Electrochemistry of cations in diopsidic melt: Determining diffusion rates and redox potentials from voltammetric curves

We have used Linear Sweep Voltammetry to measure properties of selected ions in diopsidic melt. In general, the redox reactions studied are complicated by such factors as slow reaction kinetics, monomerizations, and dimerizations, or presence of multi-step reductions that overlap on voltammetric curves. Criteria for recognizing these complications are gleaned from the literature and their application to reductions in high temperature silicate melts is discussed. Criteria that establish whether the reduction potentials obtained from the curves represent equilibrium conditions are also discussed. Diffusion coefficients were measured in diopsidic melt for cations of Eu, Mn, Cr and In. We observe that diffusion rates decrease as the size of the diffusing cation increases. Diffusional activation energies are similar to those reported for other silicate melt compositions. Enthalpies and entropies of reduction were determined for the cations V(V), Cr 3+, Mn 2+, Mn 3+, Fe 2+, Cu 2+, Mo(VI), Sn(IV) and Eu 3+. The free energies can be used to compute relative proportions of multivalent cations in various valence states as a function of temperature and fO 2. Reduction potentials also provide a means for studying the structural state of cations in the melt. Free energies of reduction in melts of different compositions cannot be modeled solely by changes in oxide ion activity, but must include contributions from changes in the activities of the cations as well.