stopper, _x_, is accurately ground (with fine emery and oil). The
capacity of the bulb is about forty-six cubic centimeters, and the
length of the neck, from _x_ to _y_, ninety millimeters. The first
condenser is a simple tube, slightly enlarged at the foot into two small
bulbs.[323] The length from _a_ to _b_ is 300 millimeters, from _b_ to
_c_ 180 millimeters, and from _e_ to _f_ thirty millimeters. The
capacity of the bulb B is twenty-five cubic centimeters, and the total
capacity of the two bulbs and tube, up to the top of C, forty-one cubic
centimeters. This condenser is immersed, up to the level of _c_, in a
beaker full of water. D is a geissler bulb apparatus, E is a calcium
chlorid tube, filled with broken glass, which acts as a tower and _g_ is
a small funnel, attached by rubber and clip to the branch =⟙= tube _h_.
Between the =⟙= tube _i_ and the wash-bottle for the carbon dioxid is
placed a short piece of glass tubing, _s_, containing a strip of filter
paper, slightly moistened with iodid of starch solution. This tube _s_
is really hardly necessary, as no chlorin escapes backwards if a
moderate current of carbon dioxid is kept passing, but it serves as a
check. A glance at the joints _o_, _p_, and _q_, which are of narrow
india-rubber tubing, is sufficient to show that, by using this
arrangement, practically no rubber is exposed to the action of the
chlorin. The tiny piece of rubber tubing at the joint _o_ may be done
away with, the narrower tube there being accurately ground into the
wider one; this makes the condensing apparatus practically perfect.
[Illustration:
FIGURE 87. MCGOWAN’S APPARATUS FOR THE IODOMETRIC ESTIMATION OF NITRIC
ACID.
]
The actual operation is performed in the following manner:
The evolution flask is washed and thoroughly dried, and the nitrate
(say, about 0.25 gram of potassium nitrate) is tapped into it from the
weighing tube. Two cubic centimeters of water are now added, and the
bulb is gently warmed, so as to bring the nitrate into solution, after
which the stopper of the flask is firmly inserted. About fifteen cubic
centimeters of a solution of potassium iodid (one in four) are run into
the first condensing tube, any iodid adhering to the upper portion of
the tube being washed down with a little water, and five cubic
centimeters of the same solution, mixed with eight to ten cubic
centimeters of water, are sucked into the geissler bulbs whilst the
glass in the tower E is also thoroughly moistened with the iodid. The
geissler bulbs should be so arranged that gas only bubbles through the
last of them, the liquid in the others remaining quiescent.
All the joints having been made tight the carbon dioxid is turned on
briskly and passed through the apparatus until a small tubeful collected
at _l_, over caustic potash solution, shows that no appreciable amount
of air is left in it. The small outlet tube _l_, is now replaced by a
calcium chlorid tube, filled with broken glass which has been moistened
with the above-mentioned iodid solution, and closed by a cork through
which an outlet tube passes, the object of this trap tube being to
prevent any air getting back into the apparatus. The brisk current of
carbon dioxid is continued for a minute or two longer, so as to
practically expel all the air from this last tube. The stream of gas is
now stopped for an instant, and about fifteen cubic centimeters of pure
concentrated hydrochloric acid, free from chlorin, run into A through
the funnel _g_ (into the tube of which it is well to have run a few
drops of water before beginning to expel the air from the apparatus),
and A is shaken so as to mix its contents thoroughly. A slow current of
carbon dioxid is now again turned on (one to two bubbles through the
wash-bottle per second), and A is gently warmed over a burner. It is a
distinct advantage that the reaction does not begin until the mixed
solutions are warmed, when the liquid becomes orange-colored, the color
again disappearing after the nitrosyl chlorid and chlorin have been
expelled. The warming should be very gentle at first in order to make
sure of the conversion of all the nitric acid, and also because the
first escaping vapors are relatively very rich in chlorin; afterwards
the liquid in A is briskly boiled. A very little practice enables the
operator to judge as to the proper rate of warming. When the volume of
liquid in A has been reduced to about seven cubic centimeters (by which
time it is again colorless) the stream of carbon dioxid is slightly
quickened and the apparatus allowed to cool a little. The burner is now
set aside for a few minutes, and two cubic centimeters more of
hydrochloric acid, previously warmed in a test-tube, run in gently
through _g_; there is no fear either of the iodid solution running back,
or of any bubbles of air escaping through _y_ if this is done carefully.
This is a precautionary measure, in case a trace of the liberated
chlorin might have lodged in the comparatively cool liquid in the tube
_h_. The carbon dioxid is once more turned on slowly and the liquid in A
is boiled again until it is reduced to about five cubic centimeters. It
is now only necessary to allow the apparatus to cool, passing carbon
dioxid all the time, after which the contents of the condensers are
transferred to a flask and titrated with thiosulfate. At the end of a
properly conducted experiment, the glass in the upper part of tower E
should be quite colorless and there should be only a mere trace of iodin
showing in the lower part of the tower, while the liquid in the last
bulb of the geissler apparatus ought to be pale yellow. During the
operation, the stopper of A and the various joints can be tested from
time to time by means of a piece of iodid of starch paper, and before
disjointing it is well to test the escaping gas (say at _m_) in the same
way, to make sure that all nitric oxid has been thoroughly expelled.
The method is capable of giving accurate results, but it can not be
preferred to the reduction or colorimetric processes.
=494. Method of Gooch and Gruener.=—The principle on which this method
rests depends on the decomposition of a nitrate in presence of a hot
saturated solution of manganous chlorid and hydrochloric acid in an
atmosphere of carbon dioxid.[324] The products of decomposition are
passed into a solution of potassium iodid and the liberated iodin is
titrated with standard sodium thiosulfate. The products of the reaction
are chlorin, nitric oxid, and possibly nitrosyl chlorid, and under
proper precautions the iodin set free is quantitively proportional to
the weight of nitrate decomposed. The manganous mixture is acted on
slowly at ordinary temperatures, but on heating, the nitrate is
decomposed with the formation of a higher manganese chlorid and nitric
oxid. When the heat is continued a sufficient length of time the chlorin
from the higher chlorids is evolved and only manganous chlorid remains.
During the heating the color of the solution passes from green to black
and at the end the green color is restored. The apparatus employed is
shown in Fig. 88.
[Illustration:
FIGURE 88. APPARATUS OF GOOCH AND GRUENER.
]
A plain pipette bent as is shown in the figure serves as the generating
flask and for the attachment on the one hand to the carbon dioxid
apparatus and on the other to the system of absorption bulbs for
containing the potassium iodid. The latter should be glass, sealed to
the evolution bulb of the pipette to prevent the action of the evolved
gases on organic materials. The point of the potassium iodid apparatus
is drawn out so as to be pushed well into the second receiver, being
held in place by a piece of rubber tubing. The third tube acts simply as
a trap to exclude the air from the absorption apparatus. The first
receiver contains in solution three grams, the second two, and the third
one gram of potassium iodid. During the reaction the first receiver is
kept cool by immersion in water. Before connecting the apparatus with
the carbon dioxid generator the solution of manganous chlorid and
afterwards the nitrate solution are drawn into the bulb of the pipette
by gentle suction. After connecting the apparatus the current of carbon
dioxid is started and kept up until all the air is expelled. Heat is
then applied to the bulb of the pipette and the distillation continued
until all the liquid has passed over. At the end of the reaction the
contents of the receivers are united by disconnecting the apparatus from
the carbon dioxid generator and passing water through the pipette. The
introduction of the manganous chlorid into the mixture does not
interfere with the titration of the iodin. This is accomplished in the
usual way with sodium thiosulfate using starch as an indicator. The
quantity of material used should contain about the amount of nitric acid
that is found in two-tenths of a gram of potassium nitrate. This method,
so similar to the preceding, is somewhat less complex, and, to that
extent, preferable to it.
ESTIMATION OF NITRIC ACID BY COLORIMETRIC COMPARISON.
=495. Delicacy of the Method.=—The remarkable delicacy of those methods
of chemical analysis, which depend on the production of a pronounced
color, which can be compared with that produced by a known quantity of a
given substance, has been long illustrated by the nesslerizing process
for the estimation of ammonia. By such methods minute qualities of
substances can be quantitively determined with great accuracy, when they
would escape all effort for their estimation by gravimetric methods.
Processes based on this principle are, therefore, peculiarly applicable
to the detection and estimation of oxidized nitrogen in waters and soil
extracts, whether they be present as nitric, nitrous, or ammoniacal
compounds.
In the following paragraphs will be given with sufficient detail for the
needs of the analyst, the principles and practice of the colorimetric
comparison methods which have been approved as best by the experience of
analysts. These methods are applicable especially to cases in which only
minute quantities of the substances looked for are present, and where
celerity of determination is especially desirable. They are, therefore,
of especial value in the analysis of rain, drainage, and irrigation
waters, and of soil extracts poor in oxidized nitrogen.
=496. Hooker’s Method.=—The quantitive action depends upon the deep
green coloration given by nitric acid, when dissolved in sulfuric acid
and carbazol.[325] Other oxidizing bodies, such as iron, chlorin,
bromin, chromic acid, etc., give the same reaction, but not in such a
prominent manner. Such bodies with the exception of chlorin and iron,
are not often found in waters. In the application of the process, iron,
if present in quantities greater than one-tenth part per one hundred
thousand, must be removed. Chlorids also, even when present in very
small quantities, interfere with the delicacy of the reaction and must
be removed. Easily destructible organic matter tends to lower the
result, but not materially, unless present in large excess. Calcium
carbonate and sulfate, soda, and other alkalies, in the quantities in
which they are usually present in water, do not affect the result. The
following reagents are required:
1. Concentrated sulfuric acid.
2. An acetic acid solution of carbazol; diphenylimid, (C₆H₄\/C₆H₄/
NH.)
3. A sulfuric acid solution of carbazol.
4. Standard solutions of potassium nitrate.
5. A solution of aluminum sulfate.
6. A solution of silver sulfate.
1. The sulfuric acid, used for all purposes in the process, should be
entirely free from nitrogen oxids. It may be readily tested by
dissolving in it a small quantity of carbazol. If the solution be at
first golden-yellow or brown, the acid is sufficiently pure; if it be
green or greenish, another and better sample must be taken. It is
essential also that the specific gravity of the acid be fully 1.84, and
it is well to ascertain that this is really the case.
2. The acetic acid solution of carbazol is prepared by dissolving
six-tenths gram in about ninety cubic centimeters of strongest acetic
acid, by the aid of gentle heat. It is allowed to cool, and is then made
up to 100 cubic centimeters by the further addition of acetic acid. The
exact strength of this solution, is of no material importance to the
success of the process, and the above proportions have been selected
principally because they are convenient. The solution will remain
unchanged for several months. The use of this solution merely
facilitates the preparation of that next described, which will not keep,
and has, consequently, to be freshly prepared for each series of
determinations.
3. The sulfuric acid solution of carbazol is easily made in a few
seconds, but it is advisable to allow it to stand from one and one-half
to two hours before using. It is prepared by rapidly adding fifteen
cubic centimeters of sulfuric acid, to one cubic centimeter of the
above-described acetic acid solution. This quantity usually suffices for
from two to three nitrate estimations. When freshly prepared it is
golden-yellow or brown; it changes gradually, however, and in the course
of one and one-half or two hours it becomes olive-green. This change is
probably due to traces of oxidizing agents, which occur in the sulfuric
and acetic acids, and which, although not present in sufficient quantity
to act immediately, gradually bring about the reaction described. The
greenish color does not interfere with the process, as might at first be
supposed; on the contrary, the solution is not sensitive to small
quantities of nitric acid until it has undergone the change to
olive-green, and it is for this reason, that it should be prepared about
two hours before required for use. This solution may be thoroughly
depended on for six hours after preparation. The intensities of color
produced by the more concentrated solutions of nitrates after this time,
gradually approach each other and become ultimately the same.
4. The standard solutions of potassium nitrate are very readily
prepared. The solutions which are to be compared directly with the
waters examined, may be prepared as required, but if many determinations
are to be made with a variety of waters, it will be found best to
prepare a complete series, differing from each other by 0.02 part
nitrogen in 100,000. This series may include solutions containing
quantities of nitrogen in 100,000 parts, represented by all the odd
numbers from 0.03 up to 0.39. It will be found convenient to prepare
them in quantities of 100 cubic centimeters at a time, from a stock
solution of potassium nitrate which contains 0.00001 gram nitrogen, or
0.000045 nitric acid in one cubic centimeter. Each cubic centimeter of
this solution, when diluted to 100 cubic centimeters, represents 0.01
nitrogen in 100,000, and consequently if it is desired to make a
solution containing 0.35 part nitrogen in 100,000, thirty-five cubic
centimeters are taken and made up to 100 cubic centimeters, and so on.
The solution of potassium nitrate (b) is best prepared from a stronger
one (a) containing 0.0001 gram nitrogen to the cubic centimeter, or
0.7214 gram potassium nitrate to the liter; 100 cubic centimeters of (a)
made up to one liter give the solution (b). It is obvious that the
series of solutions, above described, could be made directly from (a),
but by first making (b), greater accuracy is secured.
5. For purposes which will be presently described, a solution of
aluminum sulfate is required, containing five grams to the liter. The
salt used must be free from chlorin and iron; and the solution should
give no reaction when tested with carbazol.
6. The solution of silver sulfate is required for the removal of chlorin
from the water or soil extract to be examined. It is prepared by
dissolving 4.3943 grams of the salt in pure distilled water and making
up to one liter. The sulfate is preferably obtained by dissolving
metallic silver in pure sulfuric acid. The solution should be tested
with carbazol in the same way as will be presently described for water;
if perfectly pure, no reaction will be obtained. As silver sulfate is
often prepared by precipitation from the nitrate, it is very apt to
contain nitric acid, and consequently, if the source of the salt be
unknown, this test should on no account be omitted.
The analytical process is carried on as follows:
Two cubic centimeters of the water are carefully delivered by means of a
pipette into the bottom of a test-tube; four cubic centimeters of
sulfuric acid are added, and the solution thoroughly mixed by the help
of a glass rod. The test-tube is then immersed in cold water, and when
well cooled, one cubic centimeter of the sulfuric acid solution of
carbazol is added, and the whole again mixed as before. The intensity of
the color is now observed, and a little experience enables a fairly good
opinion to be formed of the quantity of nitric acid present. Suppose
that the water be roughly estimated to contain about 0.15 part nitrogen
per 100,000; in such a case solutions of potassium nitrate containing
0.11, 0.15, 0.19 part nitrogen are selected from the series. Two cubic
centimeters are taken from each, and treated, side by side, with a fresh
quantity of the water, precisely as described for the preliminary
experiment, the various operations being performed as nearly
simultaneously as possible with each of the samples, and under precisely
similar conditions. Two or three minutes after the carbazol has been
added, the intensity of the color of each is observed. If that given by
the water is matched by any of the standard solutions, the estimation is
at an end. Similarly, if it falls between two of these, the mean may be
taken as representing the nitrogen present in cases in which great
accuracy is not required. If this be done, the maximum error will be
0.02 part nitrogen, or 0.09 part nitric acid per 100,000. If greater
exactness be required, or it be found that the color given by the water
is either darker or lighter than that given by all the standard
solutions, a new trial must be made. In such a case the water must be
again tested simultaneously with the solutions with which it is to be
compared. This is rendered necessary principally for the reason that the
shade of the solutions to which the carbazol has been added is apt to
change on standing. Hence it is desirable that the water, and the
standard potassium nitrate with which it is to be compared, should have
the carbazol added at as nearly the same time as possible. When finally
the color falls between that given by any two consecutive members of the
standard potassium nitrate series, the estimation may be considered at
an end, and the mean of these solutions taken as representing the
nitrogen present.
The greatest neatness should be observed in all steps of the analysis.
The quantity of water operated upon is so small that if the greatest
care be not exercised throughout, sources of error may be readily
introduced. The test-tubes should be rinsed out with nitrate-free water
before being used and then dried. The tint should be determined by
looking through the tube and not through the length of the column of
liquid.
_Influence of Nitrites._—If the quantity of nitrous acid in the water is
known a correction can be applied for nitrates by deducting one-fifth of
the number found for nitrites when estimated as nitrates.
_Influence of Iron._—Although ferrous salts give no reaction with
carbazol, nitrates are apt to be overestimated in their presence. Oh the
other hand, ferric compounds, like other oxidizing agents, may give a
characteristic green color with carbazol. In all cases when iron is
present in any considerable quantity it is best to remove it by
rendering the water slightly alkaline, evaporating to dryness, and
redissolving the soluble residue until the solution reaches the original
volume.
_Influence of Chlorids._—The presence of chlorids furnishes by far the
most serious source of error in the process by intensifying the action
of the nitric acid. If, however, nitrates be absent chlorids give no
reaction with carbazol. The chlorids are removed by a standard silver
sulfate solution, the quantity of chlorids present having been first
determined by a standard silver nitrate solution. For this purpose an
ordinary sugar flask can be employed marked at 100 and 110 cubic
centimeters. This flask is filled to the 100 cubic centimeter mark with
the water to be examined; the necessary quantity of silver sulfate is
added and then two cubic centimeters of the solution of aluminum
sulfate, previously described, and the contents of the flask brought up
to 110 cubic centimeters by the addition of pure distilled water. The
whole is shaken up and filtered, the first portion of the filtrate being
rejected. The aluminum sulfate by reacting with the carbonates usually
present in the water and producing the precipitation of alumina,
facilitates the removal of the precipitated silver chlorid.
The above-described method on account of its delicacy is not well suited
to aqueous solutions of soils except where the quantity of nitric
nitrogen present is extremely minute.
Hooker also first suggested the use of diphenylamin for detecting the
presence of nitrates,[326] a method afterwards worked out by
Spiegel.[327]
In the variation of the method as practiced by Rideal the standard
potassium nitrate and the pure sulfuric acid mentioned below are
required, and in addition, the following reagents:[328]
(a) Silver sulfate solution containing 4.3945 grams per liter.
(b) Aluminum sulfate solution free from chlorids and iron, five grams
per liter.
(c) Carbazol solution; six-tenths gram carbazol dissolved in glacial
acetic acid and made up to 100 cubic centimeters with the glacial acid.
For use, one cubic centimeter of this solution is withdrawn by a pipette
and mixed with fifteen cubic centimeters of pure redistilled sulfuric
acid.
The process is carried out as follows: To 100 cubic centimeters of water
the amount of chlorin which has been previously ascertained is removed
by the silver sulfate solution. Two cubic centimeters of the aluminum
sulfate are added and the whole made up to a convenient volume, say
about 110 cubic centimeters. The liquid is filtered and two cubic
centimeters of the filtrate taken for an estimation of nitrates. To the
two cubic centimeters are added four cubic centimeters of concentrated
sulfuric acid and the mixture cooled.
One cubic centimeter of the carbazol solution in sulfuric acid is added
and a bright green color appears in a few moments, if nitrates are
present. Comparison is made with solutions of standard potassium
nitrate.
=497. Phenylsulfuric Acid Method.=—Rideal also proposes a variation of
the method described by Hooker, which consists in the substitution of
phenylsulfuric acid for carbazol.[329]
The solutions required are:
(a) A standard solution of potassium nitrate containing 0.7215 gram of
the pure crystallized salt in a liter of water.
(b) Phenylsulfuric acid, (acid phenyl sulfate,) prepared by dissolving
fifteen grams of pure crystallized phenol in 92.5 cubic centimeters of
pure, redistilled sulfuric acid free from nitrates and diluted with
seven and one-half cubic centimeters of water.
The process is conducted as follows:
A known volume of water, from twenty-five to one hundred cubic
centimeters, according to its richness in nitrates, is evaporated to
dryness in a porcelain dish, one cubic centimeter of phenylsulfuric acid
added then one cubic centimeter of pure water and three drops of strong
sulfuric acid and the mixture gently warmed. A yellow color shows the
presence of nitrates. Dilute to about twenty-five cubic centimeters with
water and add ammonia in slight excess. Pour into a narrow nessler tube
and add the washings and make up to 100 cubic centimeters. Imitate the
color of the solution with the standard potassium nitrate treated with
the same reagents.