fresh solution imparts a greenish tint to the yellow of the ammonium
picrate formed.
=498. Variation of Leffmann and Beam.=—The phenyl sulfate process, as
described by Leffmann and Beam, is conducted as follows:[330]
_Solutions Required._—_Acid phenyl sulfate_: 18.5 cubic centimeters of
strong sulfuric acid are added to one and one-half cubic centimeters of
water and three grams of pure phenol. Preserve in a tightly-stoppered
bottle.
_Standard potassium nitrate_: 0.722 gram of potassium nitrate,
previously heated to a temperature just sufficient to fuse it, is
dissolved in water, and the solution made up to 1000 cubic centimeters.
One cubic centimeter of this solution will contain 0.0001 gram of
nitrogen.
_Analytical Process._—A measured volume of the water is evaporated just
to dryness in a platinum or porcelain basin. One cubic centimeter of the
acid phenyl sulfate is added and thoroughly mixed with the residue by
means of a glass rod. One cubic centimeter of water, and three drops of
strong sulfuric acid are added, and the dish gently warmed. The liquid
is then diluted with about twenty-five cubic centimeters of water,
ammonium hydroxid added in excess, and the solution made up to 100 cubic
centimeters.
The reactions are:
Acid phenyl sulfate. Trinitrophenol (picric acid).
HC₆H₅SO₄ + 3HNO₃ = HC₆H₂(NO₂)₃O + H₂SO₄ + 2H₂O.
Ammonium picrate.
HC₆H₂(NO₂)₃O + NH₄HO = NH₄C₆H₂(NO₂)₃O + H₂O.
The ammonium picrate imparts to the solution a yellow color, the
intensity of which is proportional to the amount present.
Five cubic centimeters of the standard solution of potassium nitrate are
similarly evaporated in a platinum dish, treated as above, and made up
to 100 cubic centimeters. The color produced is compared to that given
by the water, and one or the other of the solutions diluted until the
tints of the two agree. The comparative volumes of the liquids furnish
the necessary data for determining the amount of nitrate present, as the
following example will show:
Five cubic centimeters of standard nitrate are treated as above, and
made up to 100 cubic centimeters, representing 0.0005 gram nitrogen.
Suppose 100 cubic centimeters of water similarly treated are found to
require dilution to 150 cubic centimeters before the tint will match
that of the standard; then
100 : 150 :: 0.005 : 0.0075
_i. e._, the water contains seven and one-half milligrams of nitrogen as
nitrate per liter.
The ammonium picrate solution keeps very well, especially in the dark. A
good plan, therefore, is to make up a standard solution equivalent to,
say, ten milligrams of nitrogen as nitrate per liter, to which the color
obtained from the water may be directly compared.
The results obtained by this method are quite accurate. Care should be
taken that the same quantity of acid phenyl sulfate be used for the
water and for the comparison liquid, otherwise different tints instead
of depths of tints are produced.
With subsoil and other waters probably containing much nitrates, ten
cubic centimeters of the sample will be sufficient for the test, but
with river and spring waters, twenty-five to one hundred cubic
centimeters may be used. When the organic matter is sufficient to color
the residue, it will be well to purify the water by addition of alum and
subsequent filtration, before evaporating. The method may also be used
to determine small quantities of nitrates in aqueous extracts of soils
when the quantity is too small for estimation by the ferrous chlorid or
reduction processes.
=499. Variation of Johnson.=—The ammonium picrate method has given very
satisfactory results as practiced by Johnson, who varies the process as
described below.[331]
The standard solution of potassium nitrate is prepared by dissolving
0.7215 gram of the pure salt in a liter of distilled water. Dilute 100
cubic centimeters of this solution to one liter with distilled water.
Ten cubic centimeters of this dilute solution contain nitrogen
equivalent to one part as nitrates in 100,000.
_The Solution of Acid Phenyl Sulfate._—This is prepared by pouring two
parts by measure of pure crystallized phenol liquefied by hot water into
five parts by measure of pure concentrated sulfuric acid and digesting
the whole in the water-bath for eight hours. After cooling, add one and
one-half volumes of distilled water and one-half volume strong
hydrochloric acid to each volume of the above mixture.
The analytical processes are carried on as follows: Ten cubic
centimeters of the water under examination and ten cubic centimeters of
the standard potassium nitrate are placed in small beakers and put near
the edge of a hot plate. When nearly evaporated they are put on the top
of the water-bath and left there until completely dry. The residue, in
each case, is then treated with one cubic centimeter of the acid phenyl
sulfate and the beakers placed on the top of the water-bath. In good
water, a red color ought not to appear for about ten minutes.
After standing about fifteen minutes, the beakers are removed, the
contents of each washed successively into 100 cubic centimeter flasks,
about twenty cubic centimeters of 0.96 per cent. ammonia added, and the
100 cubic centimeters made up by the addition of water and the yellow
liquid transferred to the nessler glass and the tints appropriately
compared.
=500. Estimation of Nitric in Presence of Nitrous Acid.=—The detection
of nitrous in presence of nitric acid can be accomplished by the method
proposed by Griess, as described further on, through the development of
azocolors, with metaphenylenediamin and other bodies, which are not
produced under similar conditions by nitric acid. The detection and
estimation of nitric in the presence of nitrous acid, however, is not so
easy. Lunge and Lwoff propose brucin for this purpose, which, contrary
to most authorities, does not give the red-yellow color with nitrous
acid.[332] The reagent is prepared by dissolving two-tenths gram of
brucin in 100 cubic centimeters of sulfuric acid, pure and concentrated.
It is almost impossible to prepare a sulfuric acid which does not give a
trace of color with brucin; but with the purest acids this trace may be
neglected.
A solution of nitrate is also prepared containing 0.01 milligram of
nitrogen as nitric acid in one cubic centimeter. It is made by
dissolving 0.0721 gram of pure potassium nitrate in 100 cubic
centimeters of distilled water, and diluting ten cubic centimeters
thereof with pure concentrated sulfuric acid to 100 cubic centimeters.
Both solutions are conveniently preserved in burettes with glass
stop-cocks. The liquid to be tested for nitric acid should be mixed with
sulfuric acid in such a way that the mixture will have a specific
gravity of one and seven-tenths. If the liquid to be tested is water,
this concentration is reached by adding three times its volume of the
strong acid. For the comparison of colors, cylinders of colorless glass
are employed, marked at fifty cubic centimeters. They should be about
twenty-four centimeters high and extend about ten centimeters above the
mark. There is placed in the cylinder one cubic centimeter of the
solution of nitrate in sulfuric acid, and the same quantity of the
brucin mixture, and it is filled to the mark with pure sulfuric acid.
The contents of the cylinder are poured into a flask and warmed at from
70°–80°, until the final yellow tint is secured, and then poured into
the cylinder again. The liquid to be tested is treated in exactly the
same way. The tints are then equalized by pouring out a part of the
contents of the deeper colored cylinder, taking account of the volume,
and filling up with pure concentrated sulfuric acid.
In this manner the content of nitric acid in the liquid under
examination can be compared directly with the solution of potassium
nitrate of known strength. The coloration is distinctly produced with
0.01 milligram in fifty cubic centimeters of liquid, at least
three-fourths of which must be sulfuric acid.
=501. Piccini Process.=—The method proposed by Piccini may also be
used.[333]
About five cubic centimeters of the nitrite solution are placed in a
small beaker, some pure urea dissolved therein and a few drops of
sulfuric acid, and then held in boiling water for three minutes. The
nitrous acid is thus completely destroyed. Ammonium chlorid may be
substituted for urea. The reaction is given on page 478. The nitric acid
present is then determined by diphenylamin or other suitable reagents.
Diphenylamin reacts both with nitrous and nitric acids, producing a
violet tint. Warington calls attention to a slight difference, however,
in its deportment with these two acids. When the solution of the reagent
is not too strong a drop of it produces but little turbidity when added
to water or to a solution containing nitric acid. When, however, nitrous
acid is present, a cream-colored turbidity is produced. The violet color
also appears at once on adding sulfuric acid when a nitrite is present,
while in the case of nitrates, more sulfuric acid is required, except
when the solution is very strong. In this connection, it must not be
forgotten that in heating nitrites with urea or ammonium chlorid in the
presence of a slight excess of sulfuric acid a trace of nitric acid may
be formed.
ESTIMATION OF NITROUS ACID BY COLORIMETRIC COMPARISON.
=502. Application of the Method.=—The most minute traces of nitrous acid
may be detected by colorimetric methods and the determination of the
quantity present may be approximated with great exactness by comparison
with a solution of a nitrite of known strength. Especially in following
the progress of nitrification is this method, in some of its forms, of
essential importance. In delicacy and celerity it has the same
advantages as the colorimetric methods for the determination of nitric
acid.
=503. Metaphenylenediamin Method.=—This process depends upon the
development of a yellow color in water containing nitrous acid on the
addition of a reagent containing metaphenylenediamin; m-C₆H₄(NH₂)₂. This
variety of the phenylenediamins is readily obtained from common
dinitrobenzene. It melts at 63° and boils at 287°. In order to preserve
the reagent in shape for use it should be prepared in the following
manner:
Dissolve two grams of the chlorid in ten cubic centimeters of ammonia,
and place the solution in a glass-stoppered flask. To this solution are
added five grams of powdered animal-black, and the whole vigorously
shaken. After allowing to settle, the shaking is repeated at intervals
of an hour, three or four times, and the flask then allowed to remain at
rest for twenty-four hours.
The supernatant liquid is generally sufficiently decolorized by this
treatment. If not, the shaking and subsidence must be repeated until a
completely colorless liquid is obtained. The solution can be kept,
indefinitely, in contact with the animal-black. Aqueous and alcoholic
solutions of the salt can not be kept.
The test is applied by mixing five drops of the reagent with five cubic
centimeters of sulfuric acid. The mixture must be colorless. To the
mixture add 100 cubic centimeters of the water to be tested, and heat on
the water-bath for five minutes. A yellow coloration indicates the
presence of nitrous acid.
The metaphenylenediamin test is fairly satisfactory in perfectly
colorless waters and aqueous extracts. Many waters and soil extracts,
however, have a yellowish tint, and this interferes in a marked way with
a proper judgment of the yellow triaminazobenzol developed in the
application of the above test.
The decoloration of such waters by means of sodium carbonate or hydroxid
and alum, is a matter of some difficulty and not wholly without action
on the nitrites which may be present. The method, therefore, is inferior
to the one next described.
=504. Sulfanilic Acid and Naphthylamin Test for Nitrous Acid.=—A very
delicate test for the presence of nitrous acid, first described by
Griess, is the coloration produced thereby in an acid solution of
sulfanilic acid and naphthylamin.[334]
Sulfuric or acetic acid may be used as the acidifying agent, preferably
the latter. The solutions are prepared as follows:
(1) Dissolve one-half gram of sulfanilic acid in 150 cubic centimeters
of dilute acetic acid.
(2) Boil one-tenth gram of naphthylamin with twenty cubic centimeters of
water, decant the colorless solution from the residue and acidify it
with 150 cubic centimeters of dilute acetic acid.
The two solutions may at once be mixed and preserved in a well-stoppered
flask. The action of light on the mixture is not hurtful, but air should
be carefully excluded because of the traces of nitrous acid which it may
contain. Whenever the mixed solutions show a red tint it is an
indication that they have absorbed some nitrous acid. The red color may
be discharged and the solution again fitted for use by the introduction
of a little zinc dust, and shaking.
The water, or aqueous solution of a soil, to be tested for nitrites, in
portions of about twenty cubic centimeters, is treated with a few cubic
centimeters of the mixed reagent and warmed to 70°–80°. If nitrous acid,
in the proportion of one part to one million be present, the red color
will appear in a few minutes. If the content of nitrous acid be greater,
_e. g._, one part in one thousand, only a yellow color will be produced,
unless a greater quantity of the reagent be used.
Leffmann and Beam recommend the following method of conducting the
determinations.[335]
Solutions required:
_Naphthylammonium Chlorid._—Saturated solution in water free from
nitrites. It should be colorless; a small quantity of animal charcoal
allowed to remain in the bottle will keep it in this condition.
_Paraamidobenzene Sulfonic Acid (Sulfanilic Acid)._—Saturated solution
in water, free from nitrites.
_Hydrochloric Acid._—Twenty-five cubic centimeters of concentrated pure
hydrochloric acid added to seventy-five cubic centimeters of water, free
from nitrites.
_Standard Sodium Nitrite._—0.275 gram pure silver nitrite is dissolved
in pure water, and a dilute solution of pure sodium chlorid added until
the precipitate ceases to form. It is then diluted with pure water to
250 cubic centimeters and allowed to stand until clear. For use, ten
cubic centimeters of this solution are diluted to 100. It is to be kept
in the dark.
One cubic centimeter of the dilute solution is equivalent to 0.00001
gram of nitrogen.
The silver nitrite is prepared in the following manner: A hot
concentrated solution of silver nitrate is added to a concentrated
solution of the purest sodium or potassium nitrite available, filtered
while hot and allowed to cool. The silver nitrite will separate in fine
needle-like crystals, which are freed from the mother-liquor by
filtration with the aid of a filter pump. The crystals are dissolved in
the smallest possible quantity of hot water, allowed to cool and
crystallize, and again separated by means of the pump. They are then
thoroughly dried in the water-bath, and preserved in a tightly-stoppered
bottle away from the light. Their purity may be tested by heating a
weighed quantity to redness in a tared, porcelain crucible and noting
the weight of the metallic silver. One hundred and fifty-four parts of
silver nitrite leave a residue of 108 parts of silver.
_Analytical Process._—One hundred cubic centimeters of the water are
placed in one of the color-comparison cylinders, the measuring vessel
and cylinder having previously been rinsed with the water to be tested.
By means of a pipette, one cubic centimeter each of the solutions of
sulfanilic acid, dilute hydrochloric acid, and naphthylammonium chlorid
is dropped into the water in the order named. It is convenient to have
three pipettes for this test, and to use them for no other purpose. In
any case the pipette must be rinsed out thoroughly with nitrite-free
water each time before using, as nitrites, in quantity sufficient to
give a distinct reaction, may be taken up from the air.
One cubic centimeter of the standard nitrite solution is placed in
another clean cylinder, made up with nitrite-free water to 100 cubic
centimeters and treated with the reagents, as above.
In the presence of nitrites a pink color is produced, which, in dilute
solutions, may require half an hour for complete development. At the end
of this time the two solutions are compared, the colors equalized by
diluting the darker, and the calculation made as explained under the
estimation of nitrates.
The following are the reactions:
Paraamidobenzene Nitrous acid. Paradiazobenzene sulfonic acid.
sulfonic acid.
C₆H₄NH₂HSO₃ + HNO₂ = C₆H₄N₂SO₃ + 2H₂O.
Naphthylammonium Azoalphaamidonaphthalene parazobenzene
chlorid. sulfonic acid.
C₆H₄N₂SO₃ + C₁₀H₇NH₃Cl = C₁₀H₆(NH₂)NNC₆H₄HSO₃ + HCl.
The last named body gives the color to the liquid.
The method pursued by Tanner, in the preparation of the reagent, is as
follows:
Sulfanilic acid is prepared by mixing thirty grams of anilin slowly,
with sixty grams of fuming sulfuric acid, in a porcelain dish. The
brown, sirupy liquid formed is carefully heated until quite dark in
color, and until the evolution of sulfurous fumes is noticed. After
cooling, the thick, semi-fluid mass is poured into half a liter of cold
water and allowed to stand for some hours. The liquid portion is then
decanted from the nearly black undissolved crystalline mass. To the
residue half a liter of hot water is added and allowed to stand until
cold, and the liquid again decanted. The undissolved portion is then
treated with one liter of hot water and filtered. The filtrate is
treated with animal charcoal to decolorize it, and allowed to stand for
twenty-four hours and again filtered, the filtrate diluted to 1,500
cubic centimeters and used as required. This solution tends to turn pink
on keeping, and thus its color interferes with the delicacy of the test,
and a small amount of animal-char is kept in a small bottle containing
the portion for immediate use, and this bottle is filled, from time to
time, from the larger one.
The solution of naphthylamin hydrochlorate is made with one gram of the
salt dissolved in 100 cubic centimeters of water. The solution is to be
occasionally filtered, and not more than 100 cubic centimeters should be
prepared at a time.
The analytical operations are carried on as follows:
A standard solution of pure potassium nitrite, made from the silver salt
in distilled water perfectly free from nitrites, is placed in a
color-glass, similar to those used in the nessler reaction, together
with a second glass containing the water to be tested. These glasses
should be marked to hold 100 cubic centimeters at the same depth. To
each of the tubes a few drops of pure hydrochloric acid are added and
two cubic centimeters of the sulfanilic solution. Afterwards, to each
tube are added two cubic centimeters of the solution of naphthylamin
hydrochlorate, and it is allowed to stand for twenty minutes, at the end
of which time the color should be fully developed. Each tube is covered
by a piece of glass in order to prevent access of air. It is unnecessary
to add that the standard solutions of nitrite of different strength
should be employed until the one is found which resembles, as nearly as
possible, the color developed in the sample of water under examination.
=505. Lunge and Lwoff’s Process for Nitrous Acid.=—The reaction of
nitrous acid with α naphthylamin, first described by Griess, may be made
reliable, quantitatively, by proceeding as below:[336]
Boil 0.100 gram of pure white α naphthylamin for fifteen minutes with
100 cubic centimeters of water, add five cubic centimeters of glacial
acetic acid, or its equivalent of dilute acid, and afterwards one gram
of sulfanilic acid dissolved in 100 cubic centimeters of hot water. The
mixture is kept in a well-closed flask. A slight red tint in the mixture
is of no significance, inasmuch as this completely disappears when one
part of it is mixed with fifty parts of the liquid to be examined. If
the coloration be very strong it can be removed by adding a little zinc
dust. One cubic centimeter of this reagent will give a distinct
coloration with 0.001 milligram of nitrous nitrogen in 100 cubic
centimeters of water.
The analysis is conducted in cylinders of white glass marked at fifty
cubic centimeters. One cubic centimeter of the above reagent is placed
in each of two cylinders with forty cubic centimeters of water and five
grams of solid sodium acetate. In one of the cylinders is placed one
cubic centimeter of a normal solution of a nitrite prepared by
dissolving 0.0493 gram of pure sodium nitrite corresponding to ten
milligrams of nitrogen in 100 cubic centimeters of water, and adding ten
cubic centimeters of this solution to ninety cubic centimeters of pure
sulfuric acid. This secures a normal solution of nitrosylsulfuric acid,
of which each cubic centimeter corresponds to 0.01 milligram of
nitrogen.
In the other cylinder is placed one cubic centimeter of the solution to
be examined, and the contents of both cylinders are well mixed so that
the nitrous acid in a nascent state may act on the reagent. The colors
are compared after any convenient period, but, as a rule, after five
minutes.
The chief improvement made by Lunge and Lwoff on the method of Griess is
in keeping the reagent in a mixed state ready for use, by means of which
any nitrous impurities in the components thereof are surely indicated.
Its advantage over the method of Ilosvay[337] consists in using the
comparative normal nitrite solution as nitrosylsulfuric acid, in which
state it is much more stable.
=506. Estimation of Nitrous Acid with Starch as Indicator.=—The method
of procedure, depending on the blue color produced in a solution of
starch in presence of a nitrite and zinc iodid when treated with
sulfuric acid, is not of wide application on account of the interference
produced by organic matter. The soil extract or water is treated in a
test-tube, with a few drops of starch solution and some zinc iodid, to
which is added some sulfuric acid. The decomposition of the nitrite is
attended with the setting free of an equivalent amount of iodin which
gives a blue coloration to the starch solution. The depth of the tint is
imitated by treating a standard solution of nitrite in a similar way
until the proper quantity is found, which gives at once the proportion
of nitrite in the sample examined. This process, however, is scarcely
more than a qualitative one.
=507. Estimation of Nitrites by the Method of Chabrier.=—In order to
make the estimation of the evolved nitrous acid more definite by the
iodin method, Chabrier has elaborated a plan for titrating it with a
reducing agent.[338]
The substance chosen for this purpose is sodium hyposulfite. In point of
fact, it is not the nitrous acid which is attacked by the hyposulfite,
but the equivalent amount of free iodin representing it. In the case of
a soil where the quantity of nitrites is usually very small, it is well
to take as much as one kilogram. The extraction should be made rapidly,
with water, free of nitrites, in order to avoid any reducing action on
the nitrates which may be present. In the case of water, from five to
ten liters should be evaporated to a small volume. The concentration
should take place in a large flask, rather than in an open dish, in
order to avoid any possibility of the absorption of nitrites produced by
combustion. When the volume has been reduced to about 100 cubic
centimeters it is transferred to a small flask and the concentration
continued until only ten or fifteen cubic centimeters are left. The
residue is filtered into a woulff bottle, F, Fig. 89, of about 100 cubic
centimeters capacity.