An electrochemical study of Ni2+, Co2+, and Zn2+ ions in melts of composition CaMgSi2O6期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	Krystyna W Semkow Ronald A Rizzo Larry A Haskin David J Lindstrom

Institution:	1. Department of Chemistry, Washington University, St. Louis, MO 63130 U.S.A.;2. Department of Chemistry, University of Wisconsin, Madison, WI 53706 U.S.A.;3. Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 U.S.A.

Abstract:	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 $E^{1} (Ni^{2+} Ni^{0}, diopside, 1500°C) = ?0.32 ± .01 V$ , $E^{1} (Co^{2+} Co^{0}, diopside, 1500°C) = ?0.45 ± .02 V$ , and $E^{1} (Zn^{2+} Zn^{0}, diopside, 1500°C) = ?0.53 ± .01 V$ . 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 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 $E1$ ( $Ni^{2+} Ni^{0}$ , 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 $E^{1} (Ni^{2+} Ni^{0}, 1525°C$ ) 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.

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