Acids, Alkalis and Salts

by George Henry Joseph Adlam

OTHER METHODS OF SALT FORMATION

Solution of Metals in Acids. Alkalis are not the only substances which neutralize acids. Speaking in a broad and general sense, we may say that an acid is neutralized when a metal is dissolved in it, because, when the point is reached at which no more metal will dissolve, all the characteristic properties of the acid are destroyed. A salt is formed in this case also.

An example will now be given to illustrate this method of salt formation. Before two pieces of metal can be united by soldering, it is necessary to clean the surfaces of the metal and the soldering iron. The liquid used for this purpose is made by adding scraps of zinc to muriatic acid (hydrochloric acid). The zinc dissolves with effervescence, which is caused by the escape of hydrogen gas. When effervescence ceases and no more zinc will dissolve, the liquid is ready for use. The acid has been “killed” or neutralized by the metal. A salt called zinc chloride has been formed. This salt can be recovered from the liquid by evaporation.

Solution of Oxides in Acids. The substances most used in commerce with the express purpose of destroying acidity are quicklime, washing soda, and powdered chalk.

Since quicklime is a compound of the metal calcium and the gas oxygen, its systematic name is calcium oxide; when it neutralizes an acid, it forms the corresponding calcium salt; for example, if it neutralizes acetic acid, calcium acetate is formed.

An instance of the neutralization of an acid by an oxide of a metal is furnished by one method of preparing blue vitriol (copper sulphate). Copper does not dissolve very quickly in dilute sulphuric acid; hence, to make blue vitriol from scrap copper, the metal is first heated very strongly while freely exposed to air. Copper and oxygen of the air combine to form the brownish black powder, copper oxide, and this dissolves very readily in sulphuric acid, making the salt, copper sulphate.

Solution of Carbonates in Acids. Washing soda and chalk belong to a different class of chemical substances. They are carbonates, that is, they are salts of carbonic acid. At first it may seem a little perplexing to the reader to learn that a salt can neutralize an acid to form a salt. It must be remembered, however, that acids differ from one another in strength, that is, in chemical activity, and that carbonic acid is a weak acid. When a salt of carbonic acid—sodium carbonate or washing soda, for example—is added to a stronger acid such as sulphuric acid, sodium sulphate is formed and carbon dioxide liberated.

As an example of the neutralization of acids by carbonates, we may mention here a practical sugar saving device. Unripe fruit is very sour because it contains certain vegetable acids dissolved in the juice. These acids are not affected by boiling; and, therefore, to make a dish of stewed fruit palatable, it is necessary to add sugar in quantity sufficient to mask the sour taste. If a pinch of bicarbonate of soda is added to neutralize the acid, far less sugar will be necessary for sweetening.

Insoluble Salts. The methods given above apply only to those salts which are soluble in water. Insoluble salts are obtained by mixing two solutions, the one containing a soluble salt of the metal, and the other, a soluble salt of the acid or the acid itself.

The formation of an insoluble salt by the interaction of two soluble substances is well illustrated in the preparation of Burgundy mixture, the most effectual remedy yet proposed for checking the spread of potato disease. This mixture contains copper carbonate, that is, the copper salt of carbonic acid. For its preparation we require copper sulphate and sodium carbonate (washing soda), a soluble carbonate. When these two substances, dissolved in separate portions of water, are mixed, copper carbonate is formed as a pale blue solid which is in such a state of fine subdivision that it remains suspended in the solution of sodium sulphate, the other product of the reaction.

The change is represented diagrammatically below. Each circle represents the atom or a group of atoms named therein. At the moment of mixing, these groups undergo re-arrangement.

Bordeaux mixture, which some gardeners prefer, is a similar preparation containing copper hydroxide instead of copper carbonate. It is made by mixing clear lime water (a soluble hydroxide) with copper sulphate.

Fig. 1

Elements and Compounds. It is scarcely possible to discuss chemical processes without having from time to time to use terms which are not in everyday use. A few preliminary definitions and explanations of terms which will be frequently used may serve to simplify descriptions, and render it unnecessary to encumber them with purely explanatory matter.

Among the many different kinds of materials known, which in the aggregate amount to several hundreds of thousands, there are about ninety substances which up to the present time have not been broken up into simpler kinds. These primary materials are called “elements,” the remainder being known as “compounds.”

The following is a list of the commonest of these elements, together with the symbols by which they are represented in Chemistry.

METALS
Aluminium	Al.
Antimony (Stibium)	Sb.
Barium	Ba.
Bismuth	Bi.
Cadmium	Cd.
Calcium	Ca.
Chromium	Cr.
Copper (Cuprum)	Cu.
Gold (Aurum)	Au.
Iron (Ferrum)	Fe.
Lead (Plumbum)	Pb.
Lithium	Li.
Magnesium	Mg.
Manganese	Mn.
Mercury (Hydrargyrum)	Hg.
Nickel	Ni.
Platinum	Pt.
Potassium (Kalium)	K.
Silver (Argentum)	Ag.
Sodium (Natrium)	Na.
Strontium	Sr.
Tin (Stannum)	Sn.
Zinc	Zn.

NON-METALS
Boron	B.
Bromine	Br.
Carbon	C.
Chlorine	Cl.
Fluorine	F.
Hydrogen	H.
Iodine	I.
Nitrogen	N.
Oxygen	O.
Phosphorus	P.
Silicon	Si.
Sulphur	S.

The first step in the building-up process consists of the union of a metallic with a non-metallic element. Such compounds are binary compounds, and are distinguished by the termination -ide added to the name of the non-metallic element; for example, copper and oxygen unite to form copper oxide, sodium and chlorine form sodium chloride, iron and sulphur form iron sulphide or sulphide of iron.

A compound containing more than two elements is distinguished by the termination -ate. Most salts fall within this category; thus we speak of acetate of lead and chlorate of potash, also of sodium sulphate and copper sulphate, the latter form being the more correct.

A difficulty arises when two bodies are composed of the same elements combined in different proportions. Then we have to resort to other distinguishing prefixes or suffixes. For this reason we meet with sulphurous acid and sulphuric acid, the corresponding salts being sulphites and sulphates.

Crystals and Water of Crystallization. When a soluble salt is to be recovered from its solution, the latter is reduced in bulk by evaporation until, either by experience or by trial, it becomes evident that the solid will be formed as the liquid cools. In some cases, when time is not an important factor, evaporation is left to take place naturally. Under either set of conditions, the substance generally separates out in particles which have a definite geometrical form. These are spoken of as crystals.

Crystals often contain a definite percentage of water, called “water of crystallization.” In washing soda, this combined water forms nearly 63 per cent. of the total weight; in blue vitriol, it is approximately 36 per cent. On being heated to a moderate temperature, the water is expelled from the solid; the substance which is left behind is called the anhydrous (that is, the waterless) salt.

CHAPTER II
SULPHURIC ACID AND SULPHATES

Key Industries. The importance of the chemical industries depends mainly on the fact that they constitute the first step in a series of operations by which natural products are adapted to our needs. The materials which are found in earth, air, and water are both varied in kind and abundant in quantity, but in their natural state they are not generally available for immediate use. Moreover, very many substances now deemed indispensable are not found ready formed in Nature.

The end product of the chemical manufacturer is often one of the primary materials of some other industry. Soda ash and Glauber’s salt are essential for making glass; soap could not be produced without caustic alkali; the textile trade would be seriously handicapped if bleaching materials, mordants, and dye-stuffs were not forthcoming. Considered in this light, the preparation of chemicals is spoken of as a “key industry.”

Furthermore, very few of these indispensable substances can be made without using sulphuric acid. This acid is, on that account, just as important to chemical industries as the products of these are to other branches of trade. It may, therefore, be looked upon as a master key of industrial life.

Primary Materials. The composition of sulphuric acid is not difficult to understand. Air is mainly a mixture of oxygen and nitrogen; and when a combustible body burns, it is because chemical action between the material and oxygen is taking place. In this way, sulphur burns to sulphur dioxide. This gas, dissolved in water, forms sulphurous acid, which changes slowly to sulphuric acid by combination with more oxygen. Hence, sulphur, oxygen, and water are the primary materials required for making sulphuric acid.

Sulphur is the familiar yellow solid commonly known as brimstone. It is found native in the earth, and is fairly abundant in certain localities, notably in the neighbourhood of active and extinct volcanoes. Italy, Sicily, Japan, Iceland, and parts of the United States are the principal sulphur-producing countries. Though very plentiful and consequently cheap, only a relatively small quantity of sulphuric acid is made directly from native sulphur, because at the time when this industry was started in England, restrictions were placed on the export of sulphur from Sicily and, consequently, the plant which was then established was adapted to the use of iron pyrites.

Iron pyrites contains about 53 per cent. of sulphur combined with 47 per cent. of iron, and when this is burnt in a good draught, nearly the whole of the sulphur burns to sulphur dioxide, leaving a residue of oxide of iron which can be used for making cast iron of a low grade.

Iron pyrites is often supplemented by the “spent oxide” from the gas works. Crude coal gas contains sulphur compounds which, if not removed, would burn with the gas and form sulphur dioxide. The production of these pungent and suffocating fumes would be a source of great annoyance, and therefore it is necessary to remove the sulphur compounds. To do this, the gas is passed through two purifiers, the first containing slaked lime and the second ferric oxide, both in a slightly moist condition. After being some time in use, the purifying material loses its efficacy; the residue from the lime purifier is sold as “gas lime,” but that from the ferric oxide purifier is exposed to the air and so “revived.” At length, however, it becomes so charged with sulphur that it is of no further use for its original work. It is then passed on to the sulphuric acid maker.

Evolution of the Manufacturing Process. In dealing with the main processes for the manufacture of acids and alkalis, reference will frequently be made to the methods of bygone times. Although as an exact science Chemistry is comparatively modern, as a branch of human knowledge its history goes back to the dawn of intelligence in man. It is agreed that the higher types of living things are more easily understood when those of a simpler and more primitive character have been studied. In like manner, the highly specialized industries of modern times become more intelligible in the light of the efforts of past generations to achieve the same object.

Basil Valentine, who lived in the fifteenth century, states that the liquid which we now call sulphuric acid was in his day obtained by heating a mixture of green vitriol and pebbles. Until quite recent times, sulphuric acid of a special grade was made by precisely the same method, except that the pebbles were dispensed with. In passing, we may remark that the common name “vitriol,” or “oil of vitriol,” is accounted for by this connection with green vitriol. The second method, quoted by Basil Valentine, consisted of the ignition of a mixture of saltpetre and sulphur in the presence of water. This is actually the modern lead chamber process in embryo.

Fig. 2. PLAN OF SULPHURIC ACID WORKS

About the middle of the eighteenth century, “Dr.” Ward took out a patent for the manufacture of sulphuric acid, to be carried on at Richmond in Surrey. He used large glass bell jars of about 40-50 galls. capacity, in which he placed a little water and a flat stone to support a red-hot iron ladle. A mixture of saltpetre and sulphur was thrown into the ladle and the mouth of the vessel quickly closed. After the vigorous chemical action was over, the ladle was re-heated and the process repeated until at last fairly concentrated sulphuric acid was produced.

The large glass vessels used by Ward were costly and easily broken. They were soon replaced by chambers about 6 ft. square, made of sheet lead, but otherwise the process was just the same. The next advance consisted in making the process continuous instead of intermittent. An enormously increased output was thereby rendered possible, and the main features of the modern process gradually developed.

The Lead Chamber Process. We can now consider the actual working of the lead chamber process, aided by the diagrammatic plan of the works shown in [Fig. 2]. Sulphur dioxide is produced in a row of kilns (A-A) by burning iron pyrites in a carefully regulated current of air. The mixture of gases which leaves the pyrites burners contains sulphur dioxide, excess of oxygen, and a very large quantity of nitrogen. To this is added the vapour of nitric acid, generated from sodium nitrate and concentrated sulphuric acid contained in the “nitre pots,” which are placed at B. The mixture of gases then passes up the Glover tower (C) and through the three chambers in succession, into the first two of which steam is also introduced. Sulphuric acid is actually produced in the chambers, and collects on the floors, from which it is drawn off from time to time. The residual gas from the last chamber is passed up the Gay Lussac tower (D), and after that is discharged into the air by way of the tall chimney (J).

Fig. 3. GENERAL VIEW OF SULPHURIC ACID WORKS

The Oxygen Carrier. We have seen that sulphur dioxide, oxygen, and water are the only substances required to produce sulphuric acid. Why, then, is the nitric acid vapour added to the mixture? As described in a former paragraph, the combining of these gases was represented as being a very simple operation. So indeed it is, for it even takes place spontaneously. Yet, as a commercial process, it would be quite impracticable without the nitric acid vapour, for although the gases combine spontaneously, they do so very slowly, and it is the nitric acid vapour which accelerates the rate of combination.

It is not known with any degree of certainty how the nitric acid acts in bringing about this remarkable change. It has been suggested that reduction to nitrogen peroxide first takes place, and that sulphur dioxide takes oxygen from this body, reducing it still further to nitric oxide, which at once combines with the free oxygen present to form nitrogen peroxide again. So the cycle of changes goes on, the nitrogen peroxide playing the part of oxygen carrier to the sulphur dioxide; and since it is continually regenerated, it remains at the end mixed with the residual gases.

Recovery of the Nitrogen Peroxide. If the gases from the last chamber passed directly into the chimney shaft, there would be a total loss of the oxides of nitrogen, and the consequence of this would be that more than 2 cwt. of nitre would have to be used for the production of 1 ton of sulphuric acid. This would be a serious item in the cost of production, and it is therefore essential that this loss should be prevented.

The recovery of the oxides of nitrogen is effected in the Gay Lussac tower, a structure about 50 ft. in height, built of sheet lead and lined with acid-resisting brick. It is filled with flints, over which a slow stream of cold concentrated sulphuric acid is delivered from a tank at the top. As the gas from the last chamber passes up this tower, it meets the stream of acid coming down. This dissolves and retains the nitrogen peroxide. The acid which collects at the bottom of the tower is known as nitrated vitriol.

The next step is to bring the recovered nitrogen peroxide again into circulation. The nitrated vitriol is raised by compressed air to the top of the Glover tower, and as it trickles down over the flints in this tower it is diluted with water, while at the same time it meets the hot gases coming from the pyrites burners. Under these conditions, the nitrogen peroxide is liberated and carried along by the current of gas into the first lead chamber. The stream of cold acid coming down the Glover tower also serves to cool the hot gases before they enter the first chamber.

In order to complete the description of the works, it is necessary to add a note on the lead chambers themselves. The sheet lead used in their construction is of a very substantial character; it weighs about 7 lb. per square foot. The separate strips are joined together by autogenous soldering, that is, by fusing the edges together. In this way the presence of another metal is avoided; otherwise this would form a voltaic couple with the lead, and rapid corrosion would take place.

The size of the chambers has varied a great deal. In the early years of the nineteenth century, the capacity of a single chamber was probably not more than 1,000 cu. ft.; at the present time, 38,000 cu. ft. is an average size, and there may be three or five of these chambers. The necessity for this large amount of cubic space is easily accounted for. The reaction materials are all gases, and a gas occupies more than one thousand times as much space as an equal weight of a solid or liquid. Moreover, oxygen constitutes only about one-fifth of the total volume of air used in burning the pyrites; the other four-fifths is mainly nitrogen, which, though it does not enter into the reaction at all, has to pass through the chambers.

Modern Improvements. Among the modern innovations in the lead chamber process, the following are worthy of note. “Atomized water,” that is, water under high pressure delivered from a fine jet against a metal plate, has certain advantages over steam. In order to bring about a more rapid mixing of the gases in the chamber, it is proposed to make these circular instead of rectangular, and to deliver the gases tangentially to the sides. Another suggestion is to replace the lead chambers by towers containing perforated stoneware plates set horizontally. By this arrangement, since the holes are not placed opposite one another, the gases passing up the tower must take a zig-zag course. This makes for more efficient mixing.