Acids. An acid is a chemical compound distinguished by the property of combining with bases in definite proportions to form Salts (q.v.). The most striking characteristics of acids are a sour taste, and the property of reddening vegetable blues. They are also mostly oxidised bodies; and at one time oxygen was thought to be essential to an acid, as the name oxygen (the acid-producer) indicates. Subsequent experience has extended the definition. There is an important class of undoubted acids that contain no oxygen; and silex, or flint, which, being insoluble, neither tastes sour nor reddens litmus-paper, is held to be an acid because it combines with bases and forms compounds like acknowledged acids. The oxygen acids, which are by far the most numerous class, are formed of elements (sulphur, nitrogen, chromium, &c.) with two or more equivalents of oxygen. The elements that form the strongest acids with oxygen are the non-metallic, and most of them have more than one stage of acid oxidation. Thus sulphur unites with oxygen to form two oxides, and , which, in combination with water, yield respectively sulphurous and sulphuric acid. Similarly, arsenic forms two oxides, and , corresponding to arsenious and arsenic acids. The higher stage of oxidation forms the stronger and more stable acid. All metals, except arsenic, that form acids with oxygen, have also, at a lower stage of oxidation, one or more oxides. To these inorganic acids containing oxygen must be added the organic acids, composed of carbon, hydrogen, and oxygen. Belonging to this extensive group are oxalic acid, ; acetic acid, ; and formic acid, . There are also acids found in animal fluids, or resulting from their decomposition, which contain nitrogen in addition to the three elements above named; such is uric acid, . The hydrogen acids are formed of hydrogen and a radical, either simple or compound. The most important of these, and the type of its class, is hydrochloric or muriatic acid, ; others are hydriodic () and hydrocyanic () acids. As all acids, however, even oxygen acids, possess acid properties—i.e. combine with bases—only when in combination with water, a new view of the constitution of acids now prevails, which makes hydrogen the real acidifying element in all acids. Thus, instead of considering vitriol as a compound of sulphuric acid and water, , the hydrated acid is held to be the real sulphuric acid, and its rational formula to be . It thus becomes analogous to hydrochloric acid, . This view has not only the advantage of bringing all acids into one class, but makes the theory of their combination with bases and of their capacity of saturation uniform and simple. Hence has arisen the most general definition of an acid—viz. that ‘acids are salts of hydrogen.’ A more intelligible definition to ordinary readers is that which is adopted by Frankland, in which an acid is described ‘as a compound containing one or more atoms of hydrogen, which become displaced by a metal when the latter is represented to the compound in the form of a hydrate.’ Thus nitric acid and sodium hydrate yield nitrate of soda and water:
in which reaction the hydrogen of the nitric acid is replaced by the sodium of the sodium hydrate (or soda), and as only one atom of hydrogen is replaced, nitric acid is said to be monobasic. When an acid admits of the displacement of two atoms of hydrogen, it is termed dibasic—as tartaric, oxalic, and sulphuric acid; and when three atoms can be replaced—as in common phosphoric acid, , in which may be replaced by or , the acid is termed tribasic. The more important acids are included in the following list:
Acids containing no oxygen: Hydrochloric, ; hydrobromic, ; hydriodic, ; hydrocyanic, ; hydrosulphuric or sulphuretted hydrogen, .
Inorganic acids containing oxygen: Boracic, ; carbonic, ; chromic, ; hypophosphorous, ; nitric, ; phos- phoric, ; phosphorous, ; sulphuric, ; sulphurous, .
Organic acids: Acetic, ; benzoic, ; citric, ; gallic, ; lactic, ; salicylic, ; tartaric, .
The most characteristic inorganic acids (hydrochloric, nitric, phosphoric, sulphuric) are used in medicine in a very dilute condition as tonics and astringents, and to allay thirst in fevers. They corrode the teeth, however, and if long administered tend to disorder digestion; so they must be used with caution. Most of the group have special, some (as hydrocyanic, oxalic) extremely poisonous actions. The stronger acids, when concentrated, are powerful caustics.