According to the maximum number of their hydrogen atoms replaceable by metals acids are termed mono-basic, dibasic, tri-basic, etc. No matter how great the excess of potassium hydroxide employed, only one hydrogen atom of acetic acid, CjH.O;, can be replaced by potassium, the
only resulting salt having the formula CjHjKO,.
Acetic acid is, therefore, said to be a mono-basic
acid. By the action of a limited amount of
potassium hydroxide on sulphuric acid (H^SOJ
a salt called the acid sulphate of potassium
(HKSOj) may be obtained; this salt is formed
by substituting the metal potassium for one of
the hydrogen atoms of sulphuric .acid. But if
an excess of potassium hydroxide is used, both
of the h3'drogen atoms of sulphuric acid are
replaced by potassium, and the salt known as
the neutral sulphate of potassium (K-SOJ is
produced. Sulphuric acid is therefore said to
be a di-basic acid. In like manner phosphoric
acid (HjPOj) is found to be a tri-basic acid, etc.
Acids containing carbon among their constit-
uent elements are called organic acids, because
some of them were originally foiuid in the
organic world. Jlost organic acids arc found to
contain one or more carboxyl groups (COOH) ;
it is the hydrogen of these groups that is replace-
able by metals. These acids are called carboxylic
acids, and their basicity is determined by the
number of carboxyl groups they contain. The
carboxylic acids are subdivided into carbocyclic
and fatty acids, according as their molecules
do or do not contain those rings of which the
so-called aromatic benzene-nucleus is the most
important. Thus benzoic acid, C'aHjC'OOH, is a
carbocyclic acid; acetic acid, CHjCOOH, is a
fatty acid. An interesting group of substances
belonging to the aromatic series and, like acids,
combining with metallic hydroxides, are not
included among the true aromatic acids because
they do not contain the carboxyl group. These
substances, called phenols (q.v. ), are found to
be weaker than the weakest carboxylic acid
known, viz., carbonic acid.
The specific strength of an acid depends, natu- rally, on its composition and chemical consti- tution. But the precise nature of that relation is as yet unknown. The correctness of the very methods of measuring the strength of acids is, according to some eminent authors, still subject to doubt. It is, however, remarkable and cannot be denied, that the dilTerent methods employed yield very nearly coincident results. 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, aiul 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 lionce possesses the same strength as nitric acid. Acetic acid is found to be very weak.
Another interesting metliod of determining tiie 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 :
C,.H„,0„ + H,0 = C'„H,,0„ + C„H,,0, Cane-sugar Dextrose Levulose For example, if equivalent quantities of nitric and hydrochloric acids are added to two equal portions of a solution of cane-svigar, it is 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- phuric 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:
CR,COOH = H -f CH:,COO I Acetic acid M
The dissociation is usually incomplete; that is to say, only a fraction of the amount of acid in solution is dissociated into ions, the rest remaining undissoeiated. So that a solution of acetic acid, for instance, contains three kinds of particles, viz., (I) positive hydrogen ions,
H; (2) negative ions, CH,COO; and (3) electrically neutral (undissoeiated) acetic acid molecules, CH.COOH. The magnitude of the fraction dissociated, or, as it is called, the degree of dissociation of an acid, depends (a) upon the amount of acid in solution; (b) upon the temperature; and (c) upon the nature of the acid. Under the same conditions of concentration and temperature the number of free ions in solutions of different acids depends U])on nothing but the nature of the acids. And as according to the electrolytic theory the capacity of an acid for conducting electricity depends upon nothing but the presence of free ions in its solution, tlie electrical conductivity of the solution may be taken as a measure, so to speak, of the nature of the acid.
Now, when the acids are tabularly arranged in the order of their electrical conductivity, it is found that the order is the same as when they are arranged according to their avidity for metallic hydroxides, or when they are arranged in the order of the rapidity with which they can effect the inversion of cane-sugar.
A remarkable relation is thus seen to exist between three phenomena having apparently no connection with one another. The common cause of these phenomena is assumed to be the presence of free hydrogen ions in an acid solution. Furthermore, on this assumption the neutralization of acids by metallic hydroxides is explained in the following manner. The fact that pure water is a non-conductor of electricity proves that its molecules are not dissociated into ions. If ions formed by the elements of
