[457] Putting [Cd^{2+}] = ‹y›, we have ‹y› × (4 ‹y›)^4 = 0.1E−17, and ‹y› = 8E−5. In view of the values of the constants, a ‹small› excess of potassium cyanide will have a much smaller suppressing effect on the cadmium-ion than on the cuprous-ion. For the excess [CN^{−}] = 0.01, [Cu^{+}] = 5E−23, [Cd^{2+}] = 10^{−10} as compared with [Cu^{+}] = 4E−8 in a 0.1 molar solution of the salt K_{3}[Cu(CN)_{3}], and with [Cd^{2+}] = 8E−5 in a 0.1 molar solution of K_{2}Cd(CN)_{4}. [458] Bromine water is a convenient agent for oxidizing cobaltous to cobaltic ions (see Chapter XV). [459] The heavy arrows «→» [See Transcriber's Note] indicate the main course the reversible actions take, ‹under the influence of the reagents used›. Since the oxidation of nickel-ion by bromine is accomplished only after the bromine has oxidized any excess of cyanide used—potassium cyanide is a powerful reducing agent (p. 89)—the addition of cyanide, beyond a very small excess, must be avoided (see laboratory instructions). [460] ‹E.g.› for the precipitation of silver, copper, nickel, cobalt and certain other metals from cyanide solutions; ‹cf.› Edgar F. Smith, ‹Electro-Analysis› (1907). [461] ‹Z. phys. Chem.›, «43», 705 (1903). ‹Vide› also Haber, ‹Z. Elektrochem.›, «11», 847 (1905). [462] 2 Fe^{2+} + Hg^{2+} → 2 Fe^{3+} + Hg ↓. If the treatment with mercuric oxide is carried to completion the final products of the reaction are ferric hydroxide, mercuric cyanide, mercury and potassium hydroxide (Rose, ‹Z. anal. Chem.›, «1», 300 (1862)): 2 K_{4}[Fe(CN)_{6}] + 7 HgO + 7 H_{2}O → 3 Hg[Hg(CN)_{4}] + 8 KOH + 2 Fe(OH)_{3} ↓ + Hg ↓ [463] Bodlaender, ‹loc. cit.› [464] The solubility-product constant of silver sulphide at 25° is 0.5E−51; for [S^{2−}] = 0.8E−5 (p. 202), we would have in the present case [Ag^{+}]^2 × [S^{2−}] = 2E−46, which is greater than the constant. ‹Vide› quantitative data by Lucas, ‹loc. cit.› [465] ‹Z. f. Elektrochem.›, «10», 433 and 773 (1904). [466] See footnote 4, p. 225. [467] Ostwald, ‹Allgem. Chemie›, Vol. II, part 1, p. 881 (1893). [468] Haber, ‹loc. cit.› [469] Then T_{Decomposition} = 10^{−4} × 10^{22} = 10^{18} seconds, and, since there are 3.15E7 seconds in a year, T_{Decomposition} = 3E10 years. [470] See p. 42. [471] Since there are still smaller "instability constants" than that of the argenticyanide-ion (‹e.g.› for the gold-cyanide-ion the constant is 1 / 10^{28}), there is a large margin of safety for the plausibility of Haber's argument. For full details, his articles (‹loc. cit.›), and the discussion (by Abegg, Bodlaender, Danneel, ‹ibid.›) aroused by them should be consulted. [472] See Le Blanc and Schick, ‹Z. phys. Chem.›, «46», 213 (1903), on measurements of the speed of ionic actions. The values obtained agree, in general, with Haber's contention.