residue sticking to the sides of the capsule is detached with
care and all the matter is thus collected in the bottom. By means of a
small funnel the contents of the capsule are introduced into the retort.
About two cubic centimeters of hydrochloric acid are used for washing
out the materials and this acid is also introduced into the retort. The
washing with hydrochloric acid is repeated three or four times, and
finally the apparatus is washed with one cubic centimeter of water,
which is also poured in by the small funnel with great care, so that
this water may form a layer over the surface of the liquid. The
apparatus is now connected and filled completely with carbon dioxid.
Since it is necessary that this gas should be completely free of air,
the flask, which generates it, is first filled with the acidulated water
from the acid flask, and the air is thus almost totally displaced by the
liquid. The evolution of carbon dioxid gas which follows, completely
frees the apparatus from air. When this is accomplished the retort is
connected with the rest of the apparatus and the gas allowed to pass for
about two minutes until the air is completely driven out of all the
connections. The current is arrested for a moment by pinching the rubber
tube which conducts the carbon dioxid into the retort, and the vessel
which is to receive the gas is then placed over the delivery-tube, this
vessel being filled with mercury and a strong solution of potash. The
communication between the retort and the carbon dioxid flask is broken
and the flask is heated slightly by means of a small lamp. The first
bubbles of gas evolved should be entirely absorbed by the potash. This
will be an indication of the complete absence of the air. When the
liquid is in a state of ebullition the nitrogen dioxid is set free. The
boiling is regulated in such a way that the evolution is regular and the
liquid of the retort may not, by a too violent boiling, pass into the
receiver. The boiling is continued until the larger part of the liquid
is distilled and only three or four cubic centimeters remain in the
retort. At this time a few bubbles of carbon dioxid are allowed to flow
through in order to cause to pass into the receiver the last traces of
nitric oxid. The gas received is left for some minutes in contact with
the potash.
Afterward in a small flask, G, the neck of which is drawn out to a fine
point, and carrying a bulb-tube, H, and a piece of rubber tubing, there
are boiled twenty-five or thirty cubic centimeters of water for five or
six minutes in order to drive all the air out of the flask, and while
the boiling is continued the rubber tubing is fastened to the drawn-out
part of the jar containing the nitric oxid. Within the rubber tubing the
drawn-out point is broken and the vapor of water is forced into the jar
and drives before it the solution of potash which has filled the
capillary part of the drawn-out tube. As soon as the point is broken,
the boiling of the flask is stopped and by its cooling the nitric oxid
passes into it. It is necessary to press the rubber tubing with the
fingers in order that the passage of the gas into the flask be not too
rapid. As the solution of potash rises in the bell-jar which contains
the nitric oxid near to the point where the rubber tubing covers its
drawn-out portion, the fingers are removed and a clamp put in their
place. There still remains a little nitric oxid in the flask and to
drive this out it is necessary to introduce five or six cubic
centimeters of pure hydrogen, which are allowed to pass over into the
receiving flask, by releasing the clamp in the same way as the nitric
oxid. The hydrogen being introduced in successive portions, finally
carries all the nitric oxid into the flask without allowing any of the
potash to enter.
The flask containing the nitric oxid is now connected with a reservoir
of oxygen. The oxygen is allowed to enter, bubble by bubble, by means of
cooling the flask by immersion in water. The transformation of nitric
oxid into nitric acid is not entirely complete for twenty-four hours. It
is necessary, therefore, to wait that long after the introduction of the
oxygen before determining the amount of nitric acid produced.
The contents of the flask are placed in a titration-jar, the flask being
washed two or three times and a few drops of tincture of litmus being
added. The nitric acid is then determined by a standard solution of
calcium hydroxid or some other standard alkali. From the titration the
content of nitric acid is calculated.
The French Committee further suggests that this method may be modified
in the way of making it more rapid by collecting the nitric acid in a
graduated tube filled with mercury and containing some potash. The
volume of the gas is determined and the pressure of the barometer and
the temperature observed, and the usual calculations made to reduce the
volume to zero and to a pressure of 760 millimeters of mercury. Each
cubic centimeter of nitric oxid thus measured corresponds to 2.417
milligrams of nitric acid. The presence of organic matter does not
interfere with the determination of nitric acid by either of the methods
given above.
=462. Modification of Warington.=—The method of procedure and
description of apparatus used, as employed by Warington, are as follows:
The vessel in which the reaction takes place is a small tubulated
receiver, A (Fig. 76), about four centimeters in diameter, mounted and
connected as shown in the illustration. The delivery-tube dips into a
jar of mercury in a trough containing the same liquid. The long supply
funnel-tube _a_ is of small bore, holding in all only one-half cubic
centimeter. The connecting tube F, carrying a clamp, is also of small
diameter and serves to connect the apparatus with a supply of carbon
dioxid.
[Illustration:
FIGURE 76. WARINGTON’S APPARATUS FOR NITRIC ACID.
]
In practice, the supply-tube _a_ is first filled with strong
hydrochloric acid and carbon dioxid passed through the apparatus until
the air is all expelled. This is indicated when a portion of the gas
collected over the mercury, is entirely absorbed by caustic alkali.
At this point the current of carbon dioxid is stopped by the clamp C,
and a bath of calcium chlorid, B, heated to 140° is brought under the
bulb A, until the latter is half immersed therein. The temperature of
the bath is maintained by a lamp. By allowing a few drops of
hydrochloric acid to enter the receiver, the carbon dioxid is almost
wholly expelled. The end of the delivery-tube is then connected with the
tube, T, filled with mercury, and the apparatus is ready for use.
The nitrate, in which the nitric acid is to be determined, in a dry
state, is dissolved in two cubic centimeters of the ferrous chlorid
solution (one gram of iron in ten cubic centimeters), one cubic
centimeter of strong hydrochloric acid is added, and the whole is then
introduced into the receiver through the supply-tube, being followed by
successive rinsings with hydrochloric acid, each rinsing not exceeding
one-half cubic centimeter. The contents of the receiver are, in a few
moments, boiled to dryness; a little carbon dioxid is admitted before
dryness is reached, and again afterwards to drive over all remains of
nitric oxid. In the recovered gas the carbon dioxid is first absorbed by
caustic potash, and afterwards the nitric oxid by ferrous chlorid. All
measurements of the gas are made in Frankland’s modification of
Regnault’s apparatus. The carbon dioxid should be as free as possible
from oxygen. The carbon dioxid generator is formed of two vessels, the
lower one consisting of a bottle with a tubule in the side near the
bottom; this bottle is supported in an inverted position and contains
the marble from which the gas is generated. The upper vessel consists of
a similar bottle standing upright and containing the hydrochloric acid
required to act on the marble. The two vessels are connected by a glass
tube passing from the side tubule of the upper vessel to the inverted
mouth of the lower vessel. The acid from the upper vessel thus enters
below the marble. Carbon dioxid is generated and removed at pleasure by
opening a stop-cock attached to the side tubule of the lower vessel thus
allowing hydrochloric acid to descend and come in contact with the
marble. A good Kipp’s generator of any approved form may also be used
instead of the simple apparatus, above described.
The fragments of marble used are previously boiled in water in a strong
flask. After boiling has proceeded for some time, a rubber stopper is
fixed in the neck of the flask and the flame removed. Boiling will then
continue for some time in a partial vacuum.
The hydrochloric acid is also well boiled and has dissolved in it a
moderate quantity of cuprous chlorid. As soon as the acid has been
placed in the upper reservoir, it is covered by a layer of oil. The
apparatus being thus charged is at once set in active work by opening
the stop-cock of the marble reservoir; the acid descends, enters the
marble reservoir, and the carbon dioxid produced drives out the air. As
the acid reservoir is kept on a higher level than the marble reservoir,
the latter is always under internal pressure, and leakage of air from
without, into the apparatus, cannot occur.
The presence of the cuprous chlorid in the hydrochloric acid not only
insures the removal of dissolved oxygen, but affords an indication to
the eye of the maintenance of this condition. While the acid remains of
an olive tint, oxygen is absent; but should the color change to a
blue-green, more cuprous chlorid must be added. All the reagents
employed should be previously boiled.
In order to secure absolute freedom from air, the following
modifications on the above process have been adopted by Warington: The
apparatus having been mounted as described, the funnel-tube attached to
the bulb retort is filled with water, and the apparatus connected with
the carbon dioxid generator. Carbon dioxid is then passed through the
apparatus until a moderate stream of bubbles rises in the mercury
trough. The stop-cock is left in this position, and the admission of gas
is controlled by the pinch-cock. The bath of calcium chlorid is so
adjusted as to cause the bulb retort to be almost entirely submerged,
and the temperature of the bath is kept at 130° to 140°. Small
quantities of water are next admitted into the bulb and expelled as
steam in the current of carbon dioxid, the supply of this gas being shut
off before the evaporation is entirely completed, so as to leave as
little carbon dioxid as possible in the apparatus. Previous to very
delicate experiments it is advisable to introduce through the
funnel-tube a small quantity of potassium nitrate, ferrous chlorid, and
hydrochloric acid, rinsing the tube with the latter reagent. Any trace
of oxygen remaining in the apparatus is then consumed by the nitric oxid
formed; and after boiling to dryness and driving out the nitric acid
with carbon dioxid, the apparatus is in a perfect condition for a
quantitative experiment.
=463. Preparation of the Materials to be Analyzed.=—According to
Warington, soil extracts may be used without other preparation than
concentration.
Vegetable juices which coagulate when heated, require to be boiled and
filtered or else evaporated to a thin sirup, treated with alcohol, and
filtered. A clear solution being thus obtained, it is concentrated over
a water-bath to a minimum volume in a beaker of small size. As soon as
cool, it is mixed with one cubic centimeter of a cold saturated solution
of ferrous chlorid and one cubic centimeter of hydrochloric acid, both
reagents having been boiled and cooled immediately before use.
In mixing with the reagents, care must be taken that bubbles of air are
not entangled, which is apt to occur with viscid extracts.
The quantity of ferrous chlorid mentioned is amply sufficient for most
soil extracts, but it is well to use two cubic centimeters in the first
experiment, the presence of a considerable excess of ferrous chlorid in
the retort being thus insured. With bulky vegetable extracts more
ferrous chlorid should be employed. To the sirup from twenty grams of
mangel-wurzel sap, five cubic centimeters of ferrous chlorid and two
cubic centimeters of hydrochloric acid are usually added.
=464. Measurement of the Gas.=—The measurement of the gas was for some
time conducted by the use of concentrated potash for absorbing the
carbon dioxid, and ferrous chlorid for absorbing the nitric oxid. The
use of the ferrous chlorid, however, was found to introduce a source of
error. The treatment of the gas with oxygen and pyrogallol over potash
has therefore been substituted by Warington for its absorption by
ferrous chlorid.