General Characters. - It is somewhat difficult to get a correct definition of an acid. Most of them have a sour taste and redden blue litmus: they combine with alkalis and destroy the power which these have of turning red litmus-paper blue.

They may be regarded as compounds of hydrogen with certain radicals, hydrogen being readily displaced by other bases. Some acids, as boric and carbolic, have no sour taste. Carbolic acid does not redden litmus-paper, but it is in reality an alcohol, although in chemical combinations it behaves like an acid.

General Preparation of Acids. - Most acids are prepared by liberating them from their alkaline salts by means of sulphuric acid. When they are volatile they are separated by distillation, and when non-volatile by crystallisation.

Sulphuric acid, which is of such importance in the preparation of other acids, is itself prepared by oxidising the fumes of sulphur by means of nitric acid. Sulphur is burnt, and the sulphurous oxide thus produced is conducted along with the vapour of nitric acid into a large leaden chamber, where it is mixed with steam. Sulphurous oxide is oxidised by the nitric acid and sulphuric oxide is formed, which uniting with the watery vapour forms sulphuric acid. The nitric acid is deoxidised in this process into nitric oxide; this unites with the oxygen of the air to form nitric peroxide, and this again supplies fresh oxygen to the sulphurous acid, NO2 + SO3 + H2O=NO + H2SO4. In this way a small quantity of nitric acid is sufficient to oxidise a large quantity of sulphuric acid; reduction and reoxidation going on alternately in the nitrous fumes. The sulphuric acid formed in the leaden chamber is drawn off and evaporated to the proper strength.

The acids which are prepared by liberation from their salts by sulphuric acid are given in the following tables: -

Volatile Acid

Prepared from

By addition of Sulphuric Acid and

Carbonic Acid . .

Any carbonate, generally Carbonate of Calcium

Conducting into water or alkaline solution, according to the purpose required.

Hydrochloric Acid, B. and U.S.P.

Sodium chloride

Distilling into water, which dissolves the acid.

Nitric Acid, B. and U.S.P.

Sodium Nitrate, or Potassium Nitrate

Distilling.

Acetic Acid, B. and U.S.P.

Crystallised Sodium Acetate

Ditto.

Glacial Acetic Acid, B. and U.S.P.

Dried Sodium Acetate

Ditto.

Dilute Hydrocyanic Acid, B. and U.S.P.

Potassium Ferrocy-anide

Distilling into water.

Sodium chloride and sodium nitrate are found native: the sodium acetate is prepared from gas liquor by saturating with sodium carbonate.

In preparing hydrocyanic acid the cyanide is not employed, but the ferrocyanide which is prepared by heating together animal refuse and iron filings and potassium carbonate.

Non-Volatile Acid

Prepared from

By addition of Sulphuric Acid and

Chromic Acid . .

Potassium Bichromate

Collection of crystals, draining, and drying.

Tartaric Acid, B. and U.S.P.

Tartrate of Calcium, made from acid tartrate of potassium

Subsequent decantation from calcium sulphate, evaporation and crystallisation.

Citric Acid, B. and U.S.P.

Citrate of Calcium, made from lemon-juice

Subsequent decantation, etc, as for tartaric acid.

Lactic Acid, B. and U.S.P.

Lactate of Calcium, obtained by peculiar fermentation of sugar

Decantation and evaporation.

Boric Acid, B. and U.S.P.

Sodium borate.

Precipitation; the boric acid, sparingly soluble in water, falls as a precipitate, and the sodium sulphate is removed by decantation or filtration.

If sulphuric acid were added to citrate or tartrate of potassium or sodium, it would be difficult to separate the acid from the sulphate. To avoid this, the citrates and tartrates of calcium are first prepared, and to these sulphuric acid is added. There results an insoluble calcium sulphate which falls as a precipitate, and the solution of citric or tartaric acid is readily separated by decantation or filtration, and evaporated to crystallisation.

Citrate of calcium is prepared by adding chalk to boiling lemon-juice, and washing the colouring matter from the precipitate by hot water. Hot is employed in preference to cold water because citrate of calcium is less soluble in it. Tartrate of calcium is prepared from the crude acid tartrate of potassium or argol, which is deposited from wine during the process of fermentation. Chalk is first added to a solution of it, whereby a neutral tartrate is formed, 2(KHC4H4O6) + CaCO3=CaC4H4O6 + K2C4H4O6 + CO2 + H2O. This is then decomposed by the addition of calcium chloride or sulphate, K2C4H4O6 + CaCl = CaC4H4O6 + 2KC1.

Exceptions to the rule that acids are prepared from salts by the addition of sulphuric acid :-

Acid

Prepared by

Sulphuric Acid . .

Combustion of sulphur and the oxidation and hydration of the resulting sulphurous acid gas by means of nitrous and aqueous vapours.

Phosphoric Acid . .

Oxidising phosphorus by heating it with diluted nitric acid until nitrous fumes have ceased to form, and then diluting it to the proper strength.

Oxalic Acid . .

Oxidising sugar by heating with nitric acid.

Sulphurous Acid .

Deoxodising sulphuric acid by means of charcoal and passing the fumes into water.

Hydrobrornic Acid .

By passing sulphuretted hydrogen into bromine and water 2Br2 + 2H2S = 4HBr + S2.

Arsenious Acid . .

Boasting arsenical ores, collecting the acid which sublimes, and purifying it by resublimation.

Benzoic Acid . .

Heating gum benzoin when the acid sublimes.

Carbolic Acid . .

Fractional distillation of coal-tar oil and subsequent purification.

Oleic Acid . .

Decomposing lead oleate by hydrochloric acid or by decomposing fats by superheated steam and separation from solid fats by pressure.

Salicylic Acid . .

By passing carbonic acid gas over sodium carbolate which is made by evaporating a mixture of caustic soda and carbolic acid to dryness.

Tannic Acid . .

Dissolving out from the fresh nut-galls in which it is contained by ether and water.

Gallic Acid . .

Dissolving it out from fermented nut-galls by hot water.

Hydrobromic acid, although volatile, is not unfrequently prepared without distillation. McLean Hamilton and Milner Fothergill's plan is to dissolve 84 1/4 grs. of potassium bromide in a fluid ounce of water, and add 99 grs. of tartaric acid to it. After standing at a low temperature for twelve hours, acid tartrate of potassium crystallises out, and leaves a solution containing about 10 per cent. of real hydrobromic acid.

General Action of Acids. - They have an affinity for electropositive or basic substances, and combine with them when they come in contact. Stronger acids drive out weaker ones from their combination with bases, setting them free; but are themselves sometimes driven out by weaker ones if these form an insoluble combination.

When they come in contact with the tissues they produce changes in a twofold manner; (1) by forming new compounds,

(2) by destroying others previously existing. The different acids possess different affinities, and the actions they exert vary with the acid and with the degree of its concentration, weak acids having their affinities easily satisfied. All the tissues of the body are alkaline, and the first effect of acids will be to neutralise the alkali, and if albumin be dissolved in it to precipitate it. If sufficient acid be present, they all, with the exception of nitric acid, again redissolve it. Acids unite with albumin in different proportions, forming acid-albumin. When mixed with blood they not only precipitate albuminous substances, but decompose haemoglobin, forming a substance which holds oxygen with more tenacity than haemoglobin. They coagulate myosin and produce instantaneous rigidity in muscles. Sulphuric and phosphoric acids have, besides their chemical affinities, a strong attraction for water, and completely decompose the tissues to which they are applied, so that they are most powerful escharotics. Nitric acid does not readily redissolve the albumin precipitated by it, and thus forms a barrier to its own action, so that it does not penetrate so deeply as sulphuric acid.

Round the tissue killed by acids inflammation ensues, and an eschar is separated. When their action is less intense they cause inflammation of the surface of the dermis, and produce vesication. Still less concentrated, they precipitate albumin from its solutions in the tissues, act as irritants, and cause contraction of the blood-vessels. This effect is removed by the alkalinity of the blood, and the irritation may be only sufficient to cause a temporary congestion subsequent to the contraction. Then the acids act only as rubefacients. As such they are used in the form of baths.