A group of acids, namely, hydrochloric, hydrofluoric, hydrobromic, hydriodic, must now be considered together with their corresponding salts. In appearance and in other physical properties they resemble one another very closely; they are, therefore, called by the general name “halogen acids.” This name is derived from the Greek word meaning “sea-salt,” which is a mixture of the salts of these acids, and from which the acids themselves can be obtained by treatment with oil of vitriol.

Hydrochloric Acid. When concentrated sulphuric acid is added to common salt, a gas is liberated which has a very pungent acid smell and taste. This is a compound of the elements hydrogen and chlorine, and therefore called hydrogen chloride. It is extremely soluble in water; a given volume of water dissolves as much as 500 times its own volume of the gas. The solution produced in this way is now called hydrochloric acid, but formerly it was known as spirits of salt, or muriatic acid.

Hydrochloric acid has all the general properties of acids. It dissolves many metals, such as zinc, iron, aluminium, and magnesium; hydrogen gas is given off, and the chloride of the metal is formed. It also dissolves limestone, marble, and all forms of calcium carbonate; carbon dioxide gas is liberated, and a solution of calcium chloride remains.

The hydrochloric acid of commerce is obtained as a by-product in the manufacture of washing soda from common salt by the method proposed by Nicholas Leblanc towards the end of the eighteenth century. In the first stage of this process, salt is mixed with sulphuric acid; this causes the liberation of hydrogen chloride gas, which, when dissolved in water, produces hydrochloric acid.

The past history of this branch of chemical industry is interesting. Until about 1870, there was no very great demand for hydrochloric acid, and in the early days of the working of the Leblanc process the soda manufacturer took no pains to recover more than he could actually sell. Consequently, a large quantity of hydrogen chloride gas was allowed to escape into the air, with results which can well be imagined. For miles around, great damage was frequently sustained by the growing crops; when it rained in the neighbourhood of the works, the gas was washed out of the air and, speaking quite literally, it rained dilute hydrochloric acid, which rapidly corroded all stone and metal work. It is not, therefore, surprising to learn that alkali makers were frequently involved in litigation, and chemical works were regarded as a great nuisance.

By the Alkali Act of 1863, chemical manufacturers were compelled to prevent the escape of more than 5 per cent. of hydrochloric acid gas; and by a subsequent Act, this limit was lowered to 0·2 grain per cubic foot. The provisions of the Acts were not difficult to carry out, because hydrogen chloride is extremely soluble in water.

The gases coming from the pans in which the salt was decomposed were led into towers (see Fig. 8) built of bricks or Yorkshire flags soaked in tar. These towers were filled up with coke or other acid-resisting material, which was kept moist by water flowing from the tank F. In this way, hydrogen chloride gas was removed and hydrochloric acid collected in tanks (not shown in the figure) at the bottom of the towers. Even then, there was no market for the greater part of the recovered acid, consequently much of it found its way into drains and streams, and so carried on its work of destruction in a less obtrusive way.

By another piece of legislation, which at first sight seems to be wholly unconnected with Chemistry, hydrochloric acid acquired a greatly enhanced value. In 1861, the tax on paper was removed, and in the next twenty years the demand for that commodity increased so much that raw material both cheaper and more abundant than rag had to be found. Esparto grass and eventually wood pulp proved successful substitutes. There is really very little difference in composition between cotton and linen rag on the one hand and wood fibre on the other, for both are mainly composed of cellulose, which is a definite chemical compound. Wood fibre is the less pure, and it is also coloured, and therefore has to be bleached before it can be used for making white paper. It was this circumstance which led to the greatly increased demand for hydrochloric acid.

At the beginning of this chapter, it was mentioned, in passing, that hydrogen chloride gas is a compound of hydrogen and chlorine. The latter element is a very active bleaching agent, and is most easily obtained by treating hydrogen chloride or its solution in water with pyrolusite (black oxide of manganese), whereby the hydrogen is oxidized, forming water, and chlorine gas is set free. Being a gas, chlorine is not convenient to handle in large quantities; it is, therefore, converted into bleaching powder, commonly but wrongly called chloride of lime.

Bleaching Powder. The manufacture of bleaching powder is carried out in the following way. Slaked lime to the depth of 3 or 4 in. is spread over the floor of a special chamber which can be made gas-tight. The lime is raked up into ridge and furrow, and the chamber is filled with chlorine. At the end of about twenty-four hours, the greater part of this chlorine will have been absorbed by the lime. The chamber is then opened, the lime is raked over to expose a fresh surface, and the process of chlorination is repeated. Generally this is sufficient; the bleaching powder should then contain about 35 per cent. of available chlorine.

The demand for bleaching powder is great and steadily increasing. The price of 35 per cent. bleaching powder has never been less than about £5 a ton,[3] so that it is perhaps unnecessary to add that the absorption of hydrogen chloride gas is now made so complete that it is well within the requirements of the second Alkali Act.

Chlorides. The salts of hydrochloric acid are called chlorides, and the most important of these is sodium chloride or common salt—a body that is so well known that it need not be described here.

Although the quantity of this substance required for domestic purposes is very large, it is, nevertheless, small by comparison with that which is used for industrial purposes. It has already been mentioned that salt is the starting-point for the manufacture of washing soda by the Leblanc process, and, in addition to this, it is employed in the glass industry to produce whiteness and transparency in certain kinds of glass; in pottery, for glazing earthenware; in soap-making, for salting out the crude soap; and in the dye trade as a mordant, and also for improving the quality of certain colours. A full account of the salt industry is given in another volume of this series.

Hydrofluoric Acid. When calcium fluoride (fluorspar, Derbyshire spar, or blue-john) is warmed with concentrated sulphuric acid in a leaden dish, hydrogen fluoride gas is evolved, and this, when dissolved in water, gives hydrofluoric acid.

The peculiar property of this substance is that it has a very marked corrosive action on glass. It cannot, therefore, be kept in glass vessels, but must be stored in bottles made of hardened caoutchouc. On the other hand, it is this same property which gives it its place in commerce. As far back as 1670 it was used for etching on glass. The process is a very simple one. The article is first coated with wax, which is then removed in places by a sharp pointed tool. When exposed to the action of the gas or its solution, corrosion takes place only where the glass has been laid bare, the other parts being protected by the wax. After a short interval, the wax can be melted off, and the design made more distinct by rubbing in some opaque cement. For general trade purposes, such as the stamping of lamp chimneys or electric light bulbs, a quicker method is required. In this case, a preparation of hydrofluoric acid which can be applied with a rubber stamp is used.

Fluorspar or calcium fluoride is the most important salt of hydrofluoric acid. It is a commonly occurring mineral, and besides its use for the preparation of the acid, it is employed in many metallurgical operations to form a fusible slag.

Hydrobromic and Hydriodic Acids are not much used, but their salts, the bromides and iodides respectively, are of great technical importance. Silver chloride, bromide, and iodide, are sensitive to light, and mixed with gelatine they form the emulsion which is spread over photographic plates and papers. Potassium bromide and iodide are also well known to photographers.

When the halogen salts of silver are exposed to light, an extremely subtle chemical change takes place, which is only made apparent when the plate or paper is developed. Then the silver salts on which the light has fallen are reduced to metallic silver, and this reduction is greatest where the light was most intense, and in other places is proportional to the light intensity. A very faint image may appear on the plate while it is in the developer, but generally the image is only brought out clearly when the plate, film, or paper is placed in “hypo” solution, which dissolves out the silver salts which have not been changed, leaving the metallic silver unaffected.