together. In the second class the reduction is accomplished first and
the distillation effected afterwards, with the addition of an alkali. In
the third class of operations the reduction is accomplished by means of
an electric current and the ammonia subsequently obtained by
distillation, or determined by nesslerizing. These processes may be
applied to rain and drainage waters, and to soil extracts. On account of
the ease with which the analyses are accomplished, the short time
required and the accuracy of the results, the reduction methods for
nitrates have already commended themselves to analysts, and are quite
likely to supersede all others for practical use where weighable
quantities of nitrates are present. For the minute traces of nitrates
found in rain and drainage waters, and in some soil extracts, the
reduction method may also be applied, but in these cases the ammonia
which is formed must be determined by colorimetry (nesslerizing) and not
by distillation. The processes about to be described are especially
applicable to the examination of soils and waters rich in nitrates.
REDUCTION IN ALKALINE SOLUTIONS.
=481. Provisional Method of the Association of Official Agricultural
Chemists.=[308]—
_Extraction of the Nitrates._—Place one kilogram of the dried soil,
calculated to water-free substance, on a percolator of glass or tin.
Moisten the soil thoroughly with pure distilled water, and allow to
stand for half an hour. Add fresh portions of pure distilled water until
the filtrate secured amounts to one liter. If the first filtrate be
cloudy before use it may be refiltered.
_Qualitative Test for Nitrates._—Evaporate five cubic centimeters of the
soil extract in a porcelain crucible, having first dissolved a small
quantity of pure brucin sulfate therein. When dry, add to the residue a
drop of concentrated sulfuric acid free of nitrates. If the nitrate
calculated as potassium nitrate does not exceed the two-thousandth part
of a milligram only a pink color will be developed; with the
three-thousandth part of a milligram a pink color with reddish lines;
with the four-thousandth part of a milligram a reddish color; with the
five-thousandth part of a milligram a distinct red color.
_Estimation of the Nitrates._—Evaporate 100 cubic centimeters of the
soil extract to dryness on a steam-bath. Dissolve the soluble portions
of the residue in 100 cubic centimeters of ammonia-free distilled water,
filtering out any insoluble residue. Place the solution in a flask, add
ten cubic centimeters of sodium amalgam, stopper the flask with a valve
which will permit the escape of hydrogen, and allow to stand in a cool
room for twenty-four hours. Add fifty cubic centimeters of milk of lime
and titrate the ammonia produced by distillation, with standard acid and
estimate as nitrogen pentoxid. Where the amount of ammonia is small,
nesslerizing may be substituted for titration.
_Preparation of Sodium Amalgam._—Place 100 cubic centimeters of mercury
in a flask of half a liter capacity; warm until paraffin will remain
melted over the surface; drop successively in the paraffin-covered
mercury, pieces of metallic sodium of the size of a pea until 6.75 grams
have united with the mercury. The amalgam contains then 0.5 per cent of
metallic sodium and may be preserved indefinitely under the covering of
paraffin.
=482. Method of the Experiment Station at Möckern.=[309]—The principle
of this reaction is based on the reducing action exercised by nascent
hydrogen on a nitrate, the hydrogen being generated by the action of
soda-lye on a mixture of zinc dust and finely divided iron.
Ten grams of nitrate are dissolved in 500 cubic centimeters of water. Of
this solution twenty-five cubic centimeters, corresponding to one-half
gram, are placed in a distillation flask of about 400 cubic centimeters
capacity, 120 cubic centimeters of water added, and about five grams of
well-washed and dried zinc dust and an equal weight of reduced iron. To
the solution are added eighty cubic centimeters of soda-lye of 32° B.
The flask is then connected with the condensing apparatus and the
distillation carried on synchronously with the reduction, the ammonia
being collected in twenty cubic centimeters of titrated sulfuric acid.
The distillation is continued from one to two hours, or until 100 cubic
centimeters have been distilled, and the remaining sulfuric acid is
titrated in the usual way. Soil extracts and sewage waters should be
concentrated until they have approximately the proportion of nitrates
given above.
=483. Method of Devarda.=—The inconvenience due to slow action and other
causes, arising from the use of pure metals in the reduction of nitrates
to ammonia, has been overcome, to some extent, by Devarda, by use of an
alloy, in a state of fine powder, consisting of aluminum, copper, and
zinc.[310] The alloy consists of forty-five per cent of aluminum, fifty
per cent of copper, and five per cent of zinc. In dissolving, the copper
is left in a finely divided state, which is a great help in distillation
in preventing bumping.
The analytical process is carried out as follows: The solution
containing the nitrate, in quantity equivalent to about one-half gram of
potassium nitrate, is placed in a flask having a capacity of about one
liter, and diluted with sixty cubic centimeters of water and five cubic
centimeters of alcohol, and then forty cubic centimeters of caustic
potash solution added of specific gravity one and three-tenths. From two
to two and one-half grams of the alloy, described above, are introduced,
and the flask attached to a condenser with a receiver containing
standard acid. The connection between the flask and the condenser is
made by means of a tube having on the limb next the flask a bulb filled
with glass beads to prevent the contents of the flask splashing over
into the receiver, and on the other limb another bulb to prevent the
acid in the receiver finding its way into the distillation flask, should
regurgitation occur. When the flask has been thus connected with the
condenser it is gently heated for half an hour, at the end of which time
the evolution of hydrogen will have slackened or ceased, and then the
distillation is begun, at first cautiously, until the zinc of the alloy
has completely dissolved, and then more vigorously, the time necessary
being about twenty minutes from the time when the contents of the flask
begin to boil. The distillate is caught in standard acid and the ammonia
determined by backward titration in the ordinary way. It is to be noted
that the strength of the alkali used is of importance, as if it be too
strong, the action on the alloy is unduly vigorous at the beginning of
the operation, and if too weak, the contents of the flask have to be
heated overmuch, the result in both cases being the formation of a fine
spray of caustic solution, which is very difficult to stop, even with
complicated washing attachments to the distilling flask. The test
analyses on pure nitrates are satisfactory. This method has been used
with satisfaction in the laboratory of the Department of Agriculture,
but does not appear to have any special advantage over the process of
Ulsch, to be described further on.
=484. Variation of Stoklassa.=—Stoklassa has subjected the method of
Devarda to a comparative test with the following methods:[311]
1. Wagner’s Schloesing-Grandeau method.
2. Lunge’s nitrometer method.
3. Stutzer’s method.
[Illustration:
FIGURE 85. STOKLASSA’S NITRIC ACID APPARATUS.
]
The reduction takes place in a copper erlenmeyer, A, Fig. 85, in which,
in addition to the solution containing the nitrate, are placed 200 cubic
centimeters of water, forty cubic centimeters of potassium hydroxid
solution of 33° B., five cubic centimeters of alcohol, and finally two
and one-half grams of the finely powdered Devarda alloy. The distillate
passes through a tube, B, filled with glass pearls and into the
condenser D, through the bulbs, C C′.
After the flask is connected with the distilling apparatus, it is gently
warmed and the reduction is ended in about twenty minutes. The ammonia
which is formed is then distilled into E, containing the standard acid,
S, requiring about twenty minutes more. The comparative results given,
show that the Devarda method is equally as accurate as any of the other
methods mentioned, giving practically theoretical results.
In so far, however, as speed of an analysis, is concerned the first
place is awarded to the Lunge nitrometer method, with which a complete
analysis can be made in from thirty to forty minutes. In the second
rank, so far as speed is concerned, the Devarda method is recommended.
All the methods give accurate results.
=485. Method of Sievert.=[312]—Two grams of potassium or sodium nitrate
are dissolved and made up to 1,000 cubic centimeters. Fifty cubic
centimeters of the solution are placed in a 600 cubic centimeter flask
and diluted with fifty cubic centimeters of water, and from eighteen to
twenty grams of caustic alkali added. After the alkali is dissolved,
seventy-five cubic centimeters of ninety-six per cent alcohol are added
and a few pieces of bone-black to prevent foaming. From ten to fifteen
grams of zinc or iron dust are then added to the flask which is closed
and connected with a =ᥩ= tube holding about 200 cubic centimeters, which
contains about ten cubic centimeters of normal sulfuric acid. This =ᥩ=
tube is kept cool by being immersed in water. The whole mixture is now
allowed to stand for three or four hours and then the alcohol is
distilled slowly and the ammonia formed by the reduction of the nitrates
is carried over with it. The distillation lasts for about two hours. The
contents of the =ᥩ= tube are carefully rinsed into a dish and the excess
of sulfuric acid titrated with one-fourth normal soda-lye.
For soil extracts and substances containing unknown quantities of nitric
acid, a preliminary test will indicate approximately the amount thereof,
and this will be an indication for the quantity to be used in the
analysis.
The method of Stutzer differs from the foregoing in the substitution of
aluminum dust instead of iron or zinc.[313] The reducing power of
aluminum, however, varies greatly according to the method in which the
metal has been prepared. Pure aluminum prepared by the electric method,
reduces the nitric acid much less vigorously than the metal prepared by
the older methods of fusion with sodium. For this reason the method of
Stutzer is not to be preferred to that of Sievert.
REDUCTION IN AN ACID SOLUTION.
=486. Variation of the Sodium Amalgam Process.=—This method is described
by Monnier and Auriol.[314]
[Illustration:
FIGURE 86. VARIATION OF THE SODIUM AMALGAM PROCESS.
]
The principle of the operation depends on the reduction of the dissolved
nitrate by titrated sodium amalgam in presence of an acid, and the
estimation of the quantity of nitric acid present from the deficit in
the volume of hydrogen. The apparatus employed is conveniently mounted
as shown in Fig. 86. The brass vessel A is movable by means of the cord
on the pulley B, in such a way as to be fixed at any required altitude.
It is filled with water and connected by a rubber tube to the cooling
tube D. Within the cooling tube there is a graduated cylinder open at
its lower end. Its upper end is connected directly with the apparatus C.
The cooling tube D has a small side tube, _c_, near its upper end, by
means of which the air can enter or escape when the position of A is
changed. The apparatus C, in which the reaction takes place, is a glass
cylinder. Its upper end is continuous with the =⟙= tube provided with
the stop-cocks _a_ and _b_. One arm of the =⟙= permits connection with
the graduated measuring tube by means of a rubber union. The lower end
of C is closed with a large hollow ground-glass stopper, carrying a
small receptacle within, so that it forms two separate water-tight
compartments, open at the top.
The sodium amalgam is prepared as follows:
In a clay crucible are heated 400 grams of mercury, and, little by
little, with constant stirring, four grams of dry sodium are added. When
cold, the amalgam is placed in a burette, having a ground-glass stopper,
and covered with petroleum. The strength of the amalgam is established
in the following manner. A small glass thimble, ground even at the top,
is filled with the amalgam and struck off even with a ground-glass
straight edge. In this way the same quantity of amalgam is taken for
each test. This measured portion of the amalgam is placed in the inner
vessel of the glass stopper to C. Ten cubic centimeters of water,
containing sixty centigrams of tartaric acid, are placed on the outer
ring of the glass stopper, which is then inserted, well oiled, in C,
closing it air- and water-tight. The tartaric acid solution also carries
a piece of litmus paper, so that its constant acidity may be insured.
The vessel A is then fixed in a position which brings the water in the
graduated burette and tube D exactly to the 0 mark. The cock _a_ is next
closed, _b_ opened, and C is inverted until all the amalgam is poured
into the solution of tartaric acid. The evolved hydrogen mixed with the
air contained in the apparatus, is passed into the graduated burette.
After fifteen minutes, the reaction is ended. The water level within and
without the graduated tube is restored and the volume of gas evolved
noted and reduced by the usual tables to 0° and 760 millimeters pressure
of the barometer.
An amalgam prepared as above will give about three cubic centimeters of
hydrogen for each gram. The thimble should hold from twelve to fifteen
grams.
The estimation of nitric acid should be made in a solution containing
about one-tenth per cent of nitrate. Ten cubic centimeters are taken, to
which six-tenths gram of tartaric acid is added, and placed in the outer
part of the glass stopper. The rest of the process is conducted exactly
as described above. The deficit in hydrogen is calculated to nitrogen
pentoxid.
The reduction by sodium amalgam is not so convenient a form of
estimating nitric acid as many of the other forms of using nascent
hydrogen. As practiced by calculating from the deficit of hydrogen,
however, it has some advantages by reason of the fact that no heating is
required. The presence of organic neutral bodies, or even those of an
acid nature, like humus, does not, therefore, interfere with the work.
Likewise, mineral bodies in solution, which are not reduced by nascent
hydrogen, do not interfere with the accuracy of the reaction.
=487. Method of Schmitt.=—In the method of Schmitt forty cubic
centimeters of glacial acetic acid are placed in a flask of 600 cubic
centimeters content, and fifteen grams of a mixture of zinc and iron
dust added.[315] To this a quantity of the solution containing the
nitrate, representing about half a gram of the pure nitrate, is added
with constant shaking, in portions which do not evolve hydrogen too
rapidly. After about fifteen minutes when the evolution of nitrogen has
somewhat diminished, an additional fifteen grams of the metal dust are
added. If the contents of the flask should become thick they can be
diluted with thirty cubic centimeters of water. The reduction is
complete in from thirty to forty minutes. The contents of the flask are
now saturated with enough soda-lye not only to neutralize the excess of
acetic acid, but to keep the zinc hydroxid also in solution. For this
purpose about 200 cubic centimeters of soda-lye of 1.25 specific gravity
are necessary. The ammonia is obtained by distillation into standard
acid in the usual way.
=488. Method of Ulsch.=—In practice the method of Ulsch has come into
general use.[316]
For the determination of nitrogen by this method half a gram of
saltpeter or four-tenths gram of sodium nitrate is taken and dissolved
in twenty five cubic centimeters of water, in a flask with a content of
about 600 cubic centimeters. Five grams of iron reduced by hydrogen, and
ten cubic centimeters of sulfuric acid diluted with two volumes of water
are then added to the flask. To avoid mechanical losses during the
evolution of hydrogen a pear-shaped glass stopper is hung in the neck of
the flask. After the first violent evolution of hydrogen has passed, the
flask is slowly heated until in about four minutes it is brought to a
gentle boil. The boiling is continued for about six minutes when the
reduction is complete. About fifty cubic centimeters of water are then
added; also an excess of soda-lye and a few particles of zinc and the
ammonia is distilled and collected in standard acid in the usual way.
The method of Ulsch can also be applied, according to Fricke, to the
analysis of nitrates contained in drinking and drainage waters, and it
is regarded by him as one of the best methods to be employed in such
investigations.[317]
The method of Ulsch in this laboratory has given entirely satisfactory
results, and is generally used in preference to other methods in cases
where a considerable quantity of nitrates is present. It is based on the
following reactions:
2KNO₃ + H₂SO₄ = K₂SO₄ + 2HNO₃
2HNO₃ + 8H₂ = 2NH₃ + 6H₂O
2NH₃ + H₂SO₄ = (NH₄)₂SO₄.
REDUCTION BY THE ELECTRIC CURRENT.
=489. Method of Williams-Warington.=—From the losses which naturally
occur during the evaporation of water, even with all the precautions
noted, Warington was led to try some method for the determination of
nitrates and nitrites in waters without previous concentration.[318] The
reduction of these bodies by the copper-zinc couple formed the basis of
these experiments, and they resulted in the following method of
manipulation, which is based on a process devised by Williams.[319]
The method consists in boiling rapidly one liter of the rain water in a
retort, with a little magnesia previously raised to a low red heat and
then washed, until 250 cubic centimeters have distilled over. The
residue is then made up to 800 cubic centimeters, transferred to a
wide-mouthed, stoppered bottle supplied with strips of copper and zinc
forming electric couples, and set aside, at a constant temperature of
from 21°–24°, for three days. A measured portion of the solution is then
distilled, and the ammonia determined in the distillate by nesslerizing.
This plan has two advantages: First, the ammonia, as well as the
nitrogen as nitrates and nitrites, can be determined in the course of
the same operation and in the same sample of water. For this purpose it
is only necessary to fit the retort to an efficient condenser and to
remove all ammonia from the apparatus by boiling distilled water in the
retort before introducing the rain water. The distillate of 250 cubic
centimeters from the rain water, as described above, is well mixed and
the ammonia determined, in from twenty-five to one hundred cubic
centimeters thereof, diluted to 150 cubic centimeters with ammonia-free
water. Second, the nitrogen, as nitrates and nitrites, is determined
directly and alone; the error of the determination is as small as
nesslerizing admits of, since it is possible, if necessary, to distill
600 cubic centimeters of the boiled rain water corresponding to 750
cubic centimeters of the original, and thus obtain a full amount of
ammonia for determination, even when the rain water has been poor in
nitrates.
The determination of nitric nitrogen, in a given sample, by the above
method gave a mean quantity of product of 0.162 part per million, while
the determination, in the same lot of samples, by the modified
Schloesing method gave 0.125 part per million. This result confirms the
supposition that in the complete evaporation necessary to the
manipulation of the Schloesing method there is a loss of nitrogen. The
amount of nitrogen as nitrates and nitrites in the rain water at
Rothamstead, for the twelve months ending April 1, 1888, was found, by
the Schloesing method, to be 0.614 pound per acre, the total rain-fall
being 21.96 inches. For the year ending April 1, 1889, by the
copper-zinc method, it amounted to 0.917 pound per acre, the total
rain-fall being 29.27 inches.
The amounts found in other localities are quite different from the
above, as for instance, the mean of seven stations in Germany for
thirteen years, beginning in 1864, showed 10.18 pounds of nitrogen per
acre. The average amount for ten years at the observatory of Mont
Sauris, near Paris, showed 12.36 pounds of nitrogen per acre. The
average for three years at Lincoln, as determined by Professor G. Gray,
shows one and six-tenths pounds of nitrogen per acre per annum. At
Tokio, in Japan, Kellner found, for one year, 1.02 pounds per acre.
=490. Determination of the Ammonia.=—The method used at Rothamstead is
to make one determination of ammonia in the whole of the distillate
obtained, the strength of which is regulated by varying the amount
introduced into the retort, so that it shall be equal to about two cubic
centimeters of the standard ammonia solution. A 150 cubic centimeter
cylinder is first filled with the rain water, and fifty cubic
centimeters of nessler reagent added. The depth of tint indicates what
quantity of rain water will be required for distillation. This having
been determined, the appropriate volume of the rain water, provided it
do not exceed 600 cubic centimeters, is placed in the retort described
above, and the distillation continued until the 150 cubic centimeter
cylinder is filled. The titration is made in the usual way.
=491. Preparation of the Copper-Zinc Couple.=—For 800 cubic centimeters
of boiled rain water, prepared as described, six strips of zinc foil,
four inches long by one and a quarter inches wide, are taken and bent at
right angles along their center to obtain stiffness. The couple is
cleansed and coated by washing in a series of five beakers containing,
respectively, dilute solution of sodium hydroxid, very dilute sulfuric
acid, a three per cent solution of copper sulfate, ordinary distilled
water, and distilled water free from ammonia. Through these five beakers
the zinc foil is successively passed. It is rinsed both after the alkali
and the acid. But after the copper has been deposited, the strips are
simply drained and carefully placed in the distilled water, it being
difficult to rinse without removing the copper. The couples should be
entirely submerged when placed in the rain water. The strips should
remain in the copper sulfate solution long enough to be well covered
with copper.
=492. Substitution of an Aluminum-Mercury Couple for
Copper-Zinc.=—Ormandy and Cohen have proposed to use an aluminum-mercury
couple for the copper-zinc in the process described above.[320]
This couple acts more quickly than the copper-zinc, and the results are
equally as accurate. Nitrites are reduced in about one hour by this
apparatus, while the zinc-copper couple of Gladstone and Tribe requires
about six times as long. Aluminum foil, free of grease, should be used.
The foil should be heated over a bunsen just before amalgamation. The
clean, very thin foil is coated with mercury by shaking with a
concentrated solution of mercuric chlorid. It should be prepared
immediately before use.
The amalgamated foil is introduced into the sample of water to be
analyzed, and left until all the aluminum is converted into oxid. The
presence of the oxid favors the prevention of bumping during the
subsequent distillation. The distilled ammonia, collected in dilute
acid, is determined by nesslerizing, the free ammonia in the sample
having been previously determined. The increase in ammonia is due to
nitrates or nitrites reduced by the couple.
IODOMETRIC ESTIMATION OF NITRIC ACID IN NITRATES.
=493. Method of De Koninck and Nihoul.=—This process is applicable only
in the absence of organic bodies and other reducing agents.
The principle on which it rests, as applied by McGowan, is as
follows:[321]
When a fairly concentrated solution of a nitrate is warmed with an
excess of pure, strong hydrochloric acid, the nitrate is completely
decomposed, and the production of nitrosyl chlorid and chlorin is
quantitative. The reaction, as shown by Tilden, is represented by the
following equation:[322]
HNO₃ + 3HCl = NOCl + Cl₂ + 2H₂O.
One molecule of nitric acid thus yields two atoms of chlorin and one
molecule of nitrosyl chlorid capable of setting free three atoms of
iodin. The iodin can be estimated in the usual manner by titration with
sodium thiosulfate. The nitrosyl chlorid is decomposed by the potassium
iodid, nitric oxid escaping.
The apparatus employed is shown in Fig. 87.