[281] ‹J. für prakt. Chem.›, «25», 431 (1882).
[282] The suspension used is prepared by saturating, with hydrogen sulphide, an aqueous solution of arsenious oxide. The latter is saturated on a steam bath, cooled to 20°, filtered and diluted with an equal volume of water before it is used.
[283] Eight grams of NH_{ 4}NO_{ 3} per 100 c.c.
[284] 0.6 c.c. of a ‹tenth›-normal solution is used, containing 1.28 gram Mg(NO_{ 3})_{ 2}, 6 aq., in 100 c.c. Precipitation was found to be incomplete with 0.5 c.c.
[285] 0.15 c.c. of a ‹tenth›-normal solution is used, containing 1.2 gram Al(NO_{ 3})_{ 3}, 4 aq., in 100 c.c. Precipitation was found to be incomplete with 0.1 c.c. of the solution.
[286] 3.3 grams (NH_{ 4})_{ 2}SO_{ 4} in 100 c.c.
[287] Freundlich found, for instance, that NaCl, KCl, BaCl_{ 2}, in equivalent concentrations, had practically the same effect on colloidal ferric hydroxide, but only ‹one-fortieth› as much of a ‹sulphate› (the precipitating ion is SO_{ 4}^{ 2−} versus Cl^{ −}) was required; ‹Z. phys. Chem.›, «44», 129 (1903).
[288] For other reasons (‹e.g.› to prevent oxidation of the sulphides), hydrogen sulphide is also used in the solution for washing the arsenic group, and ammonium sulphide in that for the zinc group (see Lab. Manual, pp. 101 and 110).
[289] Picton and Linder, ‹loc. cit.›, and Whitney and Ober, ‹loc. cit.›
[290] A. A. Noyes describes a similar experiment with sodium chloride and silver nitrate, ‹loc. cit.›
«SIMULTANEOUS CHEMICAL AND PHYSICAL EQUILIBRIUM.—THE SOLUBILITY- OR ION-PRODUCT.»
It frequently happens that we have to deal, simultaneously, with conditions of chemical and of physical equilibrium, obtaining in the same system. For instance, a gas like carbon dioxide, in contact with its saturated solution in water, is in equilibrium with the dissolved carbon dioxide, and this, in turn, is in equilibrium with its hydrate, carbonic acid. A substance may be distributed between two solvents and show a different molecular weight in the two (see p. 18); it may exist, in the one, primarily in polymeric form, and only to a slight extent in the simple form, the two forms being in equilibrium (chemical equilibrium). In the other solvent, it may exist only in its simple molecular form, and this will be in equilibrium with the same simple molecular form in the first solvent (physical equilibrium). In matters dealing with the solubility of electrolytes in water, and, therefore, in questions of their ‹precipitation› or ‹solution›, such simultaneous conditions of chemical and physical equilibrium are constantly occurring. Since qualitative analysis deals to a very considerable extent with just such precipitates of salts, acids and bases, these cases are of particular importance to us.