KINETIC-STUDY OF THE SPECIATION OF COPPER(II) BOUND TO A HYDROUS FERRIC-OXIDE

The equilibrium distribution of metal ions over distinguishable sites of a heterogeneous complexant may be determined by monitoring the rate of reaction of the equilibrated system with a ligand capable of strip ping the metal ion off the various sites. This kinetic speciation meth od has the advantage that it indicates not only the equilibrium distri bution but also the relative lability of complexes. It may also reveal lability differences among complexes of very similar thermodynamic st ability. This kinetic method has been applied to the speciation of Cu( II) in the presence of a hydrous ferric oxide colloid using a copper-s elective color-forming chelating ligand. The pH (4.5-7.0) range spans the region over which an ''adsorption edge'' is observed in convention al isotherm measurements. The kinetic speciation results are consisten t with constraints suggested by these isotherm experiments. A main poi nt of this study is that significant differences in speciation are obs erved for solutions where Cu(II) was added to already-formed colloids in contrast to those in which Cu(II) was present during the formation of colloids. Differences between equilibrium and kinetic speciation re sults may be explained by hypothesizing differential diffusional acces sibility to similar binding sites.