One of those methods consists in determining the avidity of acids for a metallic hydroxide, as shown by the proportion in which the latter is distributed between two acids when brought in contact with a mixture of the two, the amount of metallic hydroxide employed being insufficient to saturate both acids completely. For example: sodium hydroxide, sulphuric acid, and nitric acid are weighed out in such quantities that the sodium hydroxide is just sufficient to neu tralize either one of the two acids. When the three substances are now mixed together in aqueous solution, it is found that two-thirds of the sodium hydroxide have been taken up by the nitric acid and only one-third by the sulphu ric acid. The conclusion is drawn that nitric acid is twice as strong an acid as sulphuric acid. It is similarly found that hydrochloric acid, too, is twice as strong as sulphuric acid, and hence possesses the same strength as nitric acid. Acetic acid is found to be very weak.
Another interesting method of determining the relative strength of acids consists in measuring the rapidity with which various acids are capa ble of effecting the inversion of sugar ; that is to say, the decomposition of sugar into dextrose and levulose, a reaction taking place under the influence of acids, according to the following equation: = Dextrose Levulose For example, if equivalent quantities of nitric and hydrochloric acids are added to two equal portions of a solution of cane-sugar, it it found that, under the same conditions of temperature and concentration, the inversion takes place with equal rapidity in both cases; the conclusion is drawn that. nitric and hydrochloric acids are
equally strong acids. It is similarly found that these acids are about twice as strong as sul phu•ic acid, while acetic acid is found to be very weak.
When an acid is dissolved in water, its mole cules are assumed to become dissociated into ions, some of which are charged with positive, some with negative. electricity. Thus acetic acid is supposed to break up according to the following equation: —