[485] See the laboratory instructions, in regard to the precautions used, to avoid errors from this source.
[486] On the other hand, colloidal organic substances, such as casein, glue or albumen, interfere with the precipitation of even the most insoluble sulphides, by producing colloidal suspensions of the latter (see Chap. VII; cf. Müller, Allgemeine Chemie der Kolloide, p. 56 (1907)).
[487] In alcohols the hydroxide group is held by a carbon atom, whose remaining valences are satisfied by hydrogen or carbon atoms, as in ordinary or ethyl alcohol, H3C─CH2(OH).
[488] Küster, Z. Elektrochem., 4, 117 (1897).
[489] The most common organic acids contain the acid group —CO(OH), as in acetic acid, CH3CO(OH). The hydroxide group OH of the alcohols, e.g. in CH3CH2(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. CH3COOH + H2O ⇄ CH3C(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., (CH3COOH)2 or CH3C(OH)2OOCCH3), 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.
CHAPTER XIII THE ARSENIC GROUP. SULPHO-ACIDS AND SULPHO-SALTS
The analytical groups, which we have heretofore discussed, contain elements, whose oxides are preëminently base-forming. The methods of separation of these groups, from each other, involve, primarily, physical[492] differences between the groups—in the matter of the relative insolubility of analogous salts. Thus, barium, strontium and calcium carbonates are precipitated, and separated from the alkalies, by means of ammonium carbonate, not because the alkalies do not form carbonates when their salts, in solution, are treated with ammonium carbonate, but wholly because barium, strontium and calcium carbonates are very difficultly, the alkali carbonates easily, soluble in water. The hydroxides of the aluminium group and the sulphides of the zinc group are less soluble than the hydroxides and sulphides of the alkaline earths and alkalies. The sulphides of the copper and the arsenic groups, again, are still less soluble than the sulphides of the zinc group, and thus the former may be precipitated by hydrogen sulphide, even when its precipitating power is reduced by the suppression of its sulphide (and hydrosulphide) ions by the addition of a strong acid.