The kinetic study of the oxidation reaction of the tris(2,2'-bipyridine)-cobalt(II) ion and the ethylenediamine-N,N,N',N'-tetraacetato-cobaltate(II) ion by the peroxodisulfate ion.
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概要
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The kinetics of the oxidation of Co<SUP>II</SUP>–EDTA and Co<SUP>II</SUP>(bpy)<SUB>3</SUB><SUP>2+</SUP> by the peroxodisulfate ion (S<SUB>2</SUB>O<SUB>8</SUB><SUP>2−</SUP>) in acidic media have been studied spectrophotometrically; EDTA and bpy represent ethylenediamine-<I>N</I>,<I>N</I>,<I>N</I>′,<I>N</I>′-tetraacetic acid and 2,2′-bipyridine respectively. The stoichiometry of the reactions of Co<SUP>II</SUP>–EDTA with the peroxodisulfate ion was 2Co<SUP>II</SUP>–EDTA+S<SUB>2</SUB>O<SUB>8</SUB><SUP>2−</SUP>=2Co<SUP>II</SUP>(edta)<SUP>−</SUP>+2SO<SUB>4</SUB><SUP>2−</SUP>. The rate for the oxidation of Co<SUP>II</SUP>–EDTA by the peroxodisulfate ion is independent of the concentration of Co<SUP>II</SUP>–EDTA, but first-order dependent with respect to the peroxodisulfate-ion concentration. The rate law is d[Co<SUP>II</SUP>(edta)<SUP>−</SUP>]/<I>dt</I>=2<I>k</I>[S<SUB>2</SUB>O<SUB>8</SUB><SUP>2−</SUP>]. The rate-determining step must be the decomposition of S<SUB>2</SUB>O<SUB>8</SUB><SUP>2−</SUP> into 2SO<SUB>4</SUB><SUP>\newdot</SUP>. The first-order rate constant, the activation enthalpy, and the activation entropy are 5.5×10<SUP>−8</SUP> s<SUP>−1</SUP>, 24 kcal mol<SUP>−1</SUP>, and −12 eu respectively at pH 3.60, 25 °C, and <I>I</I>=0.50 (NaClO<SUB>4</SUB>). Two moles of Co<SUP>II</SUP>(bpy)<SUB>3</SUB><SUP>2+</SUP> are oxidized by one mole of the peroxodisulfate ion. For the oxidation of Co<SUP>II</SUP>(bpy)<SUB>3</SUB><SUP>2+</SUP>, the following reaction path and rate law were proposed:<BR>& Co^II(bpy)_3^2++S_3O_8^2-\oversetk_e\rightleftarrows[Co^II(bpy)_3^2+·S_2O_8^2-]\xrightarrowkCo^II(bpy)_3^3++SO_4^2-+SO_4^\newdot<BR>& Co^II(bpy)_3^2++SO_4^\newdot\xrightarrowfastCo^III(bpy)_3^3++SO_4^2-<BR>& \fracd[Co^II(bpy)_3^3+]dt=\left{\frac2k·K_e1+K_e[S_2O_8^2-]\right}[S_2O_8^2-][Co^II(bpy)_3^3+]_T<BR>At pH 3.60, 25 °C, and <I>I</I>=0.50 (Na<SUB>2</SUB>SO<SUB>4</SUB>), the rate constant, <I>k</I>, and the equilibrium constant, <I>K</I><SUB>e</SUB>, are 0.53 s<SUP>−1</SUP> and 5.6 M<SUP>−1</SUP> respectively. The difference in the rate law and mechanism between Co<SUP>II</SUP>–EDTA and Co<SUP>II</SUP>–(bpy)<SUB>3</SUB><SUP>2+</SUP> may come from the difference in charge on the cobalt(II) complexes.
- 公益社団法人 日本化学会の論文
著者
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Ohashi Kousaburo
Department Of Achemistry Faculty Of Science Ibaraki University
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Yamamoto Katsumi
Department Of Anesthesiology University Of Occupational And Environmental Health School Of Medicine
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Matsuzawa Morito
Department of Chemistry, Faculty of Science, Ibaraki University
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Hamano Eichi
Department of Chemistry, Faculty of Science, Ibaraki University
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