Other
Scientific paper
Oct 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982gecoa..46.1879s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 46, Issue 10, pp.1879-1889
Other
6
Scientific paper
Cyclic voltammetry has been done for Ni 2+ , Co 2+ , and Zn 2+ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are E * ( Ni 2+ / Ni 0 , diopside, 1500° C ) = -0.32 ± .01 V, E * ( Co 2+ / Co 0 , diopside, 1500° C ) = -0.45 ± .02 V, and E * ( Zn 2+ / Zn 0 , diopside, 1500° C ) = -0.53 ± .01 V. The electrode reactions are rapid, with first order rate constants of the order of 10 -2 cm/sec. Diffusion coefficients were found to be 2.6 × 10 -6 cm 2 /sec for Ni 2+ , 3.4 × 10 -6 cm 2 /sec for Co 2+ , and 3.8 × 10 -6 cm 2 /sec for Zn 2+ at 1500°C. The value of E * ( Ni 2+ / Ni 0 , diopside) is a linear function of temperature over the range studied, with values of -0.35 V at 1425°C and -0.29 V at 1575°C. At constant temperature the value of E * ( Ni 2+ / Ni 0 , 1525° C ) was not observed to vary with composition over the range CaO · MgO · 2SiO 2 to CaO·MgO·3SiO 2 or from 1.67 CaO·0.33MgO·2SiO 2 to 0.5 CaO·1.5MgO·2SiO 2 . The value for the diffusion coefficient for Ni 2+ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO 2 to CaO · MgO · 3SiO 2 . This is consistent with a mechanism by which Ni 2+ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni 2+ species discharging at the cathode regardless of the SiO 2 concentration in the melt.
Haskin Larry A.
Lindstrom David J.
Rizzo Ronald A.
Semkow Krystyna W.
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