[380] Amphoteric substances of a different class are also known, which have, at the same time, moderately ‹strong› acid and moderately ‹strong› basic functions. Glycocoll, H_{2}N.CH_{2}COOH, a derivative of acetic acid and ammonia, contains an acid group, the —COOH group, the hydrogen of which is approximately as ionizable as the hydrogen in the corresponding group in acetic acid, CH_{3}COOH. The ammonia residue, H_{2}N—, in glycocoll, forms with water a hydroxide, corresponding to ammonium hydroxide, which likewise is approximately as ionizable as is ammonium hydroxide. In the hydroxide of glycocoll we have, consequently, both a moderately strong acid, and a moderately strong basic, group. In this, and in similar cases, ‹salt formation between the acid and the basic groups of the amphoters takes place to as great an extent as if the functions were attributes of distinct compounds›. Glycocoll in aqueous solution is present, then, chiefly in the form of a salt, for instance, H_{3}N^{+}.CH_{2}.COO^{−}, corresponding to ammonium acetate, CH_{3}COONH_{4}. For an elaborate discussion of the equilibrium conditions in solutions of amphoteric compounds ‹vide› Walker, ‹Z. phys. Chem.›, «49» and «51». [381] The carbonates are occasionally partially hydrolyzed to basic carbonates. [382] Stokes, ‹J. Am. Chem. Soc.›, «29», 304 (1907). [383] A complication, which leads to the precipitation of alkaline earths, along with these groups, as phosphates and similar insoluble salts (not as hydroxides or sulphides), when phosphate or certain other acid ions are present, is treated in Part IV (‹q.v.›) under the systematic analysis of the groups. [384] ‹Cf.› Fresenius, ‹Quantitative Analysis›. [385] Ammonium sulphide usually precipitates a ‹pink› hydrated sulphide of manganese, probably Mn(SH)(OH). Under certain conditions of concentration and temperature, the dark ‹green› sulphide MnS is precipitated. In quantitative work the chemist aims to precipitate this green sulphide, which is more easily collected on a filter. (‹Cf.› Fresenius, ‹Quantitative Analysis›.) [386] We shall find that this property of the whole zinc group makes it possible to separate the following groups, the copper and arsenic groups, from the zinc group (see p. 158). The theory of the separation will be discussed in detail in Chapter XI. [387] ‹Vide› Noyes, Bray and Spear, ‹J. Am. Chem. Soc.›, «30», 483 (1908). [388] See footnote, p. 189.