The analytical precautions, taken to insure the precipitation, by hydrogen sulphide, of arsenic sulphide, when arsenic is present in quinquivalent form, are based on the observations described; in quantitative analysis, for the sake of securing a precipitate of ‹uniform composition›, the aim is to precipitate the pure ‹pentasulphide› and a considerable excess of hydrochloric acid is used. In qualitative analysis, where the composition of the precipitate is a matter of indifference and a large excess of acid would seriously interfere with the precipitation of certain sulphides (‹e.g.› CdS, see p. 211), a smaller excess of acid is used and the precipitation of arsenic sulphide is insured by prolonged treatment of a solution with hydrogen sulphide ‹at a high temperature›. FOOTNOTES: [492] The ‹weak basic properties› of the hydroxides of the aluminium group, as compared with those of the zinc group, ‹a chemical difference›, and the resulting great instability of the carbonates of the former group, are used in the separation of the aluminium from the zinc group, by barium carbonate; but the physical element of extreme insolubility of the trivalent hydroxides enters also as an important factor (see footnote 3, p. 194). [493] See p. 247, in regard to the behavior of arsenic acid in this respect. [494] See p. 246, footnote 3, in regard to the action of sodium sulphide on mercuric and bismuth sulphide. [495] ‹Vide› Nilson, ‹J. prakt. Ch.›, «14», 150 (1876). [496] Such a solution does not react alkaline to phenolphthaleïn. [497] Hydrogen sulphide rapidly destroys the indicator and the experiment is best carried out by preparing 50 c.c. of a saturated aqueous solution of hydrogen sulphide, containing 1 or 2 c.c. of normal hydrochloric acid, and by adding a considerable excess of methyl orange to the solution immediately before the addition of potassium hydrosulphide solution, which has been prepared as described in the text. [498] See the discussion on p. 177. See also the discussion by Remsen on acidic and basic halides, ‹Am. Chem. J.›, «11», 300 (1889) Stud. [499] In both cases acid salts, KHCO_{3} and KHCS_{3}, are also formed. [500] McCay, ‹Z. anorg. Chem.›, «29», 36 (1901). [501] On p. 238 the analogous equation for the condition of equilibrium of the anion of an oxygen acid with its components was developed. Applying the result to the ion SnO_{3}^{2−} of stannic acid, H_{2}SnO_{3}, we have: [Sn^{4+}] × [O^{2−}]^3 / [SnO_{3}^{2−}] = K. It is evident, from the form of the equation, that for the stronger oxygen acids, ‹which are most stable as acids and ionize as bases at most in traces›, the value of the constant must be extremely small.