Krystyna W. Semkow

Concentrations and behavior of oxygen and oxide ion in melts of composition CaO.MgO.xSiO2

Abstract The concentrations and behavior of oxygen and oxide ion were studied in silicate melts of composition CaO · MgO · xSiO2 (1.25 ≤ x ≤ 3) in the temperature range 1425 to 1575°C by cyclic voltammetry and chronopotentiometry. Electroreduction of oxygen is a reversible, 2 electron process involving dissociated oxygen atoms. The Henrys Law constant for O2 in molten diopside (CaO · MgO · 2SiO2) is 0.023 ± 0.004 mole/l atm at 1450°C. The diffusion coefficient for molecular oxygen in diopside melt is 4.5 ± .5 × 10−6 cm2/sec at 1450°C and the activation energy of diffusion is 80 ± 2 kcal/mole. Oxide ions produced by electroreduction of oxygen, rapidly dissociate silicate polymers, causing the concentration of free oxide ions in diopside melt to be buffered at a low level (4.7 ± .8 × 10−5 mole/l). The concentration of free oxide ion increases at higher proportions of metal oxides but remains at this value in more silicic melts. The rate of formation of oxide ions by polymerization in diopside melt is 0.021 ± .007 mole/l sec. Thermodynamic parameters (the standard free energy, enthalpy and entropy) for the oxidation of Ni, Co, and Zn in diopside melt in equilibrium with gaseous oxygen agree with those for solid oxide systems. The platinum reference electrode in molten diopside is a reversible, oxygen electrode.

An electrochemical study of Ni2+, Co2+, and Zn2+ ions in melts of composition CaMgSi2O6

Cyclic voltammetry has been done for Ni2+, Co2+, and Zn2+ 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∗ (Ni2+Ni0, diopside, 1500°C) = −0.32 ± .01 V, E∗ (Co2+Co0, diopside, 1500°C) = −0.45 ± .02 V, and E∗ (Zn2+Zn0, 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 cm2/sec for Ni2+, 3.4 × 10−6 cm2/sec for Co2+, and 3.8 × 10−6 cm2/sec for Zn2+ at 1500°C. The value of E∗ (Ni2+Ni0, 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∗ (Ni2+Ni0, 1525°C) was not observed to vary with composition over the range CaO · MgO · 2SiO2 to CaO·MgO·3SiO2 or from 1.67 CaO·0.33MgO·2SiO2 to 0.5 CaO·1.5MgO·2SiO2. The value for the diffusion coefficient for Ni2+ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO2 to CaO · MgO · 3SiO2. This is consistent with a mechanism by which Ni2+ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni2+ species discharging at the cathode regardless of the SiO2 concentration in the melt.