=199. Introductory Considerations.=—In the foregoing pages has been
given a summary of the methods most in vogue for the estimation of
nitrogen in fertilizers and fertilizing materials. There are many
cases in which the analyst may have to deal with a definite chemical
compound, and where a modified or shorter method may be used. There
are other cases in which the nitrogen may be present in two or three
definite forms, as in artificially mixed fertilizers, and where it
is desirable to show the proportions in which the various forms are
present. For these reasons it is necessary to be able to use methods by
which the percentage of nitrogen in its various forms may be relatively
as well as absolutely determined. Such a case would be presented for
instance, in that of a fertilizer containing dried blood, sodium
nitrate, and ammonium sulfate. It is evident here that the total
nitrogen could be determined by the volumetric method by combustion
with copper oxid, or by the moist combustion process adapted to
nitric nitrogen, but the method of determining the percentage of each
constituent has not yet been described.
We have to deal here with a case entirely similar to that of phosphoric
acid in a superphosphate. There is no doubt whatever of the uneven
assimilability of the different forms of nitrogen. A nitrate, for
instance, is already in condition for assimilation by plants. An
ammoniacal salt is only partly changed to a state suited to plant
nutrition while organic nitrogen is forced to undergo a complete
transformation before it becomes available to supply the needs of the
growing plant. It is important, therefore, equally to the analyst,
the merchant, and the agronomist, to know definitely the forms of
combination in which the nitrogen exists and the relative proportion of
the different combinations.
=200. Nitrogen as Ammonia.=—The most frequent form in which nitrogen
as ammonia is used for fertilizing is as sulfate. The method of
determination to be described is, however, equally applicable to all
ammonia salts. When no other form of nitrogenous compound is present
the ammonia can be easily and directly determined by distillation
with soda- or potash-lye, as described in the final part of the moist
combustion process.
To one gram of the ammonia salt add from 200 to 300 cubic centimeters
of water and thirty grams of the soda-lye used in the moist combustion
process; distil, collect the ammonia, and titrate the excess of
sulfuric acid exactly as there described.
Fresenius recommends that the ammonia expelled by distillation be taken
up by one-fifth normal sulfuric acid, the excess of which is titrated
with one-fifth normal soda, using phenolphthalein as an indicator.
If the distillate, on examination, be found to contain thiocyanate,
soda-lye cannot be used for the expulsion of ammonia, but, in its
place, caustic magnesia is applied.
In all cases where organic matter containing nitrogen is present,
caustic magnesia must be substituted for the soda solution. The
magnesia must be added in sufficient excess and the distillation
continued a little longer than is necessary when soda-lye is used.
Otherwise the details of the operation are the same.
In a mixed fertilizer containing organic nitrogen and ammonia salts,
the total nitrogen can be determined by the moist combustion process,
and the ammoniacal nitrogen by distillation with magnesia. The
difference between the two results will give the nitrogen due to the
organic matter.
To avoid any danger whatever of decomposing organic nitrogenous
compounds, the ammonia may be determined in the cold by treatment with
soda-lye, under a bell-jar containing some set sulfuric acid. The
operation must be allowed to continue for many days. Even at the end
of a long time it will be found that some ammonia is still escaping.
It may therefore be finally inferred that all the nitrogen as ammonia
is not obtained by this process, or that even magnesia may gradually
convert other nitrogenous compounds into ammonia. In this connection
the methods of determining ammonia in soils =406, 407=, and =408= of
volume one may be consulted.
=201. Method of Boussingault.=—The official French method is
essentially the original method of Boussingault with slight
modifications. It is conducted as follows:[169] In case the sample is
ammonium sulfate about half a gram is placed in a flask of half a liter
capacity, together with 300 cubic centimeters of distilled water and
two grams of caustic magnesia. The flask is connected with a condenser
of glass or metal which ends in a tube drawn out to a point and dipping
beneath the set acid in the receiver in the usual way. The acid is
colored with litmus or lacmoid tincture. The distillation is continued
until about 100 cubic centimeters have gone over. The receiver is then
removed with the usual precautions and the residual acid titrated.
Suppose twenty cubic centimeters of normal acid have been employed and
twelve and a half cubic centimeters of normal alkali be necessary to
neutralize the excess of the acid. Then the nitrogen is found by the
following equations: 20.0 - 12.5 = 7.5 and 7.5 × 0.014 = 0.105 gram =
weight of nitrogen found. Then 0.105 × 100 ÷ 5 = 21.00 = per cent of
nitrogen found.
The distilling apparatus of Aubin is preferred by the French chemists,
an apparatus so arranged with a reflux partial condenser, that nearly
all the aqueous vapor is returned in a condensed state to the flask
while the ammonia, on account of its great volatility, is carried
over into the receiver. To avoid the regurgitation which might be
caused by the concentrated ammonia gas coming in contact with the acid
the separable part of the condensing tube is expanded into a bulb
large enough to hold all the acid which lies above its mouth. By the
means of this apparatus the ammonia is all collected in the standard
acid without greatly increasing its volume and the titration is thus
rendered sharper. The employment of caustic magnesia has the advantage
of not decomposing any organic matters or cyanids that may be present.
If the sample under examination hold part of its ammonia as ammonium
magnesium phosphate it will be necessary first to treat it with
sulfuric acid in order to set the ammonia free and then to use enough
of the magnesium oxid to neutralize the excess of the sulfuric acid and
still supply the two grams necessary for the distillation. When the
sample contains a considerable quantity of organic matter it sometimes
tends to become frothy towards the end of the distillation. This
trouble can be avoided by introducing into the flask one or two grams
of paraffin.
Where carbon dioxid is given off during the distillation the contents
of the receiver must be boiled before titration, or else lacmoid must
be used as an indicator instead of litmus.
=202. Determination of Thiocyanates in Ammoniacal Fertilizers.=—The
extended use of ammonium sulfate as a fertilizer renders it important
to determine the actual constituents which may be present in samples
of this material. The following bodies have been found in commercial
ammonium sulfates: Sulfuric acid, chlorin, ammonia, thiocyanic acid,
potash, soda, lime and iron oxid. These are found in the soluble
portions. In the insoluble portions have been found silica, sulfuric
acid, lime, magnesia and iron oxid. A sample of commercial ammonium
sulfate analyzed by Jumeau contained the following substances:[170]
Per cent.
Moisture 10.5109
Ammonium sulfate 67.8453
Ammonium thiocyanate 9.3935
Sodium sulfate 9.2429
Potassium sulfate 0.9774
Calcium sulfate 0.6800
Iron thiocyanate 0.5000
Magnesium chlorid traces
Silica 0.0830
Undetermined 0.7670
The determination of the thiocyanic acid in the thiocyanate is
generally made by the oxidation of the sulfur to sulfuric acid and its
subsequent weighing in the form of barium sulfate. Jumeau has modified
the method by determining the amount of the thiocyanate by means of a
titrated liquid. The method is practiced as follows:
A solution of ammonium thiocyanate is prepared, containing eight
grams of this salt per liter, and its exact content of thiocyanate is
rigorously determined by titration with silver nitrate or by the weight
of the barium sulfate produced after the oxidation of the sulfur.
Ten cubic centimeters of the titrated liquor are taken and diluted
with water to about 100 cubic centimeters and ten cubic centimeters
of pure sulfuric acid added. Afterward, drop by drop, a solution of
potassium permanganate is added, containing about ten grams of that
salt per liter. The permanganate is instantly decolorized. There is an
evolution of hydrocyanic acid as the thiocyanate passes to the state
of sulfuric acid. A single drop in excess gives to the mixture the
well-known rose coloration of the permanganate solution which persists
for several hours. The number of cubic centimeters necessary to produce
the persistent rose tint is noted and the same operation is carried on
with from one-half to one gram of the unknown product which is to be
assayed. A simple proportion indicates the content of the thiocyanate
in the unknown body. The process is of great exactitude and permits the
rapid determination of thiocyanic acid in the presence of chlorids,
cyanids, etc., which remain without action upon the permanganate. In
case chlorids and cyanids are absent the thiocyanate can be determined
directly by silver nitrate either by weighing the precipitate or by
the process of Volhardt based upon the precipitation of the silver by
thiocyanate in the presence of a ferric salt. The end of the reaction
is indicated by the red coloration which the liquid shows when the
thiocyanate is in excess.
=203. Separation of Albuminoid from Amid and Other Forms of Nitrogen
in Organic Fertilizers.=—It may be of interest to the dealer, farmer,
and analyst, to discriminate between the albuminoid and other nitrogen
in fertilizers, such as oil-cakes. The final value of the nitrogen
for plant nourishment is not greatly different, but the immediate
availability for nitrification is a matter of some importance. The most
convenient process in such a case is the copper hydroxid separation
process as improved by Stutzer.[171] The process is conveniently
carried out in accordance with the method prescribed by the official
chemists.[172]
_Total Crude Protein._—Determine nitrogen as directed for nitrogen in
fertilizers and multiply the result by 6.25 for the crude protein.
_Determination of Albuminoid Nitrogen._—To from seven-tenths to
eight-tenths gram of the substance in a beaker add 100 cubic
centimeters of water, heat to boiling, or in the case of substances
rich in starch, heat on the water-bath ten minutes, and add a quantity
of cupric hydroxid mixture containing from one-half to six-tenths
gram of the hydroxid; stir thoroughly, filter when cold, wash with
cold water, and put the filter and its contents into the concentrated
sulfuric acid for the determination of nitrogen. The filter-papers used
must be practically free of nitrogen. Add sufficient potassium sulfid
solution to completely precipitate all copper and mercury, and proceed
as in the moist combustion process for nitrogen. If the substance
examined consist of seed of any kind, or residues of seeds, such as
oil-cake or anything else rich in alkaline phosphates, add a few cubic
centimeters of a concentrated solution of alum just before adding the
cupric hydroxid, and mix well by stirring. This serves to decompose
the alkaline phosphates. If this be not done cupric phosphate and free
alkali may be formed, and the protein-copper may be partially dissolved
in the alkaline liquid.
_Cupric Hydroxid._—Prepare the cupric hydroxid as follows: Dissolve
100 grams of pure cupric sulfate in five liters of water, and add
twenty-five cubic centimeters of glycerol; add a dilute solution
of sodium hydroxid until the liquid is alkaline; filter, rub the
precipitate up with water containing five cubic centimeters of glycerol
per liter, and then wash by decantation or filtration until the
washings are no longer alkaline. Rub the precipitate up again in a
mortar with water containing ten per cent of glycerol, thus preparing
a uniform gelatinous mass that can be measured out with a pipette.
Determine the quantity of cupric hydroxid per cubic centimeter of this
mixture.
_Amid Nitrogen._—The albuminoid nitrogen determined as above subtracted
from the total, gives that part of the organic nitrogen existing in the
sample as amids and in other allied forms.
=204. Separation of Nitric and Ammoniacal from Organic Nitrogen.=—The
nitrogen being present in three forms, _viz._, organic, ammoniacal,
and nitric, the separation of the latter two may be accomplished by
the following procedure:[173] One gram of the fertilizer is exhausted
on a small filter with a two per cent solution of tannin, using from
thirty to forty cubic centimeters in small portions. This is sufficient
to dissolve all the nitrates and the greater portion of the ammoniacal
salts, while the tannin renders insoluble all the organic nitrogenous
compounds. The filter and its contents are treated for nitrogen by the
kjeldahl process. When the distillation and titration are completed
the solution obtained by the aqueous tannin is added to the distilling
flask and the operation continued. This represents the ammoniacal
nitrogen.
The nitric acid is estimated by the ferrous iron or other appropriate
method in another portion of the substance.
This method can be used even when the fertilizer contains ammonium
magnesium phosphate. In this case digest one gram for fifteen hours in
dilute soda-lye solution, which easily dissolves the ammonium magnesium
phosphate. Filter and wash the insoluble part with the tannin solution.
The residue is treated as above. The filtered solution distilled with
soda-lye furnishes the ammonia. The nitrates are estimated by one of
the methods above mentioned.
=205. Nitric Nitrogen.=—The methods of estimating nitric nitrogen,
both when present in weighable quantities and as mere traces have
been sufficiently described in the first volume. For convenience,
however, the standard methods of procedure will be given here. The
moist combustion methods adapted to nitrates and the volumetric copper
oxid process have already been described. Of the reduction methods
the process of Ulsch is one of the easiest of application and also
reliable. As practiced by the official chemists the manipulation is
conducted as described in the first volume, page 539.
=206. Ulsch Method, Applicable to Mixed Fertilizers.=—The method of
Ulsch which is found to give good results with pure nitrates or with
nitrates in the absence of other forms of nitrogen may also be adapted
to mixed fertilizers containing nitrogen in more than one form. Street
has developed such a method and shown, by analytical data, that it is
applicable in a great number of cases.[174] The process is based on the
substitution of magnesia for soda in the distillation and is carried on
as follows:
Place one gram of the sample in a half liter flat-bottomed flask. Add
about thirty cubic centimeters of water, one gram of reduced iron, and
ten cubic centimeters of sulfuric acid diluted with an equal volume
of water, shake well, and allow to stand for a short time. This will
remove the danger of an explosion caused by the otherwise violent
action which takes place. Close the neck of the flask with a rubber
stopper through which passes a glass dropping-bulb filled with water.
The flask having been stoppered, place it on a slab to which a moderate
heat is applied. Allow the solution to come slowly to a boil and then
boil for five minutes and cool. Add about 100 cubic centimeters of
water, a little paraffin, and about five grams of magnesium oxid. Boil
for forty minutes, after which time all the ammonia will be distilled,
and collect the ammonia in set acid.
The magnesia causes a slight frothing, which can easily be controlled
by adding a little paraffin and by bringing to a boil very gradually.
Fully forty minutes are necessary to distil all the ammonia. Tests were
made after thirty minutes boiling and traces of ammonia were still
found; after forty minutes these traces entirely disappeared.
The method is a quick one. One man can easily do six determinations at
a time, and these six determinations can be made in but a little over
an hour. Magnesia gives results closely agreeing with theory and causes
a very slight frothing, which can be easily controlled. One gram of
reduced iron is sufficient in all ordinary complete fertilizers.
Magnesia is preferred to caustic soda in the distillation because it
produces less frothing and by reason of the danger of some of the
soda-lye being carried over mechanically and thus tending to produce
an error of a plus nature. In the use of magnesia, assurance must be
had that it is strongly in excess. Being less active in its effects
a longer time for the distillation must be taken than when soda-lye
is used. The modified ulsch method just described is recommended
provisionally and with the expectation that each analyst will ascertain
its true merits before allowing it to displace longer approved
processes.
=207. Method of Schlösing-Wagner.=—The Schlösing-Wagner method for
estimating nitrogen in the nitrates of fertilizers is carried out at
the Halle Experiment Station as follows:[175]
A flask, figure 14, of about 250 cubic centimeters capacity, is
provided with a rubber stopper with two holes. Through one of them
is passed the stem of a funnel carrying a glass stop-cock. The other
carries a delivery-tube leading to the receiving vessel. The end of
the delivery tube is bent so as to pass easily under the mouth of the
measuring burette and is covered with a piece of rubber tubing.
Fifty cubic centimeters of saturated ferrous chlorid solution and the
same quantity of ten per cent hydrochloric acid are placed in the
flask. The ferrous chlorid solution is obtained by dissolving nails
or other small pieces of iron in hot hydrochloric acid and it is kept
in glass stoppered flasks, of about fifty cubic centimeters capacity,
entirely filled. The content of one flask is enough for about twelve
determinations and by using the whole content of a flask as soon as
possible after opening, any danger of oxidation which would take place
in a large flask frequently opened is avoided.
[Illustration: FIGURE 14. SCHLÖSING-WAGNER APPARATUS.]
The contents of the flask are boiled until all the air is driven off.
The delivery-tube is then placed under the measuring-tube, which is
filled with forty per cent potash-lye. The measuring-tube is previously
almost filled with potash-lye and then a few drops of water added and
the tube covered with a piece of filter-paper. By a careful and quick
inversion the measuring-tube can be brought into the vessel receiving
it without any danger of air entering. The boiling is continued for
some time and when no more air escapes, the end of the delivery-tube is
brought into another freshly filled measuring-tube and the estimation
is commenced.
Ten cubic centimeters of a normal saltpeter solution, containing two
and a half grams of pure sodium nitrate in 100 cubic centimeters are
placed in the funnel and, with continued boiling, allowed to pass,
drop by drop, into the flask. When almost all has run out the funnel
is washed three times with ten cubic centimeters of ten per cent
hydrochloric acid and this is allowed to pass, drop by drop, into
the flask. When no more nitric oxid is evolved the measuring-tube is
transferred to a large jar filled with distilled water.
The solution of the substance to be examined should be taken in such
quantity as will give about the same quantity of gas as is furnished
by the ten cubic centimeters test nitrate solution before described;
_viz._, about seventy cubic centimeters. Eight or ten determinations
can be made, one following the other, and at the end another
determination with normal sodium nitrate solution should be made as
a check. At the end of the operation all of the measuring-tubes are
in the large jar filled with distilled water. The temperature of the
surrounding water will soon be imparted to the contents of each tube
and the volume of nitric oxid is read by bringing the level within
and without the measuring-tube to the same point. The percentages are
calculated for the given temperature and barometer pressure in the
usual way; or to avoid computation the volume can be compared directly
with the volume furnished by a normal nitrate solution, which is a much
simpler method.
=208. Schmitt’s Modified Method.=—The method is a modification of that
already described by the author in which a mixture of powdered zinc
and iron is used as a reducing agent.[176] The process is carried out
as follows: Ten grams of the nitrate are dissolved and the volume made
up to half a liter. Ten cubic centimeters of glacial acetic acid and
ten grams of the fine metallic powder, iron and zinc, are placed in a
flask of a capacity of about three-quarters of a liter and twenty-five
cubic centimeters of the solution of the nitrate added. The flask is
covered during the reduction to prevent loss by spraying, and after
the solution is complete, which is the case in about ten minutes,
the contents of the flask are diluted with from 200 to 300 cubic
centimeters of water, thirty cubic centimeters of caustic soda of 1.25
specific gravity added, and the whole distilled as in the kjeldahl
process. It must be noted that it is essential that the iron be finely
divided; it is mixed with the powdered zinc in equal parts. The total
nitrogen can be determined in guanos and nitrate mixtures by the
following simple alteration in procedure: One gram of the substance is
dissolved in water, five cubic centimeters of glacial acetic acid, and
from two to three grams of the mixed metallic powder added, and the
whole gently heated for ten or fifteen minutes. After the contents of
the flask have cooled, twenty-five cubic centimeters of sulfuric acid
are cautiously added in small portions, undue frothing being restrained
by the addition of a fragment of paraffin wax. The acetic acid is then
driven off by heating, and the remaining contents of the flask boiled
until the organic matter is completely decomposed, as in the kjeldahl
process. About two hours boiling is required. Neutralization and
distillation are then practiced as in the ordinary manner. The method
is also applicable to the determination of nitrates in drinking water,
provided nitrites and ammonia be absent.
=209. Krüger’s Method for Nitric Acid.=—About three-tenths gram of the
substance dissolved in water is mixed with twenty cubic centimeters of
a hydrochloric acid solution of stannous chlorid holding 150 grams of
tin per liter.[177] One and a half grams of spongy tin prepared by the
action of zinc on stannous chlorid are added. The flask containing the
mixture is heated until the tin is dissolved, by which time the nitric
acid is completely reduced. The subsequent distillation and titration
are accomplished as usual. In the case of nitro and nitroso compounds,
after the solution of the tin, twenty cubic centimeters of sulfuric
acid are added and heated until sulfuric vapors escape. After cooling,
the amido substances formed are oxidized by potassium bichromate before
the distillation takes place.
Krüger also estimates the nitrogen in benzol, pyridin, and chinolin
derivatives by dissolving them in sulfuric acid, using from two-tenths
to eight-tenths of a gram of the alkaloidal bodies and, after
cooling the solution, oxidizing by adding finely powdered potassium
bichromate.[178] About half a gram more of the potassium bichromate
should be used than is necessary for the oxidation of the substances in
solution. The entire oxidation does not consume more than from fifteen
to thirty minutes.