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Electrochemical characteristics of manganese in cyanide solutions

1963; Elsevier BV; Volume: 5; Issue: 2 Linguagem: Inglês

10.1016/0022-0728(63)80126-6

2590-2946

Stephen A. Moros, Louis Meites,

Advanced oxidation water treatment

In neutral sodium cyanide solutions, manganese(II) is reversibly reduced to the + I state at a dropping mercury electrode. The half-wave potential of the resulting polarographic wave is −1.364 ± 0.007 V vs S.C.E. in 1 F sodium cyanide, and varies with sodium cyanide concentration in a manner that can be ascribed to activity effects. The value of id/C for this wave decreases on adding sodium hydroxide, on increasing the manganese(II) concentration, and (at an anomalous rate) on increasing the drop time. These facts indicate that manganese(I) reacts with hydrogen cyanide to regenerate manganese(II), and that this reaction is fractional order with respect to manganese(I). Direct polarographic and spectrophotometric measurements of the rates of decomposition of air-free cyanide solutions of manganese (I) give for the pseudo-half-order rate constant The absorption spectra of very dilute (0.02–0.05 mF) solutions of manganese(I) in cyanide media indicate the presence of more than one absorbing species. Adding sodium hydroxide to a solution of manganese(II) in 1 F sodium cyanide results in a reaction which is probably best represented by the equation and the resulting hydroxocyano complex undergoes an irreversible 2-electron reduction to the metal at E12 = −1.72 V. The data do not appear to confirm the supposition that Mn(CN)64− is the sole important species of manganese (II) in moderately concentrated neutral cyanide solutions. Manganese(III) is the principal product of the oxidation of manganese(II) with air or oxygen in neutral cyanide media, but the reaction is not stoichiometric. At high concentrations of manganese and/or cyanide, the oxidation is incomplete even though, very prolonged; at low concentrations of both, substantial amounts of a more highly oxidized species are present. Manganese(III) in neutral cyanide media is polarographically reduced to the +1 state in two successive 1-electron steps.