[489] The most common organic acids contain the acid group —CO(‗OH‗), as in acetic acid, CH_{3}CO(‗OH‗). The hydroxide group ‗OH‗ of the alcohols, ‹e.g.› in CH_{3}CH_{2}(‗OH‗), is still found in these organic acids, but its tendency to form hydrogen-ion is very much increased by the replacement of two hydrogen atoms of the alcohols by the oxygen atom, as found in the acids. To a certain degree, the properties of the alcohol hydroxide are maintained in the properties shown by the acid hydroxide group. Thus, the organic acids, on the whole, are still rather weak acids, and their salts, in many instances, are appreciably less ionizable than the salts of strong inorganic acids. The organic acids, further, may combine, to a certain extent, with water and thus form hydrates (‹e.g.› CH_{3}COOH + H_{2}O ⇄ CH_{3}C(OH)_{3}) containing a number of hydroxide groups: the second and third hydroxide groups must have a very much smaller tendency to form hydrogen ions and ionizable salts, than has the first hydroxide group (p. 102), and the former, thus show, more nearly, the behavior of alcoholic hydroxide groups. Finally, organic acids also show a tendency to combine with themselves, forming complex acids (‹e.g.›, (CH_{3}COOH)_{2} or CH_{3}C(OH)_{2}OOCCH_{3}), from which complex salts may be derived, which may be little ionizable. The power of the organic acids to form complex ions—which they share with many inorganic acids—is most likely intimately connected with the relations described. [490] Lead acetate, itself, is less ionized than most salts and this property contributes to the solubility of lead sulphate in acetate solutions. (‹Cf.› Noyes and Bray, ‹loc. cit.›) [491] On p. 231, the same effect is discussed, in detail, in connection with the ferricyanide-ion. [p242] CHAPTER XIII «THE ARSENIC GROUP. SULPHO-ACIDS AND SULPHO-SALTS»