Cobalt ionic strength dependence

The acid-catalyzed decomposition of /i-superoxo complexes of the type [Co(III)2(/>t02)(en)4(NH3)2] involves intramolecular electron transfer with AV around 20cm moP. Reductions by [Fe(CN)e] of [Co(edta)] and [Co(NH3)5py] are outer sphere but, in the latter case, detection of an outer-sphere precursor allows evaluation of A V for electron transfer of 23.9cm moP comparable with other values/ Attempts to rationalize this suggest almost complete electron transfer in the transition state. Similar activation volumes for electron transfer are found in reduction of [(NH3)5CoOH2] by [FeCCN) ], but there is a marked discrepancy in reaction volumes for precursor formation with those of other cobalt(III) oxidants. Such differences may be due in part to solvation effects which vary with ionic strength and the necessity of studying ionic strength dependencies of activation volumes for electron transfer reactions between complex ions has been pointed out. ... 

The rate of hydrolysis depends markedly on pH and on the cobalt to ADP stoichiometric ratio it depends also on the nature of any pre-equilibration procedures. Variations in the ionic strength (NaClO ) have a minor effect. 

In the case of the octahedral robust complexes of cobalt (III) and chro-mium(III), substitution in the first sphere is hindered. This type of complex ion is, therefore, especially suitable for studying association in the second sphere. The hexammine and tris(ethylenediamine) cobalt(III) ions have especially been used for this kind of study. For the association of these ions with anions, such as sulfate and thiosulfate, the ion-pair constant is of the order of magnitude of 10 at 7 = 0, somewhat smaller for Coena" than for Co(NH3)6 21)y but strongly dependent on the ionic strength. Thus Posey and Taube 37) y from spectrophotometric measurements in the ultraviolet, obtain the following expression for the association constant of the ion pair [Co(NH3)6]S04 in solutions with y/Jvarying from 0.04 to 0.3 ... 

Co(CN)sOH2]2- + [Co(CN)5QH]3-- [Co(CN)50H2]2- + QH2 The dependences of both and pathways on pH and ionic strength have been determined. Aqueous cobalt(n) ions have been shownto react with l-nitroso-2-naphthol-3,6-disulphonate under anaerobic conditions in a four-electron reduction of the ligand to the 1-amino-derivative. At lower pH s, however, the ligand appears to act as a two-electron oxidant. ... 

Oxidation of cobalt(II) aminocarboxylates [CoHEDTA] and [CoEDDA] by IO4 is first order in metal complex but two pathways with first- and second-order dependencies on [IO4] concentration are detected, indicating intermediate mono- and bis-complex formation. Both complexes are redox active and second- and third-order rate constants are 0.16M s and 2.59M "s for [CoHEDTA] and 0.120 M s and 1.53 M s for [CoEDDA] at pH 5, 25°C, and 0.5 M ionic strength. The third-order rates are pH dependent. 

Fig. 22, Dependence of half-wave potentials on logarithm of ionic strength for kinetic reduction wave of cobalt in Co (II) — cysteine system (1), wave of Co (II) aquo complexes (2), and catalytic hydrogen wave of Co (II) — cysteine system in borate buffer solution at pH 8.4 (3).

Temperature Dependence There are a few studies that have obtained solubility constants for -Co(OH)2(s) or pink cobalt hydroxide at zero ionic strength and for reaction (2.13) (M = Co ", x = 0). Three studies have presented data for the solubility constant at 25 °C and these values are in reasonable agreement (Nasa-nen, 1942b Makovskaya and Spivakovskii, 1974 Ziemniak, Goyette and Combs, 1999). In addition, solubility constants can also be derived from the work of Ziemniak et al. for higher temperatures up to 300 °C. Figure 11.57 illustrates that the accepted solubility constants are a linear function of the reciprocal of absolute temperature. 

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