ACID. (Fe., ; GER., aura.) The term "acid" (Latin, acidus, sour) is applied in chemistry to a very large and important class of compounds, possessing certain distinctive properties. The most characteristic of these is the power of uniting with alkalies or bases to form salts which have neither acid nor alkaline properties. Every acid, in the strietest sense of the word, contains hydrogen. The difference between an acid and a salt will be readily aeon by regarding an acid as a salt containing one or more atoms of hydrogen as its base, and having the power, when brought into contact with basic sub stances, under favourable conditions, of giving up all or part of its hydrogen, and taking up an equivalent quantity of the base in its stead. In the fewest words possible, an acid is a salt of hydrogen, or a compound in which the hydrogen may be readily replaced by a base or metal, so as to form a salt. There are other characteristics which, though not essential, are common to a large number of acids ; they are (1), sourness of taste ; (2), solubility in water ; (3), the power of redden ing blue organic colouring matters, such as litmus, &c.; and (4), that of decomposing carbonates with evolution of carbou dioxide. These secondary characteristics are extremely variable. The sourness from which the acids derive their name, and which was considered by the older chemists to be their most distinctive feature, is exceedingly intense in some, as sulphuric and acetic acids ; in others, as benzoic acid, tho acidity is so feeble as to be almost imperceptible, whilst others again excite no sensation of neidity when applied to the tongue. The same variableness is extended to the solubility of acids in water. All acids, however, possess in a greater or less degree the power of reddening tincture of litmus, just as all alkalies, on the other hand, restore to the reddened litmus its blue colour ; this reaotiou with litmus forms the simplest test for acidity and alkalinity in liquid bodies.
By the chemist, then, the word "acid" is restricted to the so-called salt of hydrogen ; and to him it has no reference whatever to the sourness of the substance, so long as it fulfils the primary condition that its hydrogen is replaceable by a base or metal. It is unfortunate that a name which not only fails to convey a coireet impression to the uninitiated, but conveys a distinctly erroneous one, should have been extended to a class of bodies whose right to that name is based solely upon their fulfilment of the above condition.
The acids as a class are of very high importance in the arts and manufactures. The most important from a manufacturing point of view arc sulphuric, hydrochloric, nitric, acetic, tartaric, citric, and oxalic ; but a large number of others are manufactured and consumed, on a small scale, in the chemical industries of this country, of which acids a few of the he,t, known only will be con sidered in the following articles.
is the name given to the processes employed for the determination of the strength of acids, or of the amount of free acid contained in a given weight or volume of an acid liquid. In the processes herein described it must be understood that the liquid under examina tion is in a state of tolerable purity, i. e. freedom from foreign matter, which would tend to give rise to inaccurate results. Thus, if a sample of nitric acid contained a small quantity of hydro chloric acid, the latter would be estimated as nitric acid and would apparently increase the result, whereas it should be diminished by the amount of hydrochloric acid present. It will thus be seen that unless the sample operated upon be absolutely free from other acids, only approximate results can be obtained. It is possible to estimate with some degree of accuracy the strength of an acid solution by the temperature at which it boils, or by its specific gravity. The latter means is, indeed, most commonly employed in manufacturing operations to test the strength of eommereial acids. It lies been ascertained that the specific gravity of an acid solution almost invariably bears a uniform relation to its strength, or degree of dilution ; it is clear that if the density of absolutely pure sulphuric acid be 1.845, water being represented by 1, that a mixture of this with water in equal proportions would have a density exactly equal to the mean of those figures, and that according as acid or water predominated the density of the mixture would be higher or lower. In order to determine the density of such a mixture, and thereby its strength, or the amount of free acid which it contains, recourse is had to the small but exceedingly useful instrument known as the "hydrometer." We shall proceed to describe the principle on which this instrument depends, together with the two best-known forms and the mode of using. When a solid body is immersed in water, it is buoyed up by a pressure or force equivalent to the weight of a volume of water equal in bulk to the body immersed. In the same way, if it be plunged into a liquid of greater or less density than water, the pressure of the surrounding liquid, and the consequent buoyancy of the body, are also greater or less in proportion ; and from the difference obtained by observing the depths to which the body sinks, first in the liquid under examination, and then in pure water, by means of a graduated scale attached to the sinking body, the density of the heavier or lighter •liquid may be easily calculated, that of water representiug unity. Slog hydrometers are constructed on the same plan, and only differ from one another in the mode of graduation. They consist usually of a light glass tube, having an oval bulb A, Fig. 1, blown on the lower end. Below this bulb, which contains air, is another small bulb, B, weighted with shot or quicksilver in sufficient quantity to cause the tube to sink to a convenient depth in a liquid of the required density. Inside the tube is fixed a paper scale, the graduation of which is arbitrary. The hydrometer in common use
in this country for testing the density of acids and other liquids heavier than water is that known as " Twaddell'a." In this instrument the density of pure distilled water is represented by 0°, and the scale is graduated in such a manner that the specific gravity of the liquid may be calculated by multiply ing the number of degrees registered on the scale by 5, and adding the product to 1000 ; thus the density of a liquid testing 100° Twaddell would be 100 x 5 ± 1000 = 1500, or P 500. The reading is made by placing the instrument in the liquid and observing the figure registered on the scale at the surface. Baumes hydrometer is the form in use on the Continent ; in shape it is exactly similar to the preceding, but the stem is graduated differently. It may be used for liquids either heavier or lighter than water, the graduation in one case being slightly different from the other. As this graduation is entirely arbitrary, in order to ascertain from the number of degrees registered the actual density of the liquid tested the following tables may he conveniently referred to:— The use of the hydrometer in acidimetrieal operations constitutes the simplest and roughest test employed. When any degree of accuracy is required, the operator must have recourse to the more elaborate and lengthy processes of chemical analysis. A description of these in full would be out of place in a work like the present, and we shall content ourselves with noticing briefly the most popular methods in use in the laboratory. One of these, and perhaps the simplest, depends upon the weight of carbonic acid gas evolved from bicarbonate of soda by a known quantity of acid. The apparatus required is shown in Fig. 2, and may be readily constructed by the operator himself. It consists of a wide-mouthed flask A, furnished with a tightly-fitting cork, through which pass two glass tubes c and d. The tube c terminates in a bulb B, which is filled with chloride of calcium; the bent tube d reaches nearly to the bottom of the flask. A carefully weighed quantity of pure bicarbonate of soda is introduced into the flask and covered with distilled water. This done, a small glass test-tube containing a known volume of the acid to be examined (which must not be sufficient to decompose the whole of the alkali) is carefully lowered into the flask, in the position shown in the figure. The flask is then corked up, and accurately weighed on a delicate balance. After this, the acid in the test-tube is run out upon the alkali by causing the tube to slip into a horizontal position. By this means, a part of the alkali, equivalent to the amount of real acid in the liquid, is decomposed, the carbonic acid gas evolved escaping through the bulh-tubc B ; any moisture which may be carried upwards mechanically is absorbed by the chloride of calcium, the affinity of which substance for water is well known. When the whole of the acid has been neutralized, and the disengagement•f gas has ceased, air is sucked through the tube B in order to withdraw any gas remaining in the flask and tubes. When perfectly cool, the whole apparatus is re-weighed. The difference between the two weighings represents the weight of carbonic acid expelled, and from this the amount of real acid in the volume of liquid operated upon may readily be calculated by multiplying it by the combining weight of the acid and dividing the product by 44, the combining weight of carbonic acid gas. Thus, suppose the weight of the apparatus before the experiment be 32.355 gram., and after the experi ment 31.785 gram., the loss in weight, -NO gram., represents the amount of gas evolved from the .570 x 98 bicarbonate of soda by the acid (say sulphuric acid). Then, 44 = 1.27 gram. of real sulphuric acid, the amount contained in the volume of liquid taken for experiment. The same method applies to the estimation of any acid which decomposes carbonates, the combining weight of such acid being substituted for that of sulphuric acid used in the above example.
Another application of the same principle is a method devised by Fresenius and Will. The apparatus is shown in Fig. 3, and consists of two small flasks, A and B, A being slightly the larger. These are furnished with tightly fitting corks, through each of which pass the glass tubes a, b, and c, arranged as shown in the figure. Tho flask B is half filled with concentrated sulphuric noid, and in tho other is placed the acid to be tested, aceurately measured, and, if neceestary, diluted with water. A teat tubo is now introdncod into the flask A, in the same manner as described in the previous ease; this tube contains bicarbonate of soda, in quantity more than sufficient to neutralize the whole of the acid contained in the sample. After carefully weighing tho apparatus, the acid aud alkali are allowed to mix ; carbonic acid is evolved, passes through the sulphuric acid in the other flask, being thereby thoroughly dried, end escapes through the tube a. All effervescence having ceased, air is drawn through the two flasks,by sucking at the extremity of tho tube a, to remove any traces of carbonio acid re maining behind. When quite cool, the apparatus is re weighed, tho loss representing the amount of carbonic acid disengaged from the alkali. The calculation to find the total quantity of acid in tho volume of liquid employed is, of course, the samo as in the preceding example.
Tho estimation of acids by volumetric analysis is tho oxact converse of the method used in alkalimetry, since it depende upon tho volume of an alkaline solution of known strength required to neutralize a given volume of the acid under examination. For a description of thie process, tho reader is reforred to the article on " Alkalimetry." Works for reference :—Freeeniut?' Quantitative Analysis': Sutton's Volumetric Analysis.'