SULPHONIC ACIDS, a series of adds, both organic and inorganic, distinguished by containing the monovalent radical, SO,.OH, also written, SO.H. The inorganic mem bers of the group may be regarded as derived from S02.0H.H by replacing the final H by a monovalent atom or radical. such as chlorine; or, more generally, from n molecules of SO,.OH.H, by replacing the n final H atoms by a single n-valent atom or radical. Chlorosulphonic acid, Cl(SO,.OH), is an example of such an acid. It may be pre pared by the direct combination of sulphur trioxide, SO,, with hydrochloric acid, and also in various other ways. The number of hy drogen atoms replaced depends on the strength of the acid used and on the temperature at which the action is carried on. In organic sulphonic acids, the SO,.OH group is joined, by means of its own sulphur atom, to a carbon atom in some organic base. There arise from such combination two classes of sulphonic acids, the aliphatic and the aromatic. Only the latter series is of importance in the arts. The sul phonic acids of aromatic compounds play an important part in the coal-tar color industry, and they may be readily prepared by digesting aromatic compounds with either sulphur trioxide, sulphuric acid, or chlorosulphonic acid. The aromatic sulphonic acids are usually pre pared by boiling the sulphonation product with an excess of calcium carbonate, or, preferably, barium carbonate. The barium sulphate pre
cipitated is separated out by filtering. The fil trate contains the barium (or calcium) salt of the sulphonic acid. To this filtrate sulphuric acid is added drop by drop until no more pre cipitate forms. The solution is again filtered, and the filtrate evaporated to dryness, the acid appearing either as a syrupy liquid or in crystals, according to its constitution. If cal cium carbonate has been used traces of it will remain in the acid. and must be removed by adding alcohol, filtering, and evaporating the alcohol with gentle heat. Where the result con tains two sulphonic acids they are usually separated by conversion into sulphonic chlor ides by the addition of phosphorus pentachloride (or oxychloride) and crystallized — wLen they assume their distinctive forms.
The aromatic sulphonic acids are strongly acid in reaction, and are generally highly soluble in water, the crystalline forms as a rule being hygroscopic. When heated, they de compose gradually without melting. They are manufactured in large quantity to be converted into phenols — readily accomplished by fusion with caustic alkalies. Consult Benedikt, 'Chemistry of the Coal Tar Colors.'