[275] Precipitation is complete only when the colloids are used in the proportions required to neutralize each other's charges [Billitzer, ‹Z. phys. Chem›., «51», 140 (1905)]. The proportions to be used must be determined in each case, most simply by trial (Noyes, ‹loc. cit.›, p. 101), but quantitative methods for determining the charges, by titration, are also known (‹cf.› Billitzer, ‹loc. cit.›). [276] E. A. Schneider, ‹Z. anorg. Chem.›, «5», 80 (1894). [277] See the above discussion on silicic acid. Stannic acid has a greater tendency to form a base than has silicic acid. [278] Zsigmondy, ‹Liebig's Annalen›, «301», 361 (1898). [279] ‹Cf.› Fresenius, p. 334, or Smith's ‹Inorganic Chemistry›, p. 468. [280] Mylius, ‹Ber. d. chem. Ges.›, «36», 775 (1903); Biltz, ‹ibid.›, «37», 1116 (1904). [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.› [p139] CHAPTER VIII «SIMULTANEOUS CHEMICAL AND PHYSICAL EQUILIBRIUM.—THE SOLUBILITY- OR ION-PRODUCT.»