A glass bulb I is attached, by means of a heavy rubber tube carrying
a screw clamp, to the glass tube A, having heavy walls and a small
internal diameter, and being one meter or more in length. The tube A
is continued in the form of a =U=, the two arms being joined by very
heavy rubber tubing securely wired. The ends of the glass tubes in the
rubber should be bent so that they come near together and form the bend
of the =U=, the rubber simply holding them in place. This is better
then to have the tube continuous, avoiding danger of breaking. A tee
tube, T, made of the same kind of glass as A, is connected by one arm,
a, with the manometer B, by a heavy rubber union well wired. The union
is made perfectly air-tight by the tube filled with mercury held by a
rubber stopper. The middle arm of the tee, a′, is expanded into a bulb,
E, branching into two arms, one of which is connected with A and the
other with the delivery-tube F, by the mercury-rubber unions, MM′, just
described. The interior of the bulb E should be of such a shape as to
allow each drop of mercury to fall at once into F without accumulating
in large quantity and being discharged in mass. The third arm of the
tee a″ is bent upwards at the end and passes into a mercury sealing
tube, D, where it is connected by means of a rubber tube with the
delivery-tube from the furnace. The flow of the mercury is regulated by
the clamp C, and care should be taken that the supply does not get so
low in I as to permit air bubbles to enter A. The manometer B dips into
the tube of mercury H. A pump thus constructed is simple, flexible,
and perfectly tight. The only part which needs to be specially made is
the tee and the one in use here was blown in our own laboratory. The
bent end of the delivery-tube F may also be united to the main tube by
a rubber joint thus aiding in inserting it into the V-shaped nozzle of
the azotometer.
The azotometer used is the one devised by Schiff and modified by
Johnson and Jenkins.[134]
We prefer to get the V nozzles separately and join them to any good
burette by a rubber tube. The water-jacket is not necessary, but the
apparatus can be left exposed until it reaches room temperature.
Any form of mercury pump capable of securing a vacuum may be used,
but the one just described is commended by simplicity, economy,
effectiveness, and long use.
=158. The Pump and Combustion Furnace.=—The pump and combustion
furnace, as used in the laboratory, are shown in Fig. 10. The pump is
constructed as just described, and rests in a wooden tray which catches
and holds any mercury which may be spilled. The furnace is placed under
a hood which carries off the products of the burning lamps and the hot
air. A well-ventilated hood is an important accessory to this process,
especially when it is carried on in summer. A small mercury pneumatic
trough catches the overflow from the pump and also serves to immerse
the end of the delivery-tube during the exhaustion of the combustion
tube.
The other details of the arrangement and connections have been
sufficiently shown in the previous paragraph.
=159. Volumetric Method in this Laboratory.=—It has been found
convenient here to vary slightly the method of the official chemists
in the following respects: The tube used for the combustion is made of
hard refractory glass, which will keep its shape at a high red heat. It
is drawn out and sealed at one end after being well cleaned and dried.
It should be about eighty centimeters in length and from twelve to
fourteen millimeters in internal diameter. The relative lengths of the
spaces occupied by the several contents of the tube are approximately
as follows: Sodium bicarbonate, two; asbestos, three; coarse copper
oxid, eight; fine copper oxid, containing sample, sixteen; coarse
copper oxid, twenty-five; spiral copper gauze, ten to fifteen; copper
oxid, eight; and asbestos plug, five centimeters, respectively.
The copper oxid should be heated for a considerable time to redness in
a muffle with free access of air before using and the copper gauze be
reduced to pure metallic copper in a current of hydrogen at a low red
heat. The anterior layer of copper oxid serves to oxidize any hydrogen
that may have been occluded by the copper. When a sample is burned
containing all or a considerable part of the nitrogen as nitrates, the
longer piece of copper gauze is used.
=160. The Combustion.=—The tube having been charged and connected
with the pump it is first freed from air by running the pump until
the mercury no longer rises in the manometer. The end of the tube
containing the sodium bicarbonate is then gently heated so that the
evolution of carbon dioxid will be at such a rate as to slowly depress
the mercury in the manometer, but never fast enough to exceed the
capacity of the pump to remove it. The lamp is extinguished under the
sodium carbonate and the carbon dioxid completely removed by means of
the pump. The delivery-tube is then connected with the azotometer, and
the combustion tube carefully heated from the front end backwards, the
copper gauze and coarse copper oxid being raised to a red heat before
the part containing the sample is reached. When the nitrogen begins to
come off, its flow should be so regulated by means of the lamps under
the tube, as to be regular and not too rapid. From half an hour to
an hour should be employed in completing the combustion. Since most
samples of fertilizer contain organic matter, the nitrogen will be
mixed with aqueous vapor and carbon dioxid. The former is condensed
before reaching the azotometer, and the latter is absorbed by the
potassium hydroxid. When the sample is wholly of a mineral nature it
should be mixed with some pure sugar, about half a gram, before being
placed in the tube. When bubbles of gas no longer come over, the heat
should be carried back until there is a gradual evolution of carbon
dioxid under the conditions above noted. Finally the gas is turned
off and the pump kept in operation until the manometer again shows a
perfect vacuum when the operation may be considered finished. In the
manipulation our chief variation from the official method consists in
connecting the combustion apparatus with the measuring tube before
the heat is applied to the front end of the combustion tube. Any
particles of the sample which may have stuck to the sides of the tube
on filling will thus be subject to combustion and the gases produced
measured. Where it is certain that no such adhesion has taken place it
is somewhat safer on account of the possible presence of occluded gases
to heat the front end of the tube before connecting the combustion
apparatus with the azotometer.
=161. Method of Johnson and Jenkins.=—In the method of Johnson and
Jenkins the principal variation from the process described consists
in introducing into the combustion tube a source of oxygen whereby
any difficultly combustible carbon may be easily oxidized and all the
nitrogen be more certainly set free.[135] The potassium chlorate used
for this purpose is placed in the posterior part of the tube, which is
bent at slight angle to receive it. The sodium bicarbonate is placed
in the anterior end of the tube. The combustion goes on as already
described, and at its close the potassium chlorate is heated to evolve
the oxygen. The free oxygen is absorbed by the reduced copper oxid, or
consumed by the unburned carbon. Any excess of oxygen is recognized at
once by its action on the copper spiral. As soon as this shows signs
of oxidation the evolution of the gas is stopped. Care must be taken
not to allow the oxygen to come off so rapidly as to escape entire
absorption by the contents of the combustion tube. In such a case the
nitrogen in the measuring tube would be contaminated.
It is rarely necessary in fertilizer analysis to have need of more
oxygen than is contained in the copper oxid powder in contact with the
sample during the progress of combustion.
=162. Calculation of Results.=—The nitrogen originally present in a
definite weight of any substance having been obtained in a gaseous form
its volume is read directly in the burette in which it is collected.
This instrument may be of many forms but the essential feature of
its construction is that it should be accurately calibrated; and the
divisions so graduated as to permit of the reading of the volume
accurately to a tenth of a cubic centimeter. For this purpose it
is best that the internal diameter of the measuring tube be rather
small so that at least each ten cubic centimeters occupies a space
ten centimeters long. The volume occupied by any gas varies directly
with the temperature and inversely with the pressure to which it
is subjected. The quantity of aqueous vapor which a moist gas may
contain is also a factor to be considered. Inasmuch as the nitrogen
in the above process of analysis is collected over a strong solution
of potassium hydroxid capable of practically keeping the gas in a dry
state the tension of the aqueous vapor may be neglected.
=163. Reading the Barometer.=—Nearly all the barometers in use in this
country have the scale divided in inches and the thermometers thereunto
attached are graduated in Fahrenheit degrees. This is especially true
of the barometers of the Weather Bureau which are the most reliable
and most easy of access to analysts. It is not necessary to correct
the reading of the barometer for altitude, but it is important to
take account of the temperature at the time of observation. There is
not space here to give minute directions for using a barometer. Such
directions have been prepared by the Weather Bureau and those desiring
it can get copies of the circular.[136]
The temperature of a barometer affects its accuracy in two ways. First
the metal scale expands and contracts with changing temperatures:
Second, the mercury expands and contracts also at a much greater rate
than the scale. If a barometer tube hold thirty cubic inches of mercury
the contents will be one ounce lighter at 80° F. than at 32° F. The
true pressure of the air is therefore not shown by the observed height
of the mercurial column unless the temperature of the scale and of the
mercurial column be considered.
Tables of correction for temperature are computed by simple formulas
based on the known coefficients of expansion of mercury and brass. For
barometers with brass scales the following formula is used for making
the correction:
_t_ - 28.63
C = -_h_ --------------------- .
1.113_t_ + 10978
In this formula _t_ = temperature in degrees Fahrenheit and _h_ =
observed reading of the barometer in inches.
_Example_:—Temperature observed 72°.5
Barometer reading observed, 29.415 inches,
from which C = 0.1165, and this number, according to the
conditions of the formula, is to be subtracted from the
observed reading. The true reading in the case given is,
therefore,
29.298 inches or 744.2 millimeters.
The observed reading 747.1 “
-----
And the correction 2.9 “
Unless extremely accurate work be required the correction for
temperature is of very little importance in nitrogen determinations
in fertilizers. Each instrument sent out by the Weather Bureau is
accompanied by a special card of corrections therefor, but these are of
small importance in fertilizer work. In order then to get the correct
weight of the gas from its volume the reading of the thermometer and
barometer at the time of measurement must be carefully noted. However,
after the end of the combustion, the azotometer should be carried into
another room which has not been affected by the combustion and allowed
to stand until it has reached the room temperature.
Every true gas changes its volume under varying temperatures at the
same rate and this rate is the coefficient of gaseous expansion.
For one degree of temperature it amounts to 0.003665 of its volume.
Representing the coefficient of expansion by K the volume of the gas as
read by V, the volume desired at any temperature by V′, the temperature
at which the volume is read by _t_ and the desired temperature by _t′_,
the change in volume may be calculated by the following formula:
V′ = V[1 + K(_t′_ - _t_)].
_Example._—Let the volume of nitrogen obtained by combustion be
thirty-five cubic centimeters, and the temperature of observation 22°.
What would be the volume of the gas at 0°?
Making the proper substitutions in the formula the equation is reduced
to the form below:
V′ = 35[1 + 0.003665(0°-22°)]
or V′ = 35(1 - 0.08063) = 32.18.
Thirty-five cubic centimeters of nitrogen therefore measured at 22°
become 32.18 cubic centimeters when measured at 0°.
When gases are to be converted into weight after having been determined
by volume, their volume at 0° must first be determined; but this
volume must also be calculated to some definite barometric pressure.
By common consent this pressure has been taken as that exerted by a
column of mercury 760 millimeters in height. Since the volume of a gas
is inversely proportional to the pressure to which it is subjected, the
calculation is made according to that simple formula. Let the reading
of the barometer, at the time of taking the volume of gas, be H, and
any other pressure desired H′. Then we have the general formula:
HV
V : V′ = H′ : H; and V′ = ----.
H′
_Example_: Let the corrected reading of the barometer at the
time of noting the volume of the gas be 740 millimeters, and the
volume of the gas reduced to 0° be 32.18 cubic centimeters. What
will this volume be at a pressure of 760 millimeters?
Substituting the proper values in the formula, we have:
(32.18 × 740)
V′ = ------------- = 31.33
760
Therefore, a volume of nitrogen which occupies a space of thirty-five
cubic centimeters at a temperature of 22°, and at a barometric pressure
of 740 millimeters, becomes 31.33 cubic centimeters at a temperature of
0° and a pressure of 760 millimeters.
One liter of nitrogen at 0° and 760 millimeters pressure weighs 1.25456
grams; and one cubic centimeter therefore 0.00125456 gram. To find the
weight of gas obtained in the above supposed analysis, it will only be
necessary to multiply this number by the volume of nitrogen expressed
in cubic centimeters under the standard conditions; _viz._, 0.0125456 ×
31.33 = 0.039305 gram. If the sample taken for analysis weighed half a
gram, the percentage of nitrogen found would be 7.85.
=164. Tension of the Aqueous Vapor.=—It has been shown by experience
that when a gas is collected over a potash solution containing fifty
per cent of potassium hydroxid, the tension of the aqueous vapor is
so far diminished as to be of no perceptible influence on the final
result. To correct the volume of a gas, therefore, so collected for
this tension, would involve an unnecessary calculation for practical
purposes. If a gas thus collected should be transferred to a burette
over mercury, on which some water floats, then the correction should be
made.
At 0° the tension of aqueous vapor will support a column of mercury
4.525 millimeters high, and at 40° one of 54.969 millimeters.
The following table gives the tension of aqueous vapors in millimeters
of a mercurial column for each degree of temperature from zero to forty.
Tension of Tension of
vapor in vapor in
Temperature. millimeters. Temperature. millimeters.
0° 4.525 21° 18.505
1° 4.867 22° 19.675
2° 5.231 23° 20.909
3° 5.619 24° 22.211
4° 6.032 25° 23.582
5° 6.471 26° 25.026
6° 6.939 27° 26.547
7° 7.436 28° 28.148
8° 7.964 29° 29.832
9° 8.525 30° 31.602
10° 9.126 31° 33.464
11° 9.751 32° 35.419
12° 10.421 33° 37.473
13° 11.130 34° 39.630
14° 11.882 35° 41.893
15° 12.677 36° 44.268
16° 13.519 37° 46.758
17° 14.409 38° 49.368
18° 15.351 39° 52.103
19° 16.345 40° 54.969
20° 17.396
When a gas is in contact with water the aqueous vapor is diffused
throughout the mass, and the pressure to which the mixture is
subjected, is partly neutralized by the tension of the water vapor.
The real pressure to which the gas, whose volume is to be determined
is subjected, is therefore diminished by that tension. If for instance
a gas in contact with water show a volume of thirty-five cubic
centimeters at 22° and 740 millimeters barometric pressure its volume
is really greater than if it were perfectly dry. How much greater can
be determined by inspecting the table, for at 22° the tension of water
vapor is 19.675 millimeters of mercury. The real pressure to which
the volume of gas is subjected is therefore 740 - 19.675 = 720.325
millimeters.
If, therefore, in the example given, the nitrogen were in contact with
water, the calculation would proceed as follows:
32.18 × 720.325
V′ = ----------------- = 30.5,
760
and 30.5 × 1.25456 = 38.26.
Hence, 38.26 milligrams of nitrogen correspond to 7.65 per cent, when
half a gram of substance is taken for the combustion.
=165. Aqueous Tension in Solutions of Potassium Hydroxid.=—Even in
strong solutions of potassium hydroxid the tension of aqueous vapor is
not destroyed, but is reduced to a minimum, which is negligible in the
calculation of the percentage by weight of the nitrogen in a sample of
fertilizer. When dilute solutions of a caustic alkali are used however,
the neglect of the tension of the aqueous vapor may cause an error of
some magnitude. In such cases the strength of the solution should be
known and correction made according to the following table:[137]
Millimeters tension of aqueous vapor for KOH solutions of
|
+----------------------------+-------------------------+
Temperature. | 9.09 per 16.66 per 23.08 per 28.57 per 32.89 per |
cent. cent. cent. cent. cent.
10°.00 8.62 8.01 7.31 6.50 5.62
11°.00 9.21 8.56 7.82 6.95 6.01
12°.10 9.90 9.21 8.41 7.47 6.46
13°.00 10.50 9.77 8.92 7.93 6.86
13°.95 11.17 10.39 9.49 8.44 7.30
15°.15 12.06 11.22 10.25 9.11 7.86
16°.00 12.74 11.85 10.82 9.62 8.33
17°.00 13.57 12.63 11.54 10.26 8.88
18°.00 14.46 13.45 12.29 10.93 9.47
19°.00 15.39 14.33 13.09 11.65 10.09
20°.00 16.38 15.25 13.93 12.40 10.75
21°.00 17.42 16.22 14.82 13.20 11.44
21°.82 18.32 17.06 15.59 13.88 12.04
23°.00 19.68 18.32 16.75 14.92 12.94
24°.00 20.92 19.47 17.80 15.86 13.76
25°.00 22.19 20.67 18.91 16.85 14.62
26°.00 23.55 21.94 20.07 17.89 15.53
26°.98 24.95 23.25 21.27 18.96 16.46
27°.93 26.38 24.59 22.51 20.07 17.45
29°.00 28.08 26.18 23.96 21.38 18.59
30°.00 29.76 27.74 25.40 22.67 19.72
31°.00 31.51 29.38 26.91 24.03 20.91
32°.13 33.61 31.34 28.72 25.65 22.34
33°.00 35.30 32.93 30.18 26.97 23.50
34°.00 37.34 34.84 31.94 28.56 24.89
=166. Use of Volumetric Method.=—For practical purposes it may be said
that the volumetric determination of nitrogen in fertilizer analysis
has gone entirely out of use. For control and comparison it is still
occasionally practiced but it has had to give way to the more speedy
and fully as accurate processes of moist combustion with sulfuric
acid which have come into general use in the last decade. The student
and analyst however should not fail to master its details and become
skilled in its use. There are certain nitrogenous substances such
as the alkaloids which are quite refractory when subjected to moist
combustion. While such bodies may not occur in fertilizers it is well
to have at hand a means of accurately determining their nitrogen
content.
=167. Tables for Calculating Results.=—Where many analyses are to be
made by the copper oxid process it has proved convenient to shorten the
work of calculating analyses by taking the data given in computation
tables.[138] Before using these tables it must be known whether they
are calculated on the supposition that the gas is measured in a moist
state, partly moist, or wholly dry. Where the nitrogen is collected
over water a table must be used in which allowance has been made for
the tension of aqueous vapor. In case a saturated solution of a caustic
alkali be used in the azotometer it is customary to take no account
of the tension and the table employed must be constructed on this
supposition. In point of fact even in the strongest alkali solution
there is a certain amount of tension but this is so slight as only to
affect the results in the second place of percentage decimals. Since,
as a rule, only a few analyses are made by this method it will be
found safer to use a caustic alkali solution of given strength and to
calculate the results from the tables of aqueous tensions given above.
=168. The Soda-Lime Process.=—This process originally perfected
by Varrentrap and Will, and improved by Peligot, was used almost
exclusively by analysts until within the last decade for the
determination of nitrogen not existing in the nitric form. It is based
on the principle that when nitrogen exists as a salt of ammonia, or as
an amid, or as proteid matter, it is converted into gaseous ammonia
by combustion with an alkali. This ammonia can be carried into a set
solution of acid by a stream of gas free of ammonia and the excess of
acid remaining after the combustion is complete can be determined by
titration against a standard alkali solution. The results under proper
conditions are accurate even when a small quantity of nitric nitrogen
is present. When, however, there is any considerable quantity of this
compound in the sample the method becomes inapplicable by reason of
non-reduction of some of the nitrogen oxids produced by the combustion.
In bodies very rich in nitrogen such as urea all the nitrogen is
not transformed directly into ammonia at the commencement of the
combustion. A portion of it may unite with a part of the carbon to form
cyanogen, which may unite with the soda to form sodium cyanid. With an
excess of alkali, however, and prolonged combustion this product will
be finally decomposed and all the nitrogen be secured as ammonia.
The nascent hydrogen which unites with the nascent nitrogen during
the combustion is also derived from the organic matter which always
contains enough carbon to decompose the water formed in order to
be oxidized to carbon dioxid. While at first, therefore, during
combustion, the hydrogen may unite with the oxygen, it becomes again
free by the oxidation of the carbon and in this condition unites with
the nascent nitrogen to form ammonia. In addition to carbon dioxid,
ammonia, and free hydrogen there may also be found among the products
of combustion marsh and olefiant gases and other hydrocarbon compounds
which dilute, to a greater or less extent, the ammonia formed and help
to carry it out of the combustion tube and into the standard acid.
=169. The Official Method.=—_Reagents and Apparatus._—(1) Standard
solutions and indicator the same as for the kjeldahl method:
(2) Dry granulated soda-lime, fine enough to pass a 2.5 millimeter
sieve:
(3) Soda-lime, fine enough to pass a 1.25 millimeter sieve.[139]
Soda-lime may be easily and cheaply prepared by slaking two and
one-half parts of quicklime with a strong solution of one part of
commercial caustic soda, care being taken that there is enough water in
the solution to slake the lime. The mixture is then dried and heated in
an iron pot to incipient fusion, and, when cold, ground and sifted as
above.
Instead of soda-lime Johnson’s mixture of sodium and calcium carbonate,
or slaked lime may be used. Slaked lime may be granulated by mixing
it with a little water to form a thick mass, which is dried in the
water-oven until hard and brittle. It is then ground and sifted as
above. Slaked lime is much easier to work with than soda-lime, and
gives excellent results, though it is probable that more of it should
be used in proportion to the substance to be analyzed than is the case
with soda-lime.
(4) Asbestos, which has been ignited and kept in a glass-stoppered
bottle.
(5) Combustion tubes about forty centimeters long and twelve
millimeters internal diameter, drawn out to a closed point at one end.
(6) Large-bulbed =U= tubes with glass stop-cock, or Will’s tubes with
four bulbs.
_Manipulation._—The substance to be analyzed should be powdered
finely enough to pass through a sieve of one millimeter mesh; from
seven-tenths to one and four-tenths grams, according to the amount of
nitrogen present, are taken for the determination. Into the closed
end of the combustion tube, put a small loose plug of asbestos, and
upon it about four centimeters of fine soda-lime. In a porcelain dish
or mortar, mix the substance to be analyzed, thoroughly but quickly,
with enough fine soda-lime to fill about sixteen centimeters of the
tube, or about forty times as much soda-lime as substance, and put the
mixture into the combustion tube as quickly as possible by means of
a wide-necked funnel, rinsing out the dish and funnel with a little
more fine soda-lime, which is to be put in on top of the mixture. Fill
the rest of the tube to within about five centimeters of the end with
granulated soda-lime, making it as compact as possible by tapping
the tube gently while held in a nearly upright position during the
filling. The layer of granulated soda-lime should not be less than
twelve centimeters long. Lastly, put in a plug of asbestos about two
centimeters long, pressed rather tightly, and wipe out the end of the
tube to free it from adhering soda-lime.
Connect the tube by means of a well-fitting rubber stopper or cork
with the =U= tube or Will’s bulbs, containing ten cubic centimeters
of standard acid, and adjust it in the combustion furnace so that the
end of the tube projects about four centimeters from the furnace,
supporting the =U= tube or Will’s bulb suitably. Heat the portion of
the tube containing the granulated soda-lime to a moderate redness, and
when this is attained extend the heat gradually through the portion
containing the substance, so as to keep up a moderate and regular flow
of gases through the bulbs, maintaining the heat of the first part
until the whole tube is heated uniformly to the same degree. Continue
the combustion until gases have ceased bubbling through the acid in the
bulbs, and the mixture of substance and soda-lime has become white, or
nearly so, which shows that the combustion is finished. The process
should occupy about three-quarters of an hour, or not more than one
hour. Remove the heat, and when the tube has cooled below redness break
off the closed tip and aspirate air slowly through the apparatus for
two or three minutes to bring all the ammonia into the acid. Disconnect
the tube, wash the acid into a beaker or flask, and titrate with the
standard alkali.
During the combustion the end of the tube projecting from the furnace
must be kept heated sufficiently to prevent the condensation of
moisture, yet not enough to char the stopper. The heat may be regulated
by a shield of tin slipped over the projecting end of the combustion
tube.
It is found very advantageous to attach a bunsen valve to the exit
tube, allowing the evolved gases to pass out freely, but preventing a
violent sucking back in case of a sudden condensation of steam in the
bulbs.
=170. The Official French Method.=—The French chemists prefer to drive
out the traces of ammonia remaining in the combustion tube by means
of the gases arising from the decomposition of oxalic acid.[140] The
operation is conducted by mixing about one gram of oxalic acid with
enough of dry granular soda-lime to form a layer of four centimeters
in length at the bottom of the tube. The rest of the tube is then
charged substantially as directed above. At the end of the combustion
the oxalic acid is decomposed by heat furnishing sufficient hydrogen to
remove from the tube all traces of ammonia which it may contain. The
French chemists employ, for titration, either normal acids and alkalies
or some decimal thereof or else an acid of such strength as to have
each cubic centimeter thereof correspond to ten milligrams of nitrogen,
thus making the computation of results exceedingly simple. Such an
acid is secured when one liter thereof contains thirty-five grams of
pure monohydric sulfuric acid or forty-five grams of pure crystallized
oxalic acid. The corresponding alkaline reagent should contain, in each
liter, forty grams of pure potassium hydroxid.
=171. The Hydrogen Method.=—Thibault and Wagner recommend that
the combustion with soda-lime be conducted in an atmosphere of
hydrogen,[141] and Loges replaces this by common illuminating gas freed
from ammonia by conducting it through a tube filled with glass balls
moistened with dilute sulfuric acid.[142]
In these cases the combustion tube is left open at both ends and the
materials under the tube confined to the proper position by asbestos
plugs. The gases used act in a merely mechanical manner and their use
affords so few advantages over the method of aspirating air at the end
of the combustion as to render it unadvisable.
=172. Coloration of the Product.=—It often happens, especially in the
combustion of animal products, such as tankage and fish scrap, that the
acid securing the ammonia is deeply colored by the condensation of some
of the other products of combustion. This coloration interferes in a
very serious way with the delicacy of the indicator used to determine
the end of the reaction. In this case the liquid may be mixed with an
alkali and distilled and the ammonia secured in a fresh portion of
the standard acid as in the moist combustion process to be hereafter
described.
=173. General Considerations.=—(1) _Preparation of Sample._—In the
soda-lime method it is of great importance that the organic substances
be in a fine state of subdivision so as to admit of intimate mixture
with the alkali. In cases where fragments of hoof, horn, hair, or
similar substances are to be prepared for combustion it is advisable
to first decompose them by heating with a small quantity of sulfuric
acid. The excess of acid may be neutralized with marble dust and the
resulting mixture dried, rubbed to a fine powder, and mixed with the
soda-lime in the usual way. Care must be taken not to lose any of
the ammonia from the sulfate which may be formed in mixing with the
soda-lime in filling the tube.
(2) _Purity of Soda-Lime._—The soda-lime employed must be entirely free
of nitrogenous compounds and some blank combustions should be made in
proof of its purity.
(3) _Temperature._—The temperature of the combustion should not be
allowed to exceed low redness. At very high temperatures there would be
danger of decomposing the ammonia.
(4) _Aspiration of Air._—Before aspiring a current of air through the
tube to remove the last traces of ammonia the gas should be put out
under the furnace and the tube be allowed to cool below redness to
avoid any danger of acting on the nitrogen in the air.
=174. The Ruffle Soda-Lime Method.=—Many attempts have been made to
adapt the soda-lime method to the determination of nitric nitrogen. Of
these the process devised by Ruffle is the only one which has proved
successful.[143] The method is founded on the action of sulfurous
vapors on the nitrogen oxids produced during the combustion whereby
sulfuric acid is formed and the nascent nitrogen is joined with
hydrogen to form ammonia. By this process all the nitrogen contained
in the sample, even if in the nitric form, is finally obtained as
ammonia. In the original method the reagents employed were a mixture of
sodium thiosulfate and soda-lime and a mixture of charcoal, sulfur, and
granulated soda-lime. Subsequently the official chemists substituted
sugar for the charcoal.[144] The method was used for a long time by the
official chemists and came into general favor until displaced by the
simpler and cheaper processes of the moist combustion method adapted to
nitric nitrogen. As finally modified and used by the official chemists
the process was conducted as described below.
=175. The Official Ruffle Method.=—[145]_Reagents._—(1) Standard
solutions and indicator the same as for the kjeldahl method.
(2) A mixture of equal parts by weight of fine-slaked lime and finely
powdered sodium thiosulfate dried at 100°:
(3) A mixture of equal parts of weight of finely powdered granulated
sugar and flowers of sulfur:
(4) Granulated soda-lime, as described under the soda-lime method:
(5) Combustion tubes of hard Bohemian glass seventy centimeters long
and one and three-tenths centimeters in diameter:
(6) Bulbed =U= tubes or Will’s bulbs, as described under the soda-lime
method:
_Manipulation._—(a) Clean the =U= tube and introduce ten cubic
centimeters of standard acid.
(b) Fit the cork and glass connecting tube. Fill the tube as follows:
(1) A loosely fitting plug of asbestos, previously ignited, and then
two and five-tenths to three and five-tenths centimeters of the
thiosulfate mixture: (2) The weighed portion of the substance to
be analyzed is intimately mixed with from five to ten grams of the
sugar and sulfur mixture: (3) Pour on a piece of glazed paper or in
a porcelain mortar a sufficient quantity of thiosulfate mixture to
fill about twenty-five centimeters of the tube; then add the substance
to be analyzed, as previously prepared, mix carefully, and pour into
the tube; shake down the contents of the tube; rinse off the paper or
mortar with a small quantity of the thiosulfate mixture and pour into
the tube; then fill up with soda-lime to within five centimeters of the
end of the tube: (4) Place another plug of ignited asbestos at the end
of the tube and close with a cork: (5) Hold the tube in a horizontal
position and tap on the table until there is a gas-channel along the
top of the tube: (6) Make connection with the =U= tube containing the
acid; aspirate and see that the apparatus is tight.
_The Combustion._—Place the prepared combustion tube in the furnace,
letting the open end project a little, so as not to burn the cork.
Commence by heating the soda-lime portion until it is brought to a full
red heat. Then turn on slowly jet after jet toward the outer end of
the tube, so that the bubbles come off two or three a second. When the
whole tube is red hot and the evolution of the gas has ceased and the
liquid in the =U= tube begins to recede toward the furnace, attach the
aspirator to the other limb of the =U= tube, break off the end of the
tube, and draw a current of air through for a few minutes. Detach the
=U= tube and wash the contents into a beaker or porcelain dish; add a
few drops of the cochineal solution, and titrate.
=176. Observations.=—In our experience we have found it much more
satisfactory to adhere to the earlier directions for preparing the
mixture of thiosulfate and alkali. We much prefer to make the mixture
with soda-lime and without the previous drying of the sodium salt.
Ruffle himself says that the sodium thiosulfate should be dry but not
deprived of its water of crystallization.[146] The best method to dry
the crystal powder without depriving it of its crystal water is to
press it between blotting papers. The official method also contains
a typographical error in prescribing that the combustion tube should
have a length of thirty centimeters where evidently thirty inches were
meant. Ruffle’s original tube was twenty-two inches in length.
As is seen from the above description the method is essentially a
reduction process by the action of a powerful deoxidizer in the
presence of an alkali. The crystals of the thiosulfate salt cannot be
brought into direct contact with a pure alkali, like soda or potash,
without forming at once a wet mass which would tend to cake and
obstruct the tube. The soda-lime is therefore a mechanical device to
prevent this fusion. Where many analyses are to be made an iron tube,
for economical reasons, may be substituted for the glass; but the glass
tube permits a more intelligent observation of the progress of the
analysis.
Since charcoal has very high absorbent powers it will be found always
to contain a little nitrogen which may be in a form to generate ammonia
during the combustion. The charcoal used should therefore be previously
boiled with caustic soda or potash solution, dried, powdered, and
preserved in well-stoppered bottles. Although pure sugar is practically
free of nitrogen, even when it is used, it is advisable to occasionally
make a blank determination and thus ascertain the correction to be made
for possible contamination.
=177. Boyer’s Modification of Ruffle’s Method.=—The principle of
the method rests on the observation that if nitrates be heated in
a combustion tube with calcium oxalate and soda-lime, not more
than two-thirds of the total nitrogen appear as ammonia; but if a
certain proportion of sulfur be added the whole of the nitrogen is
recovered.[23] The process may be divided into two reactions; _viz._:
(1) Action of the calcium oxalate upon the sodium nitrate in presence
of soda-lime:
(2) The action of sulfurous acid and of calcium oxalate upon the sodium
nitrate in presence of soda-lime.
The analysis is conducted as follows: Dry and pulverize one-half gram
of nitrate (Na or K) and mix it intimately with fifty grams of the
reducing compound containing approximately ten per cent sulfur, 22.5
per cent neutral calcium oxalate, and 67.5 per cent soda-lime. The
combustion tube is charged as follows:
Length of tube fifty-five centimeters:
Diameter of tube seventeen millimeters:
Add first two grams pulverized calcium oxalate:
Add next ten grams pulverized soda-lime:
“ “ ten grams of the reducing compound:
“ “ the nitrate incorporated with fifty grams of the reducing
mixture:
Add next ten grams of the reducing mixture:
“ “ ten grams pulverized soda-lime:
The tube is then lightly closed with an asbestos plug.
The tube is heated gradually from the front backwards, the calcium
oxalate furnishing finally the gas necessary to drive out the
last traces of ammonia. The process is equally applicable to the
determination of nitrogen in all its forms or to mixtures thereof.
The method has also been applied to the mixture of ammoniacal and
organic nitrogen and to the mixture of ammoniacal, nitric, and organic
nitrogen, the combustions having been made both in an iron and a glass
tube. The amounts of material to be used vary from one-half gram to a
gram, according to its richness in nitrogen.
The combustion should be terminated in forty minutes.
When a combustion is terminated, the acid containing the ammonia is
placed in a beaker and boiled for two or three minutes to drive off the
sulfurous and carbonic acids. The titration is then conducted in the
usual manner.
The combustion can be carried on just as well in an iron tube as
in a glass one. The reagents employed, especially soda-lime, being
hygroscopic, a little water is disengaged in heating, which is
condensed at the cold extremity of the tube, and which may absorb
a little ammonia, unless special precautions are taken to have the
materials dry.