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 stripping the metal ion off the various sites. This kinetic speciation method has the advantage that it indicates not only the equilibrium distribution but also the relative lability of complexes. It may also reveal lability differences among complexes of very similar thermodynamic stability. This kinetic method has been applied to the speciation of Cu(II) in the presence of a hydrous ferric oxide colloid using a copper-selective color-forming chelating ligand. The pH (4.5-7.0) range spans the region over which an ''adsorption edge'' is observed in conventional isotherm measurements. The kinetic speciation results are consistent with constraints suggested by these isotherm experiments. A main point of this study is that significant differences in speciation are observed 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 results may be explained by hypothesizing differential diffusional accessibility to similar binding sites.