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Se0 2 + C1 2 + 2 H 2 = H 2 Se0 4 + 2 HC1.
; chlorine water.



NITROGEN



NITROGEN

PREPARATION

1. By the decomposition of ammonium chloride and
sodium nitrite. The decomposition of ammonium nitrite
results in the formation of nitrogen and water according to
equation (1) below. The instability of ammonium nitrite
precludes its use in the pure state for this reaction, hence it
is prepared and instantly decomposed by using ammonium
chloride and sodium nitrite, whose interaction produces
sodium chloride and ammonium nitrite, equation (2).

A mixture of 10 g. of ammonium chloride and 15 g.
of sodium nitrite is placed in a 200 cc. Erlenmeyer flask
fitted with a thistle-tube and a delivery-tube. Thirty cubic
centimeters of water are then added and the flask gently
heated. The nitrogen is liberated considerably below the
boiling point of water, and it is advisable not to overheat
the mixture, as frothing is likely to occur. In case the
frothing is too strong, the introduction of a few cubic cen-
timeters of water through the thistle-tube will immediately
check it. Nitrogen may be collected at the pneumatic
trough in cylinders.

(1) NH 4 N0 2 = N 2 + 2 H 2 0.

(2) NaN0 2 + NH 4 C1 = NaCl + NH 4 N0 2 .

200 cc. Erlenmeyer flask ; thistle-tube and delivery-tube ; NH 4 C1 ;
NaNO 2 .

180



NITROGEN 181

2. By the ignition of a mixture of ammonium chloride and
potassium dichromate. The decomposition of ammonium
dichrornate yielding nitrogen, water, and chromic oxide is
described in Ex. 6, p. 361. As ammonium dichromate is
rather deliquescent and not common in the laboratory, the
same results may be obtained by using a mixture of ammo-
nium chloride and potassium dichromate.

Eight grams of powdered ammonium chloride and 25 g.
of powdered potassium dichromate are intimately mixed
and placed in a 100 cc. Jena glass Erlenmeyer flask. The
flask is fitted with a very wide delivery -tube, preferably
1 cm. internal diameter, which leads to a pneumatic
trough (Fig. 128, p. 319). On heating the mixture nitrogen
is evolved and some of the ammonium chloride is sublimed,
hence the necessity of a wide delivery-tube. The Jena
glass flask will stand a very high heat without danger.

2 NH 4 C1 + K 2 Cr 2 7 = 2 KC1 + O S S + 4 H 2 + N 2 .

100 cc. Jena glass Erlenmeyer flask ; wide delivery-tube ; K 2 Cr 2 7 ;
NH 4 C1.

3. From potassium nitrate and iron powder. Iron pow-
der combines with the oxygen of potassium nitrate, liberat-
ing nitrogen.

A mixture of 10 g. of iron filings and .5 g. of pow-
dered potassium nitrate is heated in a hard-glass test-
tube fitted with a delivery-tube leading to the pneumatic
trough. When the mixture is heated, a gas is rapidly evolved
which on testing is shown to be nitrogen.

Hard-glass test-tube ; cork and delivery -tube ; KN0 3 ; Fe powder.

4. By abstraction of oxygen from air by means of phos-
phorus. The commonest source of nitrogen is air, which
consists essentially of nitrogen mixed with oxygen. To



182 CHEMICAL LECTURE EXPERIMENTS

remove the oxygen some material, such as phosphorus,
which forms a solid or non-gaseous oxide, is selected.

A flat cork, some 2.5 or 3 cm. in diameter and a centime-
ter thick, has the ring of a porcelain crucible lid pressed
into a slit in one side, and on the other side a small weight
of lead or iron is fastened to give the apparatus when floated
on water a greater stability. The float is placed on the sur-
face of the water in a pneumatic trough and a 5 mm. piece
of well-dried phosphorus laid on the crucible lid. A tubu-
lated bell-jar, with a rather wide mouth to permit of the
insertion of a candle on a wire, is
tightly corked and placed immedi-
ately above the float (Fig. 78). The
phosphorus is ignited by touching
with a hot iron wire, and the bell-
jar immediately lowered until its
mouth is sealed with water. The
phosphorus burns rapidl} 7 ", and the
FlQ 78 heat generated by the combustion

causes the gas in the bell-jar to ex-
pand and bubble out to a certain extent under the mouth of
the bell-jar. As soon as the phosphorus goes out, however,
the water will rise in the bell-jar, indicating a marked con-
traction in volume. On lowering the bell-jar until the level
of the liquids inside and outside is the same, the cork may be
removed and a burning candle on the end of a wire lowered
into the residual gas. It will be immediately extinguished.

Large cork ; crucible cover ; tubulated bell-jar ; candle on wire ; P.

5. By the removal of oxygen from the air by burning hydro-
gen. When hydrogen is burned in air water is formed, the
greater portion of which is immediately condensed. Hydro-
gen from a Kipp generator is passed through a glass tube
(Fig. 79) so bent as to rise under a tubulated bell-jar and




NITROGEN



183




FIG. 79



have its tip at least 5 cm. above the level of the water. The
glass tube is provided with a platinum tip consisting of a
small piece of foil rolled around a glass rod and inserted
in the end of the glass tube. After
lighting the hydrogen flame, which
should not be too high, the tubu-
lated bell-jar, securely corked at
the tubulature, is lowered over the
burning jet. The oxygen is rapidly
burned to form water, which de-
posits on the sides of the jar as a
mist. As it is somewhat difficult
to observe the hydrogen flame, it is
best to moisten the tip of the platinum with a little sodium
chloride solution to impart a color to the flame. The instant
the flame is extinguished the supply of hydrogen should be
cut off and the burner removed, as otherwise the admission
of unburned hydrogen will contaminate the nitrogen. The
level of the water on the inside of the bell-jar will have
risen considerably and, as in the preceding experiment, the
bell-jar should be lowered until the inner and outer levels
are the same before removing the stopper. The tubulature
should be large enough to permit the introduction of a burn-
ing candle in testing the gas. It is of the utmost impor-
tance that the hydrogen flame should be watched and the
supply of hydrogen cut off the moment the flame is ex-
tinguished. This separation of oxygen and nitrogen is
extremely simple and not difficult of comprehension by ele-
mentary students, as oxygen, hydrogen, nitrogen, and water
are familiar substances.

Tubulated bell-jar ; jet with Pt tip ; H supply ; NaCl solution.

6. By the combustion of air and ammonia on copper. When
large quantities of nitrogen are desired, by far the most sat-



184



CHEMICAL LECTURE EXPERIMENTS



isfactory method for obtaining it is that in which a mixture
of air and ammonia gas is passed over a hot copper coil,
which is alternately oxidized by the air and reduced by the
ammonia, forming water and nitrogen. The air is obtained
from' an ordinary water-blast, and the ammonia vapor from
the strongest aqua ammonia, which is placed in a gas wash-
ing-bottle (Fig. 80). The air is allowed to bubble through
the liquid and become saturated with the ammonia gas. The
gaseous mixture is then passed into a .30 cm. length of com-
bustion tubing contain-
ing a coil of copper wire
and fitted with a deliv-
ery-tube at the other end
leading to a pneumatic
trough containing acidu-
lated water. The copper
coil is heated by means of
a strong Bunsen burner
at the end nearest the
entrance of the gases. The copper will become oxidized
by the oxygen of the air, and the copper oxide thus formed
be immediately reduced by the ammonia gas, forming water
and nitrogen. The reduced copper is then immediately
oxidized and reduced, the process being a continuous
one. The air, deprived of its oxygen by the copper,
passes on as nitrogen into the pneumatic trough. To this
nitrogen is, of course, added the nitrogen liberated from
the ammonia. As the reaction proceeds, the copper coil
becomes intensely hot and the external heat may be with-
drawn. There should always be an excess of ammonia gas
in the air current, and consequently all of the spiral but
the front end should be in the reduced state. As soon
as it begins to oxidize for any distance it is an indication
that the supply of ammonia is becoming exhausted. By




FIG. 80



NITROGEN 185

observing the color of the spiral, the operation can be well
regulated. The excess of ammonia vapor carried along to
the pneumatic trough will be dissolved in the water, and
unless large quantities of nitrogen are to be prepared and
the water becomes saturated with the ammonia, no precau-
tion is necessary. It may, however, be necessary after a
while to have free sulphuric acid present to neutralize the
ammonia as it is dissolved. When very strong ammonia
water is used in the gas washing-bottle, it is found that a
good strong air-blast is necessary.

2 NH 3 + 3 CuO = 3 H 2 + 3 Cu + N 2 .

Air-blast ; gas washing-bottle j combustion-tube ; strongest NH 4 OH ;
Cu coil.

7. Oxidation of nitrogen by burning magnesium in air.

The heat of burning magnesium is sufficient to cause a
union of nitrogen and oxygen in small quantities.

An 8 cm. piece of magnesium ribbon is tied to a stout
iron wire and after ignition quickly lowered into a clean, dry
500 cc. cylinder having a layer of sand on the bottom. The
nitrogen and the oxygen of the air are caused to combine, and
a strong test for nitrous acid is obtained with iodo-starch
paper lowered into the cylinder. The layer of sand pre-
vents the cylinder from cracking if bits of burning magne-
sium fall to the bottom.

500 cc. cylinder ; Mg ribbon ; Kl-starch paper.

8. Formation of oxides of nitrogen by the combustion of
hydrogen in oxygen. The union of nitrogen and oxygen
may be brought about by the intense heat of the hydrogen
flame burning in oxygen mixed with a small quantity of air.
A clean, dry Erlenmeyer flask of 1 or 2 1. capacity is filled
with oxygen by displacement and a burning jet of hydrogen
lowered into the flask. On account of the heat generated it



186 CHEMICAL LECTURE EXPERIMENTS

is necessary to have a platinum tip on the glass tube as is
recommended in Ex. 5. The mouth of the flask being
open, a certain amount of air can enter, and a portion of
its nitrogen will combine with the oxygen to form nitrous
acid. After allowing the hydrogen to burn for a few minutes
the jet is withdrawn and moistened iodo-starch paper is
held in the flask. It is immediately colored blue, indicat-
ing the presence of nitrous acid.

Jet (Fig. 41, p. 85) ; H generator; jar of ; Kl-starch paper.



ATMOSPHERIC AIR

9. Determination of oxygen in air by potassium pyrogal-
late. One hundred cubic centimeters of air are introduced
into the eudiometer (Fig. 11, p. 26), and potassium pyrogallate
solution (Ex. 21, p. 26) allowed to flow slowly down through
the gas. The absorption will have ceased when the liquid
stops rising in the tube. Before reading off the volume of
the remaining gas the reagent should be washed out of the
tube by allowing successive portions of water to flow through
the stop-cock. It will be found that the volume has dimin-
ished 20 cc., or one-fifth.

The method is capable of yielding very accurate analyses
if the eudiometer is lowered in taking each reading till the
levels of the inner and outer liquids are the same. In exact
measurements, however, fluctuations in the temperature of
the gas must be avoided.

Eudiometer, Fig. 11, p. 26 ; potassium pyrogallate solution.

10. Quantitative determination of nitrogen in air. While
the combination of burning phosphorus and oxygen attended
by light and heat is extremely rapid, the elements do, never-
theless, unite at ordinary temperatures.



ATMOSPHERIC AIR



187



One hundred cubic centimeters of air are introduced into
a eudiometer tube. A stick of phosphorus 3 or 4 cm. long
is carefully cleaned under water and fastened to-a piece of
copper wire. The phosphorus is then thrust under the
mouth of the eudiometer tube, which
remains under water and is pushed a
considerable distance up the tube, into
the air. The copper wire is then bent
so as to rest on the bottom of the dish
and support the phosphorus in the air
(Fig. 81). The level of the water in the
tube will, of course, be depressed by as
much as the volume of the copper and
the phosphorus introduced. Hence it is
important that the eudiometer be of suffi-
cient size to allow of this expansion over
the 100 cc. After the whole apparatus
has stood over night it will be found
that a diminution in volume has taken
place. By withdrawing the phosphorus and reading off the
volume of the residual gas it will be found to be approxi-
mately 80 cc., i.e., four-fifths of the original volume.

In case a graduated tube is not at hand a linear measure-
ment of the diminution may be made.

Eudiometer tube 2 cm. in diameter ; meter stick ; P ; Cu wire.

11. Quantitative absorption of oxygen from air by metallic
copper. By measuring the quantity of air passed over a
heated copper coil and the amount of gas collected at the
pneumatic trough, the proportion of oxygen and nitrogen
present in the air may be quantitatively determined.

A stoppered cylinder is fitted with a two-holed rubber stop-
per carrying one tube leading to the bottom of the cylinder
and a glass elbow directly connected with the combustion-tube




FIG. 81



188



CHEMICAL LECTURE EXPERIMENTS



containing the copper coil. Connection is then made with a
faucet, so that by opening the valve, water may be allowed to
flow into the glass cylinder, expelling the air at the top
through the elbow into the combustion-tube. After making
all the connections the combustion-tube containing the cop-
per is brought to red heat, the expanded air being allowed to
escape at the pneumatic trough (Fig. 82). When a constant
temperature has been reached, noted by the absence of air
bubbles from the delivery-tube, an inverted graduated stop-
pered liter cylinder filled with water is placed over the




FIG. 82

delivery-tube and water allowed to flow slowly into the
first graduate. The rate must not be too rapid, as otherwise
the absorption of oxygen would not be complete. Water is
allowed to flow into the first cylinder until it has reached
the 1000 cc. graduation. The volume of gas collected at the
pneumatic trough will be found to be about 800 cc. More
exact measurement may be made by lowering the graduate
into a deep vessel until the inner and outer levels of the
water are the same. It is thus seen that from 1000 cc. of
air, approximately 800 cc. of nitrogen remain after the



AMMONIA 189

absorption of 200 cc. of oxygen. Both cylinders should be
protected from any undue heating by asbestos screens placed
between them and the burner.

Two 1000 cc. graduated stoppered cylinders ; combustion-tube ;
Cu coil.

12. Quantitative combustion of phosphorus in a confined
volume of air. The regularity of the combustion of red
phosphorus in air makes this form of phosphorus better
adapted for the experiment in which oxygen is burned out of
a confined volume of air.

A crucible lid containing red phosphorus is arranged
as in Ex. 4, and a tubulated bell-jar placed over the float.
A mark on the bell-jar is made about 2 cm. from the mouth
and the remaining volume divided into fifths. The red
phosphorus in the crucible lid is provided with a small piece
of touch-paper, which is ignited. The bell-jar with the cork
removed is lowered into water to the point marked 2 cm.
above the mouth. The cork is then inserted, and when the
phosphorus itself begins to burn the water in the bell-jar
rises as the oxygen is consumed. The contraction of the
gas (one-fifth of the original volume) is very accurately
noted by means of this apparatus. The pneumatic trough
should be deep enough to lower the jar at the end of the
experiment till the inner and outer levels are the same.

Tubulated bell-jar (graduated in fifths) ; crucible lid on cork ; red P ;
touch-paper.

AMMONIA

FORMATION AND PREPARATION

13. By the ignition of organic substances. (a) A small
quantity of gelatine or glue heated in a test-tube yields
ammonia.

Gelatine or dry glue.



190 CHEMICAL LECTURE EXPERIMENTS

(6) The presence of ammonia may be established in the
products of the dry distillation of coal (Ex. 62, p. 325) by
inserting a piece of moistened red litmus paper in the filter-
flask of the apparatus (Fig. 130, p. 325).

14. By heating organic nitrogenous matter with soda-
lime. While gelatine and similar compounds give nitrogen
directly on ignition, many organic substances containing
nitrogen uric acid, for example do not yield ammonia
by simple heating.

A small quantity of uric acid heated in a test-tube gives
no test for ammonia.

When such compounds are intimately mixed with dry
soda-lime and ignited, their nitrogen is converted into am-
monia. If uric acid is heated with an equal volume of fused
dry soda-lime in a hard-glass test-tube, ammonia is evolved.

Uric acid ; dry soda lime.

15. By heating potassium nitrate, potassium hydroxide, and
iron powder. When iron powder is heated in the presence
of potassium hydroxide, hydrogen is liberated in a manner
similar to that shown in Ex. 5, p. 42. As is seen, however,
in Ex. 3, the ignition of iron and potassium nitrate yields
nitrogen. Heating equal volumes .5 g. each of potassium
hydroxide and potassium nitrate with 20 g. of iron fillings
in a hard-glass test-tube gives a copious evolution of
ammonia.

Fe powder ; KN0 3 ; KOH.

16. From hydrogen and nitric oxide. If a mixture of
hydrogen and nitric oxide is passed over heated platinized
asbestos 5. .volumes, of. the hydrogen combine with 2 vol-
umes of the nitric oxide and ammonia is formed.

Hydrogen from a Kipp generator is passed through a glass




AMMONIA 191

tube thrust through a three-holed cork in the neck of a small
bottle containing a 2 cm. layer of sulphuric acid. A stream
of nitric oxide (Ex. 49, p. 211) passes through a second glass
tube into the bottle. Both tubes dip beneath the surface of
the sulphuric acid that their rate of bubbling may be noticed.
Hydrogen is conducted through
the whole apparatus to drive out
all air and then the nitric oxide
generator started. The current of
hydrogen should be three times
as fast as the current of nitric
oxide. The mixed gases are then ~" Fia 83

conducted through a piece of com-
bustion-tubing or a bulb-tube containing platinized asbestos
(Fig. 83). Until the asbestos is heated no ammonia is
present in the issuing gases, which redden on exposure to
the air ; but on heating the asbestos a strong test for am-
monia is immediately obtained and no red fumes are formed.

2 NO + 5 H 2 = 2 NH 3 + 2 H 2 0.

H generator ; NO generator ; wash-bottle, with 3-holed cork ; bulb-
tube ; platinized asbestos.

17. From ammonium hydroxide and potassium hydroxide.

Strong ammonium hydroxide is allowed to drop from a sep-
arating-funnel upon solid potassium hydroxide, preferably in
the stick form, in a 500 cc. Erlenmeyer flask (Fig. 3, p. 11).
The dropping-funnel is placed in a two-holed rubber stopper,
and a glass elbow conducts away the gaseous ammonia liber-
ated. In the process of the reaction the contents of the
flask become very cold from the volatilization of ammonia,
and consequently the gas is quite dry. However, in all
experiments where a perfectly dry gas is required it should
be first conducted through a U-tube containing dry quick-



192 CHEMICAL LECTURE EXPERIMENTS

lime or soda-lime. This method is by far the most conven-
ient and available one for obtaining varying quantities of
ammonia on the lecture table.

Apparatus, Fig. 3, p. 11 ; 500 cc. flask ; dropping-funnel ; stick
KOH ; con. NH 4 OH.

18. By heating ammonium hydroxide. One of the most
convenient sources of gaseous ammonia is the strongest aqua
ammonia of commerce. The simple application of heat
suffices to drive off the ammonia which, when dried, is
ready for use. The aqueous ammonia is placed in a flask
fitted with a thistle-tube and a glass elbow. On gently
warming, ammonia is driven off and passes through a gas
washing-bottle containing a small quantity of strongest
ammonia water, and then through a U-tube containing quick-
lime or fused soda-lime to dry the gas, which may be col-
lected over mercury. The gas washing-bottle with the
strong ammonia water is used to show by the bubbling the
rate at which the gas is given off. Calcium chloride cannot
be used to dry ammonia, as it forms a compound with the
gas, and hence quicklime or soda-lime is recommended.
The gas may be collected over mercury or by displacement.

Flask with thistle-tube and elbow ; gas washing-bottle ; U-tube with
soda-lime ; strongest NH 4 OH.

19. From ammonium chloride and slaked lime. Powdered
ammonium chloride and slaked lime in equal quantities
(about 40 g. of each) are placed in a 300 cc. Jena glass
Erlenmeyer flask fitted with a safety-tube (such as is shown
in Fig. 85, p. 196) and a glass elbow. A small quantity of
concentrated ammonium hydroxide or mercury is placed in
the bend of the safety-tube. On the application of gentle
heat ammonia is rapidly evolved. The gas may be dried by
conducting it through a U-tube containing quicklime or



AMMONIA 193

fused soda-lime. Owing to its low specific gravity, ammonia
can be readily collected by displacement of air according to
the method of collecting hydrogen. Almost invariably this
method of collecting the gas may be used.

2 NH 4 C1 + Ca(OH) 2 = CaCl 2 + 2 H 2 O + 2 NH 3 .

300 cc. Jena glass Erlenmeyer flask safety-tube ; soda-lime drying-
tube ; NH 4 C1 ; Ca(OH) 2 .

PROPERTIES

20. Specific gravity. Ammonia is considerably lighter
than air, resembling hydrogen in this respect.

A jar of ammonia may be opened under the mouth of an
inverted beaker suspended on the end of a balance which
has been brought into equilibrium. On allowing the ammo-
nia to rise into the beaker and expel the air the equilibrium
will be disturbed.

Lecture-balance ; inverted beaker ; jar of NH 8 .

21. Alkaline nature and tests. (a) Ammonia gas imparts
the color characteristic of alkalies to papers saturated with
solutions of litmus, cochineal, turmeric, or phenol-phthalein.
The papers may be held in a jar of ammonia or at the
mouth of a bottle containing strong ammonium hydroxide.

Ammonia, however, is a volatile, as distinguished from a
fixed, alkali, and if the papers colored by ammonia are
allowed to remain in the air, the original colors, or in case of
phenol-phthalein absence of color, will return.

Litmus, turmeric, cochineal, phenol-phthalein papers, or solutions of
these indicators ; strong NH 4 OH ; dry red litmus paper.

(6) With gaseous hydrogen chloride. If a rod moistened
with concentrated hydrochloric acid is held at the mouth of
a test-tube from which ammonia is escaping, white fumes of
ammonium chloride will be formed,
o



194 CHEMICAL LECTURE EXPERIMENTS

(c) Action on mercurous nitrate. A piece of filter-paper
dipped in mercurous nitrate solution is instantly turned
black in the presence of ammonia.

solution.



22. Solubility in water. A tall glass cylinder is filled
with ammonia by displacement, covered with a metal disk,
and opened under water by slowly sliding the disk to one
side. The water rushes up into the cylinder with almost
explosive violence. On account of this rapid solubility of
the gas in water the use of a metal, rather than a glass, cover
is recommended, as the latter might be broken by the rapid
inrush of water.

Cylinder of NH 3 ; metal disk.

23. Solubility in water producing a fountain. A 2 1.

round-bottomed flask is filled with ammonia by displace-
ment, the gas entering through a long tube pushed through
a two-holed rubber stopper leading to the bottom of the
flask, which is supported in an inverted position. When
completely filled with ammonia, the cork and the tube are rap-
idly withdrawn and another two-holed rubber stopper, with
fittings described beyond, is rapidly inserted in the neck of
the flask. Through one of the holes in the cork a long glass
tube, whose end has been drawn out to a 2 mm. opening, is
thrust until the jet is about in the centre of the flask (Fig.
84). The other end of the tube, which must extend some
25 or 30 cm. beyond the cork, is plugged with a small piece
of rubber tubing and a bit of glass rod, and dips into a crys-
tallizing dish filled with a slightly acid solution of litmus.
Through the other hole of the cork is thrust an ordinary
medicine dropper which has been filled with water to within
a few millimeters of the end of the jet. It is important to
have the rubber bulb as well as the glass portion of the dropper



AMMONIA



195




filled with water. The whole apparatus is firmly supported



Online LibraryFrancis Gano BenedictChemical lecture experiments → online text (page 13 of 29)