[458] Bromine water is a convenient agent for oxidizing cobaltous to cobaltic ions (see Chapter XV).
[459] The [heavy arrows] → 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 Fe2+ + Hg2+ → 2 Fe3+ + 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 K4[Fe(CN)6] + 7 HgO + 7 H2O →
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 [S2−] = 0.8E−5 (p. [202]), we would have in the present case [Ag+]2 × [S2−] = 2E−46, which is greater than the constant. Vide quantitative data by Lucas, loc. cit.
[465] Z. f. Elektrochem., 10, 433 and 773 (1904).