The kinetics of the oxidation of CoII–EDTA and CoII(bpy)32+ by the peroxodisulfate ion (S2O82−) in acidic media have been studied spectrophotometrically; EDTA and bpy represent ethylenediamine-N,N,N′,N′-tetraacetic acid and 2,2′-bipyridine respectively. The stoichiometry of the reactions of CoII–EDTA with the peroxodisulfate ion was 2CoII–EDTA+S2O82−=2CoII(edta)−+2SO42−. The rate for the oxidation of CoII–EDTA by the peroxodisulfate ion is independent of the concentration of CoII–EDTA, but first-order dependent with respect to the peroxodisulfate-ion concentration. The rate law is d[CoII(edta)−]/dt=2k[S2O82−]. The rate-determining step must be the decomposition of S2O82− into 2SO4\ewdot. The first-order rate constant, the activation enthalpy, and the activation entropy are 5.5×10−8 s−1, 24 kcal mol−1, and −12 eu respectively at pH 3.60, 25 °C, and I=0.50 (NaClO4). Two moles of CoII(bpy)32+ are oxidized by one mole of the peroxodisulfate ion. For the oxidation of CoII(bpy)32+, the following reaction path and rate law were proposed:
& Co^II(bpy)_3^2++S_3O_8^2-\oversetk_e\ightleftarrows[Co^II(bpy)_3^2+·S_2O_8^2-]\xrightarrowkCo^II(bpy)_3^3++SO_4^2-+SO_4^\ewdot
& Co^II(bpy)_3^2++SO_4^\ewdot\xrightarrowfastCo^III(bpy)_3^3++SO_4^2-
& \fracd[Co^II(bpy)_3^3+]dt=\left{\frac2k·K_e1+K_e[S_2O_8^2-]\ight}[S_2O_8^2-][Co^II(bpy)_3^3+]_T
At pH 3.60, 25 °C, and I=0.50 (Na2SO4), the rate constant, k, and the equilibrium constant, Ke, are 0.53 s−1 and 5.6 M−1 respectively. The difference in the rate law and mechanism between CoII–EDTA and CoII–(bpy)32+ may come from the difference in charge on the cobalt(II) complexes.
Spektrophotometrische Untersuchung von Kinetik und Mechanismus zur Diskussion des Verhaltens negativ und positiv geladener Co(II)‐Komplexe gegen‐ über dem Oxidationsmittel.
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