Acids, Alkalis and Salts

CHAPTER VI
PHOSPHORIC, BORIC, AND SILICIC ACIDS

The acids which are grouped in this chapter are not in themselves of much interest, though some of their salts are extremely important compounds.

Bone. Much of the refuse bone, sooner or later, reaches the marine store, and from that point starts on a career of usefulness in the industrial world.

“Green bone,” as it is then called, may have fat adhering to it or confined in its hollow interior as marrow. This is recovered by treatment with benzine, and after that the bone is subjected to the action of superheated steam in order to convert cartilage into glue. In some cases, the residue is then ground up to make bone meal, which is valuable as a manure because of the calcium phosphate which it contains. In this way, the phosphate returns again to the animal kingdom, for it supplies plants with the phosphates that they require, and from the vegetable kingdom it passes to animals and helps to build up bone again.

Calcium Phosphate and Bone Black. Instead of being ground up, bone may be heated in a retort in much the same way as coal is treated for the manufacture of coal gas; bone oil is distilled off, and a non-volatile residue, called bone black or animal charcoal, remains. This contains about 90 per cent. of calcium phosphate and 10 per cent. of finely divided carbon disseminated throughout the mass. It has the peculiar property of absorbing colouring matter, and is used for this purpose in the sugar industry and in the preparation of fine chemicals.

Phosphoric Acid. After being some time in use, bone black loses the property of absorbing colouring matter; and though it can be “revived” several times by heating it strongly in a closed retort, it ultimately becomes spent and of no further use to the sugar refiner. It is then heated again, this time in an open vessel, until all the carbon is burnt away. The residue is now a greyish solid consisting mainly of calcium phosphate. This, supplemented with native phosphate, which is probably fossilized bone, is used for the preparation of phosphoric acid.

The salt is decomposed by sulphuric acid in wooden vats; calcium sulphate is formed, and ultimately settles on the bottom of the vat, leaving a clear supernatant liquid, which is a dilute solution of phosphoric acid. This liquid is drawn off and evaporated to a syrup. This is “syrupy” phosphoric acid. On being still more strongly heated, the syrup loses still more water, and a semi-transparent glassy-looking substance, called metaphosphoric acid, remains.

Superphosphate. All fertile soils, especially those on which wheat is to be grown, must contain a certain amount of phosphate. With this, as with all other plant foods, the actual percentage weight required in the soil is very small indeed, but it is necessary that it should be disseminated throughout the soil. Even distribution is very difficult to secure in the case of a substance like calcium phosphate, which is practically insoluble in water.

To get over this difficulty, calcium phosphate is converted into a mixture known as “superphosphate” by the following process. Bone ash or the mineral phosphate is finely ground and thoroughly mixed by machinery with two-thirds its weight of sulphuric acid from the lead chambers. After a time, this mixture sets to a hard mass, containing principally gypsum and calcium tetrahydrogen phosphate. It is then ground up finely and is ready for use.

The special modification of calcium phosphate contained in superphosphate is soluble in water. It is, therefore, carried into the soil in solution, and in this way very evenly distributed. In the soil it reacts with the lime or chalk which is present, and is gradually reconverted into insoluble calcium phosphate.

The manufacture of superphosphate is a very important industry. The weight of the substance produced annually in Great Britain alone is not far below a million tons.

Basic Slag. In the Bessemer process for converting iron into steel, cast iron is melted up in a vessel called a converter and, by the aid of a powerful blast blown through the molten iron, most of the impurities are burnt off. If, however, phosphorus and sulphur are present, they are not removed if the converter has a silica (acid) lining. The original Bessemer process was, therefore, modified by Thomas and Gilchrist, and the converter for this kind of iron is lined with dolomite and lime (basic lining). Phosphorus is then converted into phosphate and retained by the lining, which is subsequently removed, ground up finely, and sold as “basic slag.”

Boric Acid, or boracic acid, is familiar because it is used in medicine as a mild antiseptic; it is also employed as a preservative for food. It is a white crystalline compound, sparingly soluble in water. It has no well-marked taste, and causes only a partial change in the colour of litmus solution; it is, therefore, one of the weak acids. It does not dissolve metals, but it displaces carbon dioxide from carbonates, forming salts.

Borax, the best known salt of boric acid, is used in laundry work and also for making some enamels, for when it is strongly heated it loses water, and ultimately melts down to a clear “glass” in which the oxides of metals will dissolve, yielding transparent substances which are beautifully coloured according to the nature of the oxide used. This property is often made use of in chemical analysis in what is known as the “borax-bead” test.

Boric acid is a natural product; the method by which it is obtained is of some interest, because it is so simple, and because it shows how mere traces can be gradually accumulated until a very fair total is ultimately obtained. Moreover, the method is copied directly from Nature.

In the early years of the nineteenth century, certain jets of natural steam, called suffioni, which issue from the earth in Tuscany, were found to contain the vapour of boric acid. These jets of steam are of volcanic origin. The quantity of boric acid in the vapour is very small indeed; nevertheless, by the method which is adopted, it can be profitably recovered, and more than a ton of the solid is daily produced.

In the same country there are many lagoons, the water of which contains boric acid. It was rightly conjectured that this boric acid came from jets of steam which issued from the earth in the bed of the lagoon. This suggested the idea of building up an artificial lagoon around a group of jets.

Series of about five of these collecting basins (Fig. 9) are formed, each one at a slightly lower level than the one which precedes it. The first basin is filled with water from an adjacent spring, and this is allowed to remain for twenty-four hours. A sluice is then opened and the liquid contained in the first basin flows down to the second, where it remains for another day, and so on until it reaches the last basin of the series. The liquid by this time is almost fully charged with boric acid, but it contains only about 2 per cent., because the acid is so sparingly soluble in water.

From the last basin (A), the liquid runs into large vats (B, D), where the suspended impurities settle down. The solution of boric acid is then concentrated by causing it to flow over a broad inclined plane made of corrugated lead or through a series of shallow vessels heated by jets of natural steam. The hot liquid flows into another vat (C), and, as it cools, boric acid crystallizes out and is removed by perforated ladles.

The mother liquor from which the crystals have been withdrawn is, of course, a cold saturated solution of the acid, and this is returned to the top of the incline to flow down again and lose more water. The boric acid is finally transferred to drying chambers, which are also heated by the natural steam.

Native borax or “tinkal” comes from Thibet and also from Ceylon. In California, a large quantity of borax is obtained from a borax lake, and also from the mud of marshes in its neighbourhood.

Silica. The element silicon does not occur in the free state in Nature, neither has any particular use been found for it, and therefore it is not often isolated except to provide a lecture specimen. The compounds of silicon, however, are both plentiful and important, especially silica, the oxide, and the silicates or salts of silicic acid.

The commonest forms of silica are sand, flint, and quartz. Silver sand is composed of small crystals of pure silica, while flint is the amorphous variety of the same substance. Quartz, or rock crystal, is a very hard and transparent mineral. It forms six-sided prisms ending in pyramids. It is distinguished from other common transparent minerals, such as calcspar, by the fact that it cannot be scratched even with a good knife or file, and that a drop of hydrochloric acid has no action on it. The melting point of silica is very high.

Sometimes silica is very delicately coloured with minute traces of metallic oxides and other substances, and these forms, because of their rarity and beauty, are more highly valued. Smoky quartz, cat’s-eye, and amethyst are some of the coloured varieties of quartz. Opal, agate, jasper, onyx, and chalcedony are, in the chemist’s classification, merely coloured flints.

In recent years, chemical apparatus has been made from pure fused silica. This can only be worked in the oxy-hydrogen blow-pipe flame or in the electric furnace; nevertheless, crucibles, flasks, beakers, and retorts can be made. Silica ware has several advantages over glass, notably, that water has no action upon it at all; moreover, its coefficient of expansion is so very small that a piece of apparatus made of silica can be suddenly heated or cooled without risk of fracture; indeed, it can be made red-hot and cooled immediately by plunging into cold water.

Quartz or silica fibres, used for suspending magnets and other bodies in very delicate physical apparatus, are made in the following way. Molten silica is attached to the bolt of a crossbow, which is then released above a carpet of black velvet. As the bolt flies forward, it draws out the silica into a filament, which is so fine that it would be difficult to find were it not for the velvet background.

Silicic Acid itself is only of theoretical interest. It is obtained by adding hydrochloric acid to a solution of potassium or sodium silicate. It is a gelatinous substance of somewhat indefinite composition. It has no effect on litmus, no taste, and no solvent action; in fact, it is only recognizable as an acid because it dissolves in alkalis, forming salts called silicates. It is one of the weakest acids known.

The natural silicates are very abundant and varied; orthoclase or potash felspar, and albite or soda felspar, are those which most commonly occur. The former is potassium aluminium silicate, and the latter, sodium aluminium silicate. Iron is generally present in both as an impurity. The weathering of the felspars, in conjunction with the action of water, has produced the clays. In this way, pure white China clay has been formed from felspars which contain little or no iron, and the coarser kinds of clay from others containing a greater proportion of foreign substances.

Mica, which is used for making lamp chimneys, is a potassium aluminium silicate. Asbestos, meerschaum, beryl, garnet, jade, and hornblende are all silicates of various metals.

Glass is a complex mixture of insoluble silicates with excess of silica. The varieties in common use are soda glass, Bohemian glass, and lead glass (which is also called flint glass). Soda glass is mainly a mixture of calcium and sodium silicates, and is distinguished by its low melting point, which makes it easy to work at moderate temperatures. It appears in commerce as plate glass, window glass, and common bottles. Bohemian glass contains calcium and potassium silicates, and has a high melting point. It is used for making chemical apparatus. Lead or flint glass contains the silicates of lead and potassium; this is a dense glass, but at the same time rather soft. It takes a high polish and is used for making ornamental or cut-glass ware.

Remembering that glass is composed of the salts of silicic acid, the reader will readily understand that the mixture from which it is made must contain acidic and basic constituents. The acidic or acid-forming material is in every case silica or sand. This must be pure, and for all but the commonest kind of bottle or window glass, it must be free from iron, otherwise the glass will have a more or less pronounced greenish colour. It must also be fine and even grained. Formerly, the glass sands used in this country came from Holland and Belgium, but now supplies from several British sources are being successfully used.

The basic portion of the glass mixture differs according to the kind of glass required. An average mixture for soda glass contains sand, 20 parts; salt cake (sodium sulphate), 10 parts; quicklime, 5 parts; charcoal, 1 part. For Bohemian glass, pearl ash (potassium carbonate) takes the place of salt cake, and no charcoal is necessary because the materials used are finer. For lead glass, the mixture is still further modified by the use of litharge, or more often red lead, in place of lime.

The ingredients are well mixed and thoroughly dried. Waste glass from a previous batch is also added. The mixture is heated to about 1200° C. in large pots made of Stourbridge clay, and the heating is continued for as much as sixteen hours, and until the whole of the material in the pot is molten and fairly mobile. Scum or glass-gall is removed, and when gas bubbles have disappeared, the temperature is allowed to fall to 700°-800°, when the glass becomes sufficiently viscous for subsequent working. The semi-fluid mass is then blown, moulded, or drawn, according to the kind of article that is required.

The physical properties of glass will now be considered in order that we may be able to account for its extended use. Such an inquiry as this, especially in the case of materials in common use, is often interesting, because it frequently happens that the special property upon which we set so much value is an abnormal one and, consequently, the feature which we take for granted is precisely the one into which we should inquire most closely.

The most striking feature of glass is its transparency. This property is abnormal, if glass is a solid. Consider what happens in most cases. A substance like nitre melts easily and in the molten state is perfectly transparent; when it cools, crystals form and, though these individually may be transparent, yet the solid mass is opaque. The reason for this is that the solid is not optically homogeneous, and therefore a ray of light cannot pass through it in a straight line. At each facet of a crystal light is deviated and reflected, and in the end is almost wholly scattered. Consequently, an object, even if it can be seen at all, can be discerned only in a blurred and indistinct fashion through such a medium.

There are very good reasons, however, for supposing that glass is not a true solid but an extremely viscous liquid. If glass is heated, it softens and begins to flow very sluggishly at first, but afterwards more readily. There is no abrupt change, as there generally is in passing from the solid to the liquid state. Similarly in cooling, there is no point at which it is possible to say that the glass is solidifying. The view that this substance is really a liquid is perhaps a little startling at first, but it becomes less so when we observe that a long glass rod supported at its ends in a horizontal position sags in the middle and is permanently deformed.

To avoid that change which would be technically called solidification by a scientist, the article which has been fashioned in glass is cooled down very slowly and gradually. This part of the process is called annealing; it may occupy some days in extreme cases, and it points to the fact that experience has shown that it is necessary to guard against some change which would normally take place if this precaution were neglected.

The change in glass which annealing is intended to prevent is known as devitrification. In spite of all precautions, this does occur sometimes, and specimens of old window glass are often seen to have lost their transparency completely and to have an opalescent sheen. In these cases, the silicates have crystallized.

An extreme case of badly annealed glass is illustrated by Rupert’s drops, a scientific curiosity of very old standing. These are “tears” of glass made by dropping the molten substance into water. When the tail of the drop is nipped off, the whole thing is shattered to powder with something like explosive violence. Clearly there is a very great internal strain, due to the fact that the outer parts have solidified and contracted, while the inner part is still warm and dilated.

Another remarkable feature of glass is the ease and simplicity with which it can be fashioned into articles of various shapes. As a plastic material, molten glass almost ranks with clay. This again is due to the property of passing through a viscous state, that is, one which is intermediate between a solid and a liquid.

Water Glass, or soluble glass, is mainly sodium silicate. It is made by fusing sand or powdered flint with caustic or with mild soda; sometimes, by digesting crushed flint or chert with caustic soda solution under considerable pressure in autoclaves or specially constructed boilers. In the latter case, no extraction is necessary; but in the former, the residue is treated with water and the solution evaporated until it becomes a viscous transparent liquid.

This liquid is used in various ways in industry. It is added to the cheaper varieties of yellow soap, and is employed as a mordant in dyeing and printing calico. An artificial sandstone is made by mixing sand, calcium chloride, and sodium silicate; the two last-named substances interact to form calcium silicate, which is insoluble in water. For domestic purposes, water glass is best known in connection with the preserving of eggs. When the film of water glass dries on the surface of the egg shell, the latter becomes impervious to air.

CHAPTER VII
ORGANIC ACIDS

Organic Chemistry. About a century ago, when the science of Chemistry was still in its infancy, several substances were known which could then only be obtained from animals or plants. The composition of these substances was not understood, and they were not classified; moreover, since none of them had ever been prepared artificially, it was supposed that it was impossible to do this—the reason given was that “vital force” was necessary for their production. In time, however, some of the most typical animal and vegetable products were prepared in the laboratory, and the belief in vital force disappeared.

In later times it was proved that substances like sugar, starch, urea, indigo, and a great many more, all contain the element carbon. At the present time, more than 100,000 compounds of this element are known; and since they resemble one another, and at the same time differ in several important respects from the compounds of other elements, it is both natural and convenient that they should be classed together and studied separately. This branch of Chemistry is called organic. It must not, however, be supposed that all organic compounds are necessarily produced by some living organism. A great many are, but there are many more which are purely synthetic products.

Inorganic Chemistry includes all the other elements and their derivatives. The element carbon, and also some of its simpler compounds, such as carbon monoxide, carbon dioxide, carbonic acid, and carbonates, are more appropriately placed in the inorganic section.

The acids which have been considered up to this point are all inorganic acids, and those which follow are organic. Sulphuric, nitric, and hydrochloric acids are often distinguished as the mineral acids in contradistinction to oxalic, citric, tartaric, and some others which were first obtained from unripe fruits and therefore called vegetable acids.

Organic acids have all the general properties of the class, but they are much weaker than the mineral acids mentioned above. This is shown by their solvent action on metals, oxides, and carbonates, which is in all cases slight.

Vinegar is the trade name for what is essentially a dilute solution of acetic acid which has been made by the acetous fermentation of saccharine fluids containing weak alcohol. In addition to acetic acid, vinegar contains minute quantities of a large number of compounds. Some of these help to produce that agreeable flavour and aroma which distinguishes vinegar from diluted acetic acid. The nature and quantity of the flavouring constituents depend mainly upon the nature of the alcoholic solution used.

Since the acetic acid in vinegar is always produced by fermentation, all processes for the manufacture of vinegar are essentially arrangements for promoting the vigorous growth and development of Mycoderma aceti, the organism which produces the vinegar ferment.

Like all other plants, Mycoderma aceti will flourish only under certain favourable conditions. In the first place, it requires nourishment, and therefore certain nitrogen compounds and salts must be present in the alcoholic solution. These are contained in wines, beer, cider, and malt liquors, but not in spirits of wine, which is pure alcohol distilled from liquids which have undergone vinous fermentation. If the plant is placed in dilute spirits of wine, only a very little acetic acid is formed, and then the action ceases because the solution does not contain the necessary food substances. Temperature also has a very marked effect on growth, the most favourable range being between 68° and 95° F.

Alcohol is changed to acetic acid by the process of oxidation, and therefore, in making vinegar, arrangements have to be made to bring together weak alcohol and air in the presence of the plant. The ferment which is secreted by the plant then causes an acceleration of the reaction. There is a considerable amount of similarity between fermentation and contact action. In this connection, it is interesting to note that the conversion of alcohol into acetic acid can also be brought about by exposing a mixture of alcohol vapour and air to the action of platinum black; in fact, there is one process for making vinegar in this way.

French Vinegar. New wine, especially that which contains a low percentage of alcohol, is liable to many kinds of “sickness.” It may turn bitter, it may turn sour, or it may undergo what is called lactic fermentation. In either case, it becomes unsaleable as a beverage. Wine which has turned sour is the best material for making vinegar, and when this is done by the French or slow process, a product with a very fine bouquet is obtained.

The methods adopted are very simple. Formerly, the wine was poured into barrels leaving the top portion empty, and providing for a current of air over the surface. The barrels were often set up in rows in the open air in an enclosure which was then known as a “vinegar field.” The process of souring which had already begun went on naturally, and in the course of a few months, nearly the whole of the alcohol was converted into acetic acid.

The process now in use in some of the French factories is somewhat similar. Large casks holding about 100 gallons are set up in a room, and provision is made for keeping the temperature uniform. Each cask is first acidulated by allowing strong vinegar to stand in it until the vinegar plant has developed on the surface. The casks are then filled up very gradually by adding a few gallons of wine every eight or ten days. When the cask is full, a fraction of the contents is drawn off and replaced by wine. The process then becomes continuous, until it is necessary to clean out the generator and start again.

In recent times, the manufacture of wine vinegar has been carried out on more scientific principles. The vinegar plant is actually cultivated and examined microscopically before being used, in order to make sure of the absence of moulds and bacteria, which set up other fermentations, producing substances which affect adversely the taste and aroma of the finished product. The cultivated ferment is then added to the wine in shallow vessels and the process is carried on as described above.

Malt Vinegar. A dilute solution of alcohol which is made from malt by fermentation with yeast contains the nutritive substances necessary for the growth of the vinegar plant, and can therefore be used as a starting-point for the manufacture of vinegar. Sprouted barley or malt is mixed with oats, barley, rice, or other starch-containing material. The mixture is mashed with warm water and then fermented with yeast, giving what is called “raw spirit.” This is converted into vinegar by the “quick” process.

The vinegar generator (Fig. 10) is a large barrel divided into three compartments by two perforated partitions. The lower disc is fixed about one-third of the way up the barrel, and near it holes are bored to admit air. The upper disc, fixed near the top of the barrel, is perforated with a large number of small holes which are partially stopped up with short threads or wicks, which hang from the under side. The space between the two discs is packed with specially prepared beech shavings, which have been left to stand in strong vinegar until they are covered with the vinegar plant.

The weak spirit is delivered into the upper portion of the barrel and is distributed in very small drops by the threads; it then passes slowly over the vinegar plant, to which the air also has free access. When it reaches the bottom, it overflows into a reservoir and is again passed through the generator; this is repeated until the product contains the desired amount of acetic acid.

The principle of the quick vinegar process is the same as that employed in making wine vinegar. The speed of the reaction is, however, greatly increased by having the ferment spread over a very large surface and by the free circulation of air. It is possible to make wine vinegar by the quick process, but it is not done, because the product is inferior in taste and aroma to that made by the slow process.

Both wine vinegar and malt vinegar when freshly prepared have a stupefying and unpleasant odour. Before the product is ready for the market, it has to be matured in barrels. During this process, a small quantity of alcohol which still remains in the vinegar combines slowly with some of the acetic acid, producing acetic ester, a substance which has a pleasant fruity odour.

The colour of wine vinegar is natural, but vinegar which is produced by the quick process is colourless or only faintly coloured. Since the public has a preference for vinegar which is brown in colour, the product of the quick process is coloured artificially, either by adding caramel or by preparing the weak spirit from malt which has been slightly charred in drying.

Industrial Acetic Acid. The solutions of acetic acid dealt with above would be too dilute for any industrial purpose; moreover, the acid can be obtained much more cheaply by the distillation of wood. When wood is subjected to a high temperature, it is converted into charcoal and, at the same time, an inflammable gas, an acid liquid, and tar are given off, and can be collected in suitable vessels. The following table, on page 73, gives the relative amounts of the various substances obtained from wood by dry distillation. The quantities are those derived from one cord, that is, 125 cu. ft.

The aqueous liquid that distils over contains methyl alcohol (wood spirit), acetone, and acetic acid. The crude mixture is known as pyroligneous acid. This is neutralized with milk of lime or soda ash, which converts acetic acid into calcium or sodium acetate, but has no action on the methyl alcohol and acetone which are also present. The mixture is then distilled, when methyl alcohol, acetone, and water pass over into the distillate, leaving the acetate in the retort.

To obtain the free acid from the acetate, the latter is well dried and then distilled with concentrated sulphuric acid. Acetic acid, being the more volatile of the two acids, distils over, and is nearly pure.

The method of removing the last traces of water depends upon the fact that acetic acid solidifies at 17° C. The acid, which is nearly, but not quite, free from water, is cooled until a portion solidifies. The part which still remains liquid is poured away, and the process is repeated until a residue is obtained which solidifies as a whole. This is glacial acetic acid, so called because it is a mass of glistening plates which look like newly-formed ice.

The Acetates

Aluminium Acetate, made by dissolving alumina in acetic acid, is the “red liquor” which is used as a mordant in dyeing. It is a colourless liquid, but is called “red liquor” because it is used with dyes which give a red colour.

Ferrous Acetate, made in a similar way from scrap iron and acetic acid, is the “black liquor” used in dyeing.

Verdigris, or basic copper acetate, is a valuable pigment. It is made by interposing cloths soaked in vinegar between plates of copper. After the action has been allowed to go on for a long time, the plates are washed with water and the verdigris is scraped off. The finest verdigris is made in France in the wine-producing district around Montpellier. Here, instead of cloths soaked in vinegar, the solid residue from the wine presses is spread in layers between the copper plates. The product made in this way is called vert de Montpellier.

Verdigris, like all the copper compounds, is extremely poisonous. It is very liable to be formed on the surface of copper utensils used for cooking purposes.

Lead Acetate, or sugar of lead, is used in large quantities in the colour industry for making various reds and yellows. It is prepared by dissolving the metal or its oxide (litharge) in acetic acid.

The slow action which acetic acid vapour has upon the metal lead finds a very interesting application in what is known as the Dutch process for the manufacture of white lead[4] for paint. The metal is cast into grids or spirals, which are placed on the shoulders of the specially made pots sketched in Fig. 11. A little dilute acetic acid is poured into each of the pots, which are then arranged side by side on a thick layer of tan bark, stable manure, or other material which will heat by fermentation. The first layer of pots is then boarded over; another layer of pots is placed upon this, and so on, tier upon tier, until the shed is quite full. The heat developed by the fermenting material vaporizes the acetic acid, and this vapour corrodes the lead, forming basic lead acetate. The carbon dioxide which is also produced during fermentation converts the acetate into the carbonate, which falls as a heavy white powder into the pots.

Future Supply of Acetic Acid. When all the operations involved in the production of acetic acid from wood, from the felling of the tree to the final separation of the glacial substance, are taken into consideration, it will be readily understood how it is that this acid has never been cheap when compared with other acids used on an equally large scale. In addition to this, the competition for wood for paper-making and for the very numerous cellulose industries is rapidly increasing. It is, therefore, not surprising to learn that chemists have turned their attention towards the discovery of newer and cheaper methods of making acetic acid.

Such a process seems to have been worked out in Germany. The starting-point is acetylene gas made by the action of water on calcium carbide. When this gas is passed through sulphuric acid containing suspended mercuric oxide or dissolved mercury salt, the acetylene is oxidized first to aldehyde and then to acetic acid.

If this process should prove to be successful, it will form the starting-point of a new and important industry, for, apart from the large amount of acetic acid which is used in commerce, there is the production of the very important solvent known as acetone, which can be made from acetic acid by a very simple operation.

Tartaric Acid. Grape juice contains a large quantity of potassium hydrogen tartrate dissolved in it; when the liquid is fermented and alcohol is formed, this salt crystallizes out because it is not soluble in alcohol. After the new wine has been poured off, the salt is found as a brownish crystalline residue adhering to the sides of the vat. Also the salt goes on crystallizing after the wine is put into barrels, and forms an incrustation on the sides. This is called the lees or sediment of wine. In commerce, the substance is known as argol (sometimes spelt argal), and also tartar of wine.

Crude argol is purified by dissolving it in water and destroying the colour by boiling with animal charcoal. When the clear liquid obtained from this mixture by filtration is evaporated, a white crystalline substance separates out. This is potassium hydrogen tartrate or cream of tartar.

Tartaric acid is obtained from cream of tartar. The salt is dissolved in water and nearly neutralized with milk of lime. Insoluble calcium tartrate is precipitated, and potassium tartrate remains in solution. A further quantity of calcium tartrate is obtained by adding calcium chloride to the solution just mentioned. The two precipitates of calcium tartrate are then mixed and decomposed by dilute sulphuric acid, and after the calcium sulphate is filtered off, tartaric acid is obtained as a solid by evaporating the clear liquid.

The general properties of tartaric acid are well known. It is soluble in water, giving a solution which has a pleasantly acid taste.

Citric Acid. The sharp flavour of many unripe fruits is due to the presence of citric acid; the juice of lemons contains 5-6 per cent. of the acid. The free acid is obtained in a manner precisely similar in principle to that described for tartaric acid.

Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely distributed in the vegetable kingdom. These compounds are present in wood sorrel (Oxalis acetosella), in rhubarb, in dock, and in many other plants. The acid is made on a large scale by mixing pine sawdust to a stiff paste with a solution containing caustic soda and potash. The paste is spread out on iron plates and heated, care being taken not to heat the mixture to the point at which it chars. The mass is then allowed to cool, and is mixed with a small quantity of water to dissolve out the excess of alkali. This is recovered and used again.

Sodium oxalate, which is the main product of the reaction described above, is dissolved in water and treated with milk of lime, whereby insoluble calcium oxalate is obtained, which is subsequently decomposed with sulphuric acid, yielding oxalic acid.

Potassium hydrogen oxalate is sometimes called salts of sorrel, and potassium quadroxalate, salts of lemon. The most familiar use of the latter substance is in the removal of ink stains.

Oxalic acid and its salts are poisonous. The free acid has sometimes been mistaken for sugar with fatal results.

Formic Acid (L. formica, an ant) is found both in the vegetable and in the animal kingdom. If the leaf of a stinging nettle is examined with a microscope, it is seen to be covered with long pointed hairs having a gland at the base. This gland contains formic acid. When the nettle is touched lightly, the fine point of the hair punctures the skin, and a subcutaneous injection of formic acid is made, which quickly raises a blister.

The inconvenience which arises from the stings of bees and wasps, also from the fluid ejected by ants when irritated, is due to formic acid. The remedy in each case is the same; the acid must be neutralized as quickly as possible with mild alkali, such as washing soda.

Formic acid was first made by distilling an infusion of red ants. It is now made from glycerine and oxalic acid.

The Fatty Acids. Animal fats and vegetable oils are similarly constituted bodies. They are composed mainly of three chemical compounds known as stearin, palmitin, and oleïn. Of these, stearin and palmitin are solids at ordinary temperatures, while oleïn is a liquid. Hard fats like those of mutton and beef are composed mainly of stearin; fats of medium hardness contain stearin, palmitin, and some oleïn; while oils such as cod-liver oil and olive oil are nearly pure oleïn.

Stearin, palmitin, and oleïn are analogous in composition to salts. Their proximate constituents are glycerine and certain organic acids, stearic, palmitic, and oleïc respectively.

In order to obtain the fat free from tissue which it contains in its natural state, it is tied up in a muslin bag and heated in boiling water. The fat is squeezed out through the meshes of the fabric and floats on the surface of the water as an oil which solidifies on cooling. This clarified fat is called tallow.

All fats and vegetable oils can be resolved into their two constituents, the acid and the glycerine. This can be brought about by heating the fat with water to about 200° C. This operation must be carried out in a vessel capable of withstanding pressure and closed with a safety valve; otherwise, the requisite temperature could not be obtained. After this treatment, there is left in the vessel an oily layer which solidifies on cooling and an aqueous layer which contains the glycerine. The solidified oily layer is the fatty acid. In the case of mutton or beef tallow, it would be mainly a mixture of stearic and palmitic acids. This mixture is used to make “stearin” candles. The acids themselves are wax-like solids without any distinctive taste. Stearic acid melts at 69° C. and palmitic at 62° C. They have no perceptible action on the colour of litmus, neither have they any solvent action on metals or carbonates. We should not recognize these substances as acids at all were it not for the fact that they combine with alkalis, forming salts.

The salts of the fatty acids are called soaps. To make soap, the fat is boiled with caustic alkali or caustic lye, as it is more often called. This breaks the fat up primarily into the acid and glycerine; but in this case, instead of obtaining the acid as the final product as we did above by heating with water under pressure, we get the sodium or potassium salt of the acid according to the alkali used. When caustic soda is used, the product is a hard soap; when caustic potash is used, it is a soft soap. The treatment of fats in this way with caustic alkalis is called “saponification.”

CHAPTER VIII
MILD ALKALI

Caustic and Mild. There are two classes of alkalis distinguished by the terms caustic and mild. If a piece of all-wool material is boiled with a solution of caustic soda or potash, it dissolves completely, giving a yellow solution. Mild alkali will not dissolve flannel, though it may have some slight chemical action causing shrinkage. Partly for this reason, and partly because commercial washing soda often contains a little caustic soda, woollen garments must not be boiled or even washed in hot soda water.

The disintegrating action of the caustic alkalis is also illustrated by the use of caustic soda in the preparation of wood pulp for paper making. Tree trunks are first torn up and shredded by machinery; but notwithstanding the power of modern machinery, the fibre is not nearly fine enough for the purpose until it has been “beaten” with a solution of caustic soda, whereby the pulp is brought to a smooth and uniform consistency like that of thin cream.

Mild Soda and Potash. Until the middle of the eighteenth century, it was thought that the soluble matter extracted from the ashes of all plants was the same. In 1752 it was shown that the substance obtained in this way from plants which grew in or near the sea differed from that from land vegetation by producing a golden yellow colour when introduced into the non-luminous flame of a spirit lamp, while that from land plants gave to the flame a pale lilac tinge. The former substance is now known as mild soda, and the latter as mild potash.

At this point it is well to make it clear to the reader that there are two bodies commonly called soda, and two called potash. One of each pair is caustic and one mild.

By a simple chemical test it is easy to distinguish a mild from a caustic alkali. When a little dilute acid is added to the former, there is a vigorous effervescence caused by the escape of carbon dioxide, but no gas is given off when a caustic alkali is treated in the same way. The liberation of carbon dioxide on the addition of acids shows that the mild alkalis are carbonates.

Washing Soda is so well known, that very little description of its external characteristics is necessary. It is a crystalline substance, easily soluble in water. The crystals, when freshly prepared, are semi-transparent; but after exposure to air for some time, they are found to lose their transparency and to become coated with an opaque white solid which crumbles easily. This change in appearance is accompanied by a loss in weight.

Crystals of soda melt very easily on the application of heat and, on continued heating, the liquid seems to boil. When this operation is carried out in a vessel attached to a condenser, the vapour that is given off from the melted soda condenses to a clear colourless liquid which, on examination, proves to be water. When no more water collects in the receiver, the vessel contains a dry, white solid, which by any chemical test that may be applied is shown to be the same as washing soda, but it contains no water of crystallization and has a different crystalline form. This substance is anhydrous sodium carbonate, or soda ash as it is called in commerce. When soda ash is mixed with water, it combines with about twice its own weight of that liquid, forming soda crystals again.