CHLORINE.
Chlorine is a substance of very great commercial importance on account
of its extensive use as a bleaching agent. Again it is a constituent of
common salt, and in this form of combination it is both of great value
as an article of food, and it is recognized as widely distributed. Thus
it exists in salt, whether that substance is in the brine of the ocean
or of mineral springs, or whether it occurs as a solid rock—as indeed
it does in some parts of the world. At Wieliczka, in Austria, mines of
solid salt have been worked for hundreds of years. So also at Cardona,
in Spain, are what may be called quarries of this valuable mineral;
while Cheshire, in England, furnishes immense solid deposits from which
salt is obtained to supply the enormous industrial establishments using
this substance for the production of chlorine and of compounds of
sodium.
Chlorine was first recognized as a distinct substance, by a European
chemist, Carl Wilhelm Scheele, known only to his neighbors as a humble
apothecary. Scheele was born at Stralsund, a seaport town of Pomerania,
situated on the little strait which leaves the island of Rügen in the
Baltic Sea. He spent the principal portion of his life in Sweden, and
on this account is often referred to as a Swedish chemist. Though
living in great obscurity, he yet made many discoveries in chemistry
which have rendered his name, otherwise almost unknown, one of the most
brilliant in the annals of this science.
It is related of Scheele that the King of Sweden, Gustavus III.,
while on a journey outside of his own dominions, heard so much of the
fame of this chemist, unknown to him before, that he regretted having
previously done nothing for him. He therefore commanded that Scheele
receive the honor of being created chevalier. “Scheele?” “Scheele?“
said the minister charged with this duty. “This is very singular; what
in the world has Scheele done?” The order was peremptory however, and
Scheele was knighted. But, as the reader may perhaps divine, the honor
designed for the acute discoverer fell upon another Scheele—not upon
that Scheele unknown at court but illustrious among the scientists of
Europe.
It was this obscure apothecary then, who added to the list of his
other investigations a study of the properties of what was ordinarily
considered a dull and uninteresting earthy substance called black
magnesia. This study was repaid by the revelation of no less than four
hitherto unknown substances: oxygen, barium, manganese, and finally
chlorine. Scheele obtained the chlorine in the year 1774 exactly as
it is done at the present day, namely, by bringing together the two
substances, now called chlorohydric acid and black oxide of manganese,
but then known as muriatic acid and black magnesia.
Scheele believed, and other celebrated chemists concurred in the
opinion, that the greenish gas that he discovered was a compound
substance. It was not until thirty-six years later, that is 1810, that
the distinguished English chemist, Sir Humphry Davy, demonstrated that
this gas is not a compound, but is in fact a simple or elementary
substance; and it was he who gave to it the name chlorine, a name
derived from a Greek word (χλωρός, chloros, meaning light green),
conveying an obvious and convenient reminder of one striking property
of the thing referred to.
[Illustration: SIR HUMPHRY DAVY, Bart.:
Born in Penzance, England, Dec. 17, 1778; died in Geneva, Switzerland,
May 29th, 1829.
“Davy, when not yet thirty-two years old, occupied, in the opinion of
all those who could judge of such labors, the first rank among the
chemists of this or any other age.”]
How Chlorine is Obtained.
The preparation of chlorine is a very simple matter. It may be
accomplished by placing some powdered black oxide of manganese, an
abundant mineral substance, in any deep glass vessel, and then adding
to it four or five times its weight of chlorohydric acid. Anyone who
performs the experiment will soon perceive the greenish gas rising
higher and higher in the vessel, and will soon discover its choking
and corrosive odor. Moreover the chlorine gas, which is two and a half
times as heavy as air, accumulates within the flask and stays there
some time. This is the process which has already been referred to as
that which first revealed the gas to Scheele, and this process, with
but slight modification, is that which to-day furnishes the enormous
quantities of chlorine demanded by modern industries.
The Characteristics of Chlorine.
The three most striking properties of chlorine are its noticeable
weight—greater than that of the air—its greenish color, and its
exceedingly irritating odor. Its influence on the animal organism is
very violent: more than one example can be produced of fatal results
following the inhalation of too large quantities of the gas. Thus
Pelletier, a French chemist, died at Bayonne from the effects of
inhaling a considerable quantity of chlorine, and Roe, a young Irish
chemist of Dublin, lost his life from the same cause, while studying
the properties of this gas.
Chlorine, as a _chemical agent_, manifests its activities in connection
with two principal properties, namely: its affinity for hydrogen
and its affinity for the metals. By this statement it is meant that
chlorine manifests a strong tendency to combine with hydrogen, and to
combine with metals, whenever these substances are accessible to it.
When it combines with hydrogen it forms the important compound
designated by the formula H Cl and called by the chemist chlorohydric
acid, but known in commerce as muriatic acid.
When chlorine combines with the metals it forms chlorides of them.
Thus with the metal sodium it forms the compound designated by the
formula NaCl and called by the chemist indifferently sodic chloride or
chloride of sodium; these will be recognized as the chemical names for
the important and well-known substance, common salt.
Chlorine and Hydrogen Combine.
Chlorine and hydrogen have a very strong tendency to combine with each
other. They manifest this tendency in a variety of ways. Thus, if the
two gases are prepared in a dark room, they may be there safely mixed
together in a glass vessel; but if the sunlight is allowed to enter and
fall upon the vessel, there is danger of its being shattered by the
explosive violence with which the hydrogen and chlorine immediately
unite. As a result of this combination, chlorohydric acid is produced.
The chemical change is represented by the following equation:
=H₂= + =Cl₂= = =2HCl=
One molecule of One molecule of Two molecules of
Hydrogen, Chlorine, Chlorohydric acid,
2 71 73
parts by weight. parts by weight. parts by weight.
\___________________________________/ \_______________/
| |
73 73
Again, when chlorine is brought in contact with vegetable or animal
substances, containing hydrogen, it proceeds to withdraw that hydrogen
for its own benefit, even though these vegetable and animal compounds
are thereby destroyed.
Although this operation, as well as the foregoing one, produces
chlorohydric acid, yet neither method is suitable for a determinate
preparation of that substance. It is usually better to prepare
chlorohydric acid in another way. Thus it is easily produced by the
action of sulphuric acid upon common salt.
Experimental Preparation of Chlorohydric Acid.
Any one who will take a little trouble may prepare chlorohydric acid in
the way indicated.
The experiment should be conducted as follows:
Place a small amount of common salt (NaCl) in a small retort; to it add
enough concentrated sulphuric acid to make a thin paste; connect the
neck of the retort with a clean test-tube containing a few drops of
water. Now gently heat the retort; chlorohydric acid will be formed and
will distil from the retort and condense in the receiver.
The chemical change is represented by the following equation:
=NaCl= + =H₂SO₄=
One molecule of One molecule of
Sodic chloride, Sulphuric acid,
58½ 98
parts by weight. parts by weight.
\____________________________________/
|
156½
= =HCl= + =HNaSO₄=
One molecule of One molecule of
Chlorohydric acid Hydro-Sodic sulphate
36½ 120
parts by weight. parts by weight.
\_______________________________________/
|
156½
The product of the foregoing experiment may be tested in three ways and
so shown to be in fact chlorohydric acid.
_First_: Take a minute drop on a glass rod and apply it to the tongue
and observe the sour or acid taste.
_Second_: Take a drop on a glass rod and touch it upon blue
litmus-paper. It should turn the paper red.
_Third_: Pour a few drops of the liquid into a solution of argentic
nitrate (that is, nitrate of silver) in a test-tube or other convenient
vessel: a white precipitate of argentic chloride will be formed.
[Illustration: FIG. 19.—Section of furnace used for manufacture of
chlorohydric acid. Common salt and sulphuric acid are placed in the
large retort A; upon heating, chlorohydric acid passes into the
receivers C, C, C.]
The method of producing chlorohydric acid just described and
illustrated, is followed in the manufacture of the substance for
general chemical purposes. It is also employed for the production of
the enormous quantities of it incidentally used in the manufacture of
bleaching-powder.
Experiments with Common Salt.
Chlorine has already been shown to combine with the metal silver
producing the compound designated by the formula AgCl, and called
argentic chloride and also chloride of silver. This substance may also
be prepared very easily somewhat as follows:
Make a solution of nitrate of silver. Prepare it either by dissolving
in water the crystals sold by apothecaries, or by dissolving a small
piece of silver in nitric acid. Then make a second clear solution,
by dissolving common salt in ordinary water. Add the salt solution
cautiously, drop by drop to the silver solution. There immediately
appear thick masses of white flakes which sooner or later fall to the
bottom of the vessel. These flakes consist of the argentic chloride
(AgCl), also called chloride of silver, already referred to.
The chemical change is represented by the following equation:
=AgNO₃= + =NaCl=
One molecule of One molecule of
Argentic nitrate, Sodic chloride,
169½ 58½
parts by weight. parts by weight.
\___________________________________/
|
228
= =AgCl= + =NaNO₃=
One molecule of One molecule of
Argentic chloride, Sodic nitrate,
143 85
parts by weight. parts by weight.
\____________________________________/
|
228
This white precipitate produced in this experiment possesses some
special interest from its use in photography. In fact chloride of
silver, as a thin film upon the surface of the photographic paper, is
the principal substance which, by its sensitiveness to light, produces
the photographic picture. In fact any one, who tries the experiment
last described, will soon observe, upon preserving the chloride of
silver so produced, that it rapidly grows dark upon exposure to
sunlight.
Bleaching-Powder.
The substance known as bleaching-powder may be spoken of in a general
way as consisting of lime saturated with chlorine. This description
points very justly to the method of producing the substance, but gives
no idea of the chemical arrangement of the constituents. Scheele
early noticed that chlorine gas possessed decided bleaching power, and
the French chemist, Berthollet, soon called attention to the possible
applications of the substance in the bleaching industries. But its
annoying odor made it impracticable to use chlorine on any large scale
in the state of gas, and forbade the use of it even when dissolved in
water. At length, twenty years after the discovery of the gas—that is
in 1798—the plan of absorbing chlorine in lime was hit upon, and here
may be discovered the beginnings of the bleaching-powder industry,
now one branch of the alkali trade, the greatest chemical industry
conducted by man. This bleaching-powder, at first a mere chemical
curiosity, is now manufactured by the thousands of tons, and is used in
the bleaching of cotton and linen goods, both in the form of cloth and
in the form of the various kinds of paper.
In another place reference is made to the vast proportions attained
by the alkali industry, meaning the manufacture of certain compounds
of sodium, the one produced in largest quantities being doubtless
sodic carbonate (Na₂CO₃), commonly called soda-ash. In trade this
substance is called an alkali because of certain alkaline properties
it possesses, but more strictly speaking it is called a salt—sometimes
an alkaline salt. In chemistry the single term alkali is reserved for
certain compounds called hydrates, of which indeed sodic hydrate—having
the formula NaOH, and often called caustic soda—is an appropriate
example. This latter compound is at present manufactured on a
large scale in connection with soda-ash. Now although the Leblanc
process—which has long been used for manufacturing soda-ash—is at
present meeting with a powerful and successful rival, yet the older
process has still a strong hold upon life in the fact that it gives
rise, as a convenient incidental product, to vast quantities of
chlorohydric acid. The meaning will be better understood when it
is explained that the first step of the Leblanc process is to add
sulphuric acid to common salt. Two substances are here produced:
the one is sodic sulphate, to be carried forward until it is turned
into sodic carbonate; the other substance is chlorohydric acid, a
compound largely used in the arts, and especially in the manufacture of
bleaching-powder.
[Illustration: FIG. 21.—Bleaching of pulp for manufacture of paper.]
[Illustration: FIG. 22.—Apparatus for producing bleaching-powder (by
passing chlorine gas, generated in A, upon quicklime spread upon the
shelves).]
In the production of bleaching-powder, the first step is to mingle
this chlorohydric acid and manganese dioxide. Chlorine gas is thus
generated, much as it is when the experiment is conducted on a small
scale as already described. The chlorine so generated is passed into
a chamber provided with shelves and containing slaked lime. Hereupon
the lime absorbs the chlorine, giving rise to a new substance called
bleaching-powder—also known as chloride of lime. From what has been
said it is evident that chemists know perfectly well what elementary
substances enter into this compound. But there are decided differences
of opinion as to the exact way in which the atoms are arranged.
Bleaching-powder is generally considered to be a chemical union of
calcic hypochlorite and calcic chloride with the addition of calcic
hydrate. The following representation may serve as a formula for the
compound:
CaCl₂O₂ + CaCl₂ + CaO₂H₂
(Calcic hypochlorite.) (Calcic chloride.) (Calcic hydrate.)
[Illustration: FIG 20.—Representation of the old method of bleaching
cotton and linen goods on lawns.]
[Illustration: FIG. 23.—Apparatus for “souring” cotton cloth by
passing it into dilute acid, before submitting it to the action of
bleaching-powder.]
The use of bleaching-powder offers certain advantages. The following
are some of them:
—The compound is itself white.
—It is a powder which can be easily handled, packed and transported.
—With reasonable precautions, the active bleaching agent chlorine is
retained by the powder in available form for a considerable length of
time.
—In actual use in the process of bleaching, the entire amount of
chlorine originally stored up in the powder may be liberated in contact
with the goods to be bleached.
—The liberation of this chlorine is easily affected. The addition of
almost any acid will accomplish it: even the carbon dioxide of the
atmosphere will suffice.
In the bleaching of cotton goods chlorine is not the only agent relied
upon, though it seems to be an essential one. At least three other
substances are employed to contribute to the bleaching. Each of them
either removes some colors or stains from the goods, or so modifies
them that the solution of bleaching-powder—one of the last agents to be
employed—can the easier finish its work. The three substances referred
to are milk of lime, diluted sulphuric acid, and sodic carbonate, also
called soda-ash.
The pieces of cloth, being sewed together into continuous strips many
miles in length, pass from one liquor to another, with washings in
water at proper times, until finally, after being fully whitened by the
chlorine preparation and then receiving the final washing in water,
they emerge from the works, completely bleached.
READING REFERENCES.
Alkali Trade, in its Various Branches.
=Claus=, C.—Chem. News. xxxviii, 263. (Ammonia soda.)
=Davis=, G. E.—Chem. News. xxxii, 164, 174, 187, 198, 210, 238.
=Hargreaves=, J.—Chem. News. xlii, 322.
=Kingzett=, Charles T.—The Alkali Trade. London, 1877.
=Lunge=, G.—Jour. of Chem. Soc. of London, xliv, 524, 528.
=Mactear=, J.—Chem. News. xxxv, 4, 14, 17, 23, 35; xxxvii, 16.
=Schmidt=, T.—Chem News. xxxviii, 203. (Ammonia soda.)
=Weldon=, W.—Chem. News. xlvii, 67, 79, 87.
(Present condition of soda industry.)
Bleaching Powder.
=Jurisch=, K.—Jour. of Chem. Soc. of London. xxxi, 350.
=Kingzett=, C. T.—_loc. cit._ xxviii, 404.
=Kopfer=, F.—_loc. cit._ xxviii, 713.
=Lunge=, G.—Chem. News. xliii, 1.
=Stahlschmidt=, C.—Jour. of Chem. Soc. of London. xxxi, 279.
=Wolters=, W.—_loc. cit._ xxviii, 404.
Chlorine Industry, Future of
=Hurter=, F.—Jour. of Chem. Soc. of London. xlvi, 225.
Chlorine, Preparation of
=Berthelot.=—Annales de Chimie et de Physique.
5 Sér. xxii, 464.
Davy, Sir Humphry.
=Davy=, John.—Collected Works and Memoirs of Sir H. Davy.
London, 1839.
=Paris=, John A.—Life of Sir Humphrey Davy. London, 1831.
=Brougham=, H.—Lives of Men of Letters, etc. London, 1845.
p. 448.
=Cooke=, J. P.—Scientific Culture. Boston, 1881. p. 11.
Salt Mines of Europe.
=Harper’s= Magazine, i, 759.
Scheele, C. W.
=Hoefer=, F.—Histoire de la Physique et de la Chimie. Paris,
1872. p. 497.