quantity of carbon monoxid produced during the absorption with
pyrogallol; this error becomes negligible if the oxygen be only used in
small excess. The amount of oxygen employed can be regulated by the use
of Bischof’s gas delivery-tube. This may be made of a test-tube having a
small perforation half an inch from the mouth. The tube is partly filled
with oxygen over mercury, and its mouth is then closed by a finely
perforated stopper made from a piece of wide tube and fitted tightly
into the test-tube by means of a covering of rubber. When this tube is
inclined, the side perforation being downwards, the oxygen is discharged
in small bubbles from the perforated stopper, while mercury enters
through the opening. Using this tube, the supply of oxygen is perfectly
under control and can be stopped as soon as a fresh bubble ceases to
produce a red tinge on entering. Warington concludes his description by
stating that in the reaction proposed by Schloesing the analyst has a
means of determining a very small quantity of nitric acid with
considerable accuracy, even in the presence of organic matter; but to
accomplish this, the various simplifications consisting in the omission
of the stream of carbon dioxid, and the collection of the gas over
caustic soda must be abandoned, and special precautions must be taken to
exclude all traces of oxygen from the apparatus.
=465. Spiegel’s Modification.=—Spiegel noticed inaccuracies in the
results of the ferrous chlorid method of estimating nitric acid when
carbon dioxid is used, which sometimes amounted to three per cent of the
nitric acid present in the sample. The following suggestions are made by
him for the improvement of the process:[297]
As regards the use of carbon dioxid in the operation, the first
difficulty consists in obtaining it entirely free from air. By the use
of small pieces of marble, which, before being placed in the Kipp
apparatus, are kept for a long while in boiling water, a product is
obtained which, after thirty minutes of moderate evolution, leaves only
a trace of unabsorbed gas in contact with potash-lye. The apparatus used
is illustrated in Fig. 77.
[Illustration:
FIGURE 77. SPIEGEL’S APPARATUS FOR NITRIC ACID.
]
A is a round flask of about 150 cubic centimeters capacity, furnished
with a well-fitting rubber stopper provided with two holes, one for the
entrance of the funnel-tube B and the other for the delivery-tube C. The
tube B ends about two centimeters above the bottom of A and carries a
bulb-shaped funnel at its top capable of holding about fifty cubic
centimeters. The gas-tube D is ground into the bulb of B as shown in the
figure.
After the flask had been filled with the solution to be examined, carbon
dioxid is conducted through D and the flask is heated to boiling until
the gas which escapes through C no longer contains any air. The
measuring tube is brought over the end of the delivery-tube C, in the
usual manner, but not shown in the figure. In the funnel of B are placed
twenty cubic centimeters of previously prepared and boiled ferrous
chlorid solution and this liquid is allowed to flow partly into A by
lifting slightly the gas-tube, D. About forty cubic centimeters of
concentrated, boiled hydrochloric acid are afterwards added to it in the
same way. As soon as the liquid in the flask A is again boiling, the
stream of carbon dioxid is shut off and allowed to flow again only
towards the end of the operation, when the contents of the flask are
reduced almost to dryness. As will be seen from the above directions no
unboiled liquids of any kind are to be used as reagents in the apparatus
described. If the flask A were made much smaller the efficiency of this
apparatus would be increased. It appears to have few, if any, advantages
over Warington’s process.
=466. Schulze-Tiemann Method.=—The modification of Schulze-Tiemann in
the ferrous salt method consists chiefly in the omission of the use of
carbon dioxid, and in the simplified form of apparatus, which permits
rapid work and gives, also, according to some authorities, very exact
and reliable results.[298] The extract, representing 500 grams of the
fine soil, is reduced by evaporation to 100 cubic centimeters and placed
in a glass flask, _A_ (Fig. 78), of 500 cubic centimeters capacity. The
flask is closed with a rubber stopper, carrying two bent glass tubes
which pass through it. The tube _a b c_ is drawn out into a point at _a_
and reaches about two centimeters below the surface of the rubber
stopper. The tube _e f g_ passes just to the lower surface of the rubber
stopper. The two tubes mentioned are connected, by means of rubber tubes
and pinch-cocks, with the tubes _d_ and _h_. The pinch-cocks at _c_ and
_g_ must be capable of closing the tubes air-tight. The end of the tube
_g h_ passes into a crystallizing dish, _B_, and is bent upward to a
point passing two to three centimeters into the measuring tube _C_. The
point within the tube is covered with a piece of rubber tubing. The
measuring tube _C_ is divided into tenths of a cubic centimeter, and
together with the crystallizing dish _B_, is filled with a ten per cent
solution of boiled soda-lye, which is obtained by dissolving 12.9 parts
of sodium hydroxid in 100 parts of water.
[Illustration:
FIGURE 78. SCHULZE-TIEMANN’S NITRIC ACID APPARATUS.
]
The liquid which is to be examined for nitric acid, the pinch-cocks
being opened and the tube _g h_ not dipping into the crystallizing dish,
is boiled for one hour in order to drive the air out of the flask _A_.
The end of the tube _e f g h_ is then brought into the crystallizing
dish containing the sodium hydroxid solution so that the steam escaping
from the flask _A_, escapes partly through the tube _b c d_, and partly
through the tube _f g h_, not allowing, however, the bubbles to enter
the measuring tube _C_. To determine whether the air is all expelled,
the pinch-cock at _g_ is closed and the soda-lye will thereupon rise to
_g_ in case no air interferes. It is best to close the tube at _g_ first
with the thumb and finger and then the rise of the soda-lye to that
point can be determined by the impulse felt. The tube is then firmly
closed by means of the pinch-cock _g_. The rest of the steam is allowed
to escape through the tube _a b c d_, and the evaporation is continued
until the contents of the flask are evaporated to about ten cubic
centimeters. The flask into which the tube _c d_ dips, is filled with
freshly boiled water. The lamp is removed from the flask _A_, the
pinch-cock is closed, whereupon the tube _c d_ becomes filled with the
freshly boiled water. The measuring tube _C_, filled with freshly boiled
soda-lye is closed with the thumb and brought into the dish _B_, care
being taken that no bubble of air enters. It is placed over the end of
the tube _g h_.
The pressure of the external air will now flatten the rubber tubes at
_c_ and _g_. The flask at the end of _c d_ holding freshly boiled water
is then replaced with one filled with a nearly saturated solution of
ferrous chlorid containing some hydrochloric acid. The flask containing
the ferrous chlorid solution should be graduated so that the amount
which is sucked into the flask _A_ can be determined. The pinch-cock _c_
is opened and from fifteen to twenty cubic centimeters of the ferrous
chlorid solution allowed to flow into _A_. The end of the tube _c d_ is
then placed in another flask containing strong hydrochloric acid, and
the latter allowed to flow into the tube in small quantities at a time
until all the ferrous chlorid is washed out of the tube _b c d_ into
_A_. At the point _b_ there is sometimes formed a little bubble of
hydrochloric acid in the state of gas, which by heating the flask _A_
completely disappears.
The flask _A_ is next warmed gently until the rubber tubes at the
pinch-cocks begin to assume their normal condition. The pinch-cock at
_g_ is now replaced by the thumb and finger, and as soon as the pressure
within the flask _A_ is somewhat stronger, caused by the nitric oxid gas
evolved from the mixture, it is allowed to pass through the tube
_e f g h_ and escape into the measuring cylinder _C_. By a manipulation
of the finger and thumb at _g_, it is possible to prevent regurgitation
of the sodium hydroxid into _A_, and at the same time to relieve the
pressure of the nitric oxid in _A_, which would be difficult to do by
means of the pinch-cock alone.
The boiling of the liquid is continued until there is no longer any
increase of the volume of gas in the measuring cylinder _C_. After the
end of the operation the tube _g h_ is removed from the dish _B_ and the
measuring tube _C_ is closed by means of the thumb while its end is
still beneath the surface of the soda-lye, and it is shaken until all
traces of any hydrochloric acid, which may have escaped absorption, are
removed. It is then placed in a large glass cylinder filled with water
at the temperature at which the volume of gas is to be read. After being
kept at this constant temperature for about half an hour the volume of
the nitric oxid can be read. For this purpose the measuring cylinder _C_
is sunk into the water of the large cylinder until the level of the
liquids within and without the tube is the same. The usual correction
for pressure of the atmosphere, as determined by the barometer, and for
the tension of the aqueous vapor at the temperature at which the reading
is made, is applied. The correction is made by means of the following
formula:
V′ = (V × 273 × (B − f)) ÷ ((273 + t) × 760)
In this formula V′ denotes the volume of the gas at the temperature of
zero, and at 760 millimeters barometric pressure; V the volume of the
gas as read at the barometric pressure observed, B, and the temperature
observed, _t_, while _f_ denotes the tension of the aqueous vapor in
millimeters of mercury pressure at the observed temperature _t_. The
tension of the aqueous vapor at temperatures from zero to 26°, expressed
in millimeters of mercury, is given in the following table:
─────┬─────────────
Temp.│ Tension in
° │mm. mercury.
─────┼─────────────
0│ 4.6
1│ 4.9
2│ 5.3
3│ 5.7
4│ 6.1
5│ 6.5
6│ 6.9
7│ 7.4
8│ 8.0
9│ 8.5
10│ 9.1
11│ 9.7
12│ 10.4
13│ 11.1
14│ 11.9
15│ 12.7
16│ 13.5
17│ 14.4
18│ 15.3
19│ 16.3
20│ 17.4
21│ 18.5
22│ 19.6
23│ 20.9
24│ 22.2
25│ 23.5
26│ 25.0
─────┴─────────────
From the gas volume reduced by the above formula the nitric acid is
calculated as follows:
One cubic centimeter of nitric oxid weighs at 0° and 760 millimeters
barometric pressure 1.343 milligrams.
Since two molecules of NO (molecular weight sixty) correspond to one
molecule of N₂O₅ (108) we have the following equation: 60 : 108 = 1.343
: x. Whence x = 2.417 milligrams, the weight of nitric acid
corresponding to one cubic centimeter of nitric oxid.
[Illustration:
FIGURE 79. DE KONICK’S APPARATUS.
]
[Illustration:
FIGURE 80. END OF DELIVERY-TUBE.
]
=467. DeKonick’s Modification of Schloesing’s Method.=—This modification
consists in an arrangement of the gas delivery-tube, whereby the
regurgitation of the water in the measuring burette into the evolution
flask is prevented by a device for sealing the delivery-tube with
mercury.[299] The apparatus is arranged as shown in Fig. 79. The flask
in which the decomposition takes place is provided with a long neck,
into which a side tube is sealed and bent upwards, carrying a small
funnel attached to it by rubber tubing. The piece of rubber tubing
carries a pinch-cock, by means of which the solution containing the
nitrate and hydrochloric acid can be introduced into the flask. The
small gas delivery-tube is arranged as shown in the figure, and carries
at the end next the burette a device shown in Fig. 80. The cork
represented in this device has radial notches cut in it, so as to permit
of a free communication between the water in the burette and in the
pneumatic trough. The open end of the burette, when the apparatus is
mounted ready for use, rests on the notched surface of the cork, and the
end of the delivery-tube is placed in the crystallizing dish resting on
the bottom of the pneumatic trough.
The end of the delivery-tube, as indicated, has fused onto it a vertical
tube open at both ends and six to seven centimeters in length, and
carrying the notched cork already described. The crystallizing dish in
the bottom of the pneumatic trough is filled with mercury until the
point of union of the delivery-tube with the vertical end is sealed to
the depth of a few millimeters. As the gas is evolved it bubbles up
through the mercury into the measuring tube and the displaced water
passes out through the notches in the cork. Should any back pressure
supervene the mercury at once rises in the delivery-tube which is of
such a length as to prevent its entrance into the flask. The operation
can then be carried on with absolute safety.
To make an estimation there are placed in the flask about forty cubic
centimeters of ferrous chlorid solution containing about 200 grams of
iron to the liter, and also an equal volume of hydrochloric acid of one
and one-tenth specific gravity. The side tube is also filled up to the
funnel with the acid. The contents of the flask are boiled until all air
is expelled, which can be determined by holding a test-tube filled with
water over the end of the delivery-tube. The solution containing the
nitrate is next placed in the funnel, the pinch-cock opened and the
liquid allowed to run into the flask by means of the partial vacuum
produced by stopping the boiling and allowing the mercury to rise in the
delivery-tube. All the solution is washed into the flask by successive
rinsings of the funnel with hydrochloric acid, being careful to allow no
bubble of air to enter. The contents of the flask are again raised to
the boiling-point and the nitric oxid evolved collected in the
nitrometer. The solution examined should contain enough nitrate to
afford from sixty to eighty cubic centimeters of gas. Without refilling
the flask, from eight to nine determinations can be made by regenerating
the ferrous chlorid by treatment with zinc chlorid. Care must be
exercised not to add the zinc chlorid in excess, otherwise ammonia and
not nitric oxid will be produced. The side tube and funnel must also be
carefully freed from zinc chlorid by washing with hydrochloric acid.
=468. Schmidt’s Process.=—In the case of a water, or the aqueous extract
of a soil, according to the content of nitric acid, from fifty to one
hundred cubic centimeters are evaporated to thirty cubic centimeters,
and the residue sucked into the generating flask of the apparatus, Fig.
81, and, with the rinsings with distilled water, evaporated again to
from twenty to thirty cubic centimeters, and the flask then connected,
as shown in the figure, to a Schliff measuring apparatus, B.[300] This
apparatus is previously filled to _i_ with mercury, and the bulb _g_
connected with _k_ by a rubber tube.
[Illustration:
FIGURE 81. SCHMIDT’S APPARATUS.
]
The apparatus is then filled with a twenty per cent, previously boiled
and still warm, caustic soda solution until the bulb _g_ is partially
filled when raised a little above the cock _h_. Then _h_ is closed and
_g_ held, by an appropriate support, on about the same level with _h_.
The cock at _b_ is then closed and _e_ opened. Meanwhile the ebullition
in the flask is continued, and the air bubbles rising in the Schliff
apparatus are removed, from time to time, by carefully opening _h_ and
raising _g_. When bubbles no longer come over, the cock at _e_ is closed
and at _b_ opened, and the steam issuing at _a_ is conducted through a
mixture of ferrous chlorid and strong hydrochloric acid to free it, as
far as possible, from air. When the contents of the flask have been
evaporated to about five cubic centimeters, _b_ is closed and the lamp
at once removed.
By carefully opening _b_ about ten cubic centimeters of a mixture of
ferrous chlorid and hydrochloric acid are allowed to enter the flask,
when _b_ is closed and the flask slowly heated until the positive
pressure is restored. The pinch-cock _e_ is then opened and the contents
of the flask evaporated nearly to dryness. The cock _e_ is again closed
and the flame removed. Another quantity (fifteen cubic centimeters) of
ferrous chlorid and hydrochloric acid solution is sucked into the flask
and the process of distillation repeated, whereby the whole of the
nitric oxid is collected in _h_. The nitric oxid evolved is measured in
the usual way and calculated to nitric acid, one cubic centimeter of
nitrogen dioxid being equal to 2.417 milligrams of nitric acid.
=469. Merits of the Ferrous Chlorid Process.=—The possibility of an
accurate determination of nitrates; by decomposition with a ferrous salt
in presence of an excess of acid, has been established by many years of
experience and by the testimony of many analysts. The method is
applicable especially where the quantity of nitrate is not too small and
when organic matter is present. In the case of minute quantities of
nitrate, however, the process is inapplicable and must give way to some
of the colorimetric methods to be hereafter described.
In respect of the apparatus modern practice has led to the preference of
that form which does not require the use of carbon dioxid for displacing
the air. Steam appears to be quite as effective as carbon dioxid and is
much more easily employed. That form of apparatus should be used which
is the simplest in construction and has the least cubical content.
The measurement of the evolved gas is most simply made by collecting
over lye in an azotometer, reading the volume, noting the reading of the
barometer and thermometer and then reducing to standard conditions of
pressure and temperature by the customary calculations. Where a very
strong lye is used the tension of the aqueous vapor may be neglected.
While every analyst should have a thorough knowledge of the ferrous
chlorid method and the principles on which it is based it can not be
compared in simplicity to the later methods with pure nitrates which are
based on the conversion of the nitric acid into ammonia by the action of
nascent hydrogen. In accuracy, moreover, it does not appear to have any
marked advantage over the reduction methods.
=470. Mercury and Sulfuric Acid Method.=—This simple and accurate method
of determining nitric acid in the absence of organic matter is known as
the Crum-Frankland process.[301]
The method rests on the principle of converting nitric acid into nitric
oxid by the action of mercury in the presence of sulfuric acid. The
operation as at first described is conducted in a glass jar eight inches
long by one and a half inches in diameter filled with mercury and
inverted in a trough containing the same liquid. The nitrate to be
examined, in a solid form, is passed into the tube together with three
cubic centimeters of water and five of sulfuric acid. With occasional
shaking, two hours are allowed for the disengagement of the gas, which
is then measured.
=471. Warington’s Modification.=—A graduated shaking tube is employed
which allows the nitrate solution and oil of vitriol to be brought to a
definite volume. The nitrate solution, with rinsings, is always two
cubic centimeters and enough sulfuric acid is added to increase the
volume to five cubic centimeters. The sulfuric acid should give no gas
when shaken with distilled water. Any gas given off in the apparatus
before shaking, is not expelled but is included in the final result. The
persistent froth sometimes noticed where some kinds of organic matter
are present, is reduced by the addition of a few drops of hot water
through the stop-cock of the apparatus. The nitric oxid is finally
measured in Frankland’s modification of Regnault’s apparatus.
This method, accurate for pure nitrates, unfortunately fails in the
presence of any considerable amount of organic matter.
According to Warington’s observations the presence of chlorids is no
hindrance to the accurate determination of both nitric and nitrous acids
by the mercury method. This simplifies the operation as carried on by
Frankland who directs that any chlorin present, be removed before the
determination of the nitric acid is commenced.
=472. Noyes’ Method.=—In the analyses made by Noyes for the National
Board of Health, the Crum-Frankland method was employed.[302] The
apparatus used was essentially that which is now known as Lunge’s
nitrometer and it will be described in the next paragraph. No correction
is made by Noyes for the tension of aqueous vapor in the measurement of
the nitric oxid because of the moderate dilution of the sulfuric acid by
the liquid holding the nitric compounds in solution. The chlorin was not
removed from the dry residue of the evaporated water as its presence in
moderate quantity does not interfere with the accuracy of the process.
In order to obtain the amount of nitrogen in the form of nitrates, the
total volume of nitric oxid must be diminished by that due to nitrites
present, which must be determined in a separate analysis. The method of
manipulation is given in the following paragraph.
[Illustration:
FIGURE 82. LUNGE’S NITROMETER.
]
=473. Lunge’s Nitrometer.=—The apparatus employed by Noyes, in a
somewhat more elaborate form, is known as Lunge’s nitrometer.[303] This
apparatus is shown in Fig. 82. It consists of a burette, _a_, divided
into one-fifth cubic centimeters. At its upper end it is expanded into a
cup-shaped funnel attached by a three-way glass stop-cock. Below, the
burette is joined to a plain tube, _b_, of similar size, by means of
rubber tubing. The apparatus is first filled with mercury through the
tube _b_, the stop-cock being so adjusted as to allow the mercury to
fill the cup at the top of _a_. The cock is then turned until the
mercury in the cup flows out through the side tube carrying the rubber
tube and clamp. The three-way cock is closed, and the solution
containing the nitrate placed in the cup. By lowering the tube _b_ and
opening the cock the liquid is carefully passed into _a_, being careful
to close the cock before all the liquid has passed out of the cup. By
repeated rinsings with pure concentrated sulfuric acid, every particle
of the nitric compound is finally introduced into _a_, together with a
large excess of sulfuric acid. The total volume of the introduced liquid
should not exceed ten cubic centimeters. The mixture of the mercury,
nitric compound, and sulfuric acid is effected by detaching _a_ from its
support, compressing the rubber connection between _a_ and _b_, placing
_a_ nearly in a horizontal position, and quickly bringing it into a
vertical position with vigorous shaking.
After about five minutes the reaction is complete, and the level of the
liquids in the two tubes is so adjusted as to compensate for the
difference in specific gravity between the acid mixture in _a_ and the
mercury in _b_; in other words, the mercury column in _b_ should stand
above the mercury column in _a_ one-seventh of the length of the acid
mixture in _a_. This secures atmospheric pressure on the nitric oxid
which has been collected in _a_. The measured volume of nitric oxid
should be reduced to 0° and 760 millimeters barometric pressure. Each
cubic centimeter of nitric oxid thus obtained corresponds to 1.343
milligrams NO; 2.417 milligrams N₂O₅; 4.521 milligrams KNO₃; 1.701
milligrams N₂O₃; 2.820 milligrams HNO₃; and 3.805 milligrams NaNO₃.
=474. Lunge’s Improved Apparatus.=—Lunge has lately improved his
apparatus for generating and measuring gases and extended its
applicability.[304] The part of it designed to measure the volume of a
gas is the same in all cases. For generating the gas, the apparatus
varies according to the character of the substance under examination.
The measuring apparatus is shown in Fig. 83. It is composed essentially
of three tubes, conveniently mounted on a wooden holder with a box base
for securing any spilled mercury. The support is not shown in the
illustration.
The tubes A, B, C, are mutually connected by means of a three-way tube
and rubber tubing with very thick walls to safely hold the mercury
without expansion. In the middle of the measuring tube A, is found a
bulb of seventy cubic centimeters capacity. Above and below the bulb the
tube is divided into tenths of a cubic centimeter, and its diameter is
such, _viz._, 11.3 millimeters, that each cubic centimeter occupies a
length of one centimeter. The upper end of A is closed with a glass cock
with two oblique perforations, by means of which communication can be
established at will, either through _e_ with the apparatus for
generating the gas, or through _d_ with the absorption apparatus, or the
opening be completely closed.
[Illustration:
FIGURE. 83.
LUNGE’S IMPROVED APPARATUS.
]