Abstract
The rate of back-extraction of tris(acetylacetonato)cobalt(III), Co(acac)3, in carbon tetrachloride containing acetylacetone, Hacac, by washing with aqueous 4 mol dm−3 perchloric acid-sodium perchlorate solutions is studied. In the early stage, a rapid back-extraction which is due to the two-phase distribution of the complex is observed; the transient distribution equilibrium established is dependent on the acid concentration. This is explained in terms of the formation of protonated species, Co(acac)3H+ and Co(acac)3(H+)2. When the two phase agitation is continued, the back-extraction proceeds gradually. The rate is dependent on the acid concentration but it is not dependent on the Hacac concentration. Most cobalt back-extracted in this stage is found to be in the bivalent state. The rate-determining step is concluded to be the dissociation of the first acetylacetonate ion from the complex in the aqueous phase, which is followed by the reduction of cobalt(III) to cobalt(II). The rate constants of the dissociation of acetylacetonate ion from the Co(acac)3H+ and from the Co(acac)3(H+)2 are one and two orders higher than that from the Co(acac)3.
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