This method answers well with all the stronger acids (excepting oxalic acid), even when dilute; and it has the advantage of not being affected by the presence of a neutral metallic salt with an acid reaction, as sulphate of copper, or of zinc.

Besides this process a solution of lime in sugar may be used, as proposed by M. Peligot, and made as follows:—

Pure caustic lime is carefully slaked by sprinkling with water, and 50 grains (or grammes), made up by water to a milky solution, and 100 grains of pure sugar candy dissolved in 1000 grains of water, are added, and the whole well shaken. It is allowed to settle in a closed bottle, and the clear solution poured off and diluted, until 1000 grains neutralise exactly 100 grains of pure hydrochloric acid of sp. gr. 1·1812. Of course it only answers with acids whose calcium salts are readily soluble in water.

b. Gravermetrically:—

The test-liquors or standard solutions of the above methods are made up so as to weigh exactly 1000 grains, instead of to ‘measure’ 100 acidimeter divisions. Every grain of the test-liquor thus represents 1⁄10th gr. of alkali; and every 10 gr., 1 gr. of alkali; or respectively, 1⁄10th per cent. and 1 per cent. The vessel used for containing the solutions is carefully weighed whilst empty, and 1000 gr. being placed in the opposite scale, the test-solution, containing exactly one equivalent of base, is poured in, and the whole made up with distilled water (if necessary) so as to restore the balance to an equilibrium. After the process of neutralisation, the acidimeter, with its contents, is again placed in the scales; its previous weight still remaining there. The number of grains required to restore the equilibrium of the balance (i.e., the loss of weight), gives the exact weight of the test-liquor consumed. In all other respects the process is the same as in the ‘volumetrical method’ already described.

Another method for estimating the strength of the sample of acid is by weighing the amount of carbonic acid expelled during saturation. (Method of Fresenius and Will.) This depends on the weight of gaseous carbonic acid which a given weight of the acid-sample under examination is capable of expelling from pure bicarbonate of soda (or of potash), which is estimated by the loss of weight in the acidimeter, or apparatus, after the gas, rendered perfectly dry by passing through sulphuric acid, has escaped into the air.

Table I.—Weights of the respective acids equivalent to the given weight of the principal bases, hydrogen being taken as unity.

17gr.of pure ammonia.[8]are exactly neutralised by51 Acetic acid (anhydrous).
31”anhydrous soda.[9]60 Acetic acid (crystallised or glacial).
40”hydrate of soda.[9]99 Arsenious acid (dry).
53”dry carbonate of soda.[10]35 Boracic acid (anhydrous).
143”crystallised carbonate of soda.[11]62 Boracic acid (crystallised).
84”crystallised bicarbonate of soda.22 Carbonic acid (dry).
47”anhydrous potassa.[9]67 Citric acid (crystallised).
56”hydrate of potassa.[9]85 Gallic acid (dried at 212°).
69”dry carbonate of potassa.[10]94 Gallic acid (crystallised).
100”crystallised bicarbonate of potassa.1271⁄2 Hydriodic acid (dry or gaseous).
50”pure chalk or pure marble.27 Hydrocyanic acid (anhydrous).
28”pure caustic lime.361⁄2 Hydrochloric acid (dry or gaseous).
37”hydrate of lime (fresh).1091⁄2 Hydrochloric acid (liquid, sp. gr. 1·162).
44”dry carbonic acid (when the bicarbonate of potassa or soda is used for testing in the process of Fresenius and Will).1661⁄2 Iodic acid.
22”dry carbonic acid (when a dry carbonate is used).54 Nitric acid (anhydrous).
671⁄2 Nitric acid (liquid, sesquihydrated, sp. gr. 1·5033 to 1·504).
72 Nitric acid (liquid, binhydrated, sp. gr. 1·486).
90 Nitric acid (liquid, sp. gr 1·42).
36 Oxalic acid (anhydrous).
63 Oxalic acid (crystallised).
72 Phosphoric acid (anhydrous).
81 Phosphoric acid (glacial).
50 Succinic acid (dry or anhydrous crystals).
59 Succinic acid (ordinary crystals).
40 Sulphuric acid (anhydrous).
49 Sulphuric acid (liquid, monohydrated, sp. gr. 1·8485).
75 Tartaric acid (crystallised).
12 Tannic acid (carefully dried).

[8] 1000 water-grains measure of pure liquor of ammonia, sp. gr. 0·992, contains exactly 17 gr., or 1 equiv. of pure gaseous ammonia. A standard liquor of this strength may be most conveniently prepared by cautious dilution of a stronger solution, until a hydrostatic bead, corresponding to the sp. gr., floats indifferently in the middle of the new solution, at 60° Fahr. By keeping two hydrostatic beads in the solution—the one made barely to float, and the other barely to sink—we shall always be able to detect any change of strength or temperature which it may suffer; since the “loss of a single hundredth part of a grain of ammonia per cent., or the difference of a single degree of heat, will cause the beads to” vary their positions. To preserve its integrity it must be kept in a well-stoppered bottle. (See below.)

[9] These substances, as well as ‘test-solutions’ containing them, must be perfectly free from carbonic acid, and must be carefully preserved to prevent the absorption of carbonic acid from the atmosphere. Mohr states that a dilute solution of either of them is best preserved in a flask or bottle well closed with a cork fitted with a small bulb tube (resembling a chloride of calcium tube), filled with a finely triturated mixture of sulphate of soda and caustic lime, and bearing a very thin open tube in the exit aperture. Fresenius, and most other foreign chemists, prefer ‘test-solutions’ of pure soda. With test-solutions containing caustic alkalies, exact neutralisation of an acid is not only more easily effected, but more readily perceived, particularly when either solution is tinted with litmus.