Chapter X: 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.

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.

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.

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.)

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.