11 research outputs found
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Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments
The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry
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Solubility behavior of quartz and corundum in supercritical water: A quantitative thermodynamic interpretation
Dissolution reaction equilibria for {alpha}-quartz (SiO{sub 2}) and corundum ({alpha}-Al{sub 2}0{sub 3}) in pure, supercritical water are quantified using a density-dependent thermodynamic model. The database of existing solubility literature for {alpha}-quartz (0.2-10 kb, 200--575{degrees}C) is shown to be consistent with the presence of two hydrolyzed SI(IV) ion forms: Si(OH){sub 4}(aq) and Si{sub 2}O(OH){sub 6}(aq); the corundum database (1-20 kb, 400--700{degrees}C) is consistent with Al(OH){sub 3}(aq) and Al(OH){sub 4}{sup {minus}}. A third Si(IV) ion hydroxocomplex, Si{sub 2}O{sub 2}(OH){sub 5}{sup {minus}}, is indicated at lower pressures (0.03-0.10 kb). The characteristic sigmoidal nature of the solubility isobars is explained by dimerization of Si(OH){sub 4}(aq) (at high densities) or the formation of anionic hydrolysis products, Si{sub 2}0{sub 2}(OH){sub 5}{sup {minus}} and Al(OH){sub 4}{sup {minus}}, in the low density region (p < 0.01 gm/cc). By means of the evaluated equilibria, thermochemical properties of Si{sub 2}O(OH){sub 6}(aq) and Si{sub 2}O{sub 2}(OH){sub 5}{sup {minus}} are made available for the first time
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Nickel(II) Oxide Solubility and Phase Stability in High Temperature Aqueous Solutions
A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of nickel(II) oxide (NiO) in deoxygenated ammonium and sodium hydroxide solutions between 21 and 315 C. Solubilities were found to vary between 0.4 and 400 nanomolal (nm). The measured nickel ion solubilities were interpreted via a Ni(II) ion hydroxo- and amino-complexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. Two solid phase transformations were observed: at temperatures below 149 C, the activity of Ni(II) ions in aqueous solution was controlled by a hydrous Ni(II) oxide (theophrastite) solid phase rather than anhydrous NiO (bunsenite); above 247 C, Ni(II) activities were controlled by cubic rather than rhombohedral bunsenite
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Immiscibility in the Fe3O4-FeCr2O4 Spinel Binary
A recent thermodynamic model of mixing in spinel binaries, based on changes in cation disordering (x) between tetrahedral and octahedral sites, is investigated for applicability to the Fe{sub 3}O{sub 4}-FeCr{sub 2}O{sub 4} system under conditions where incomplete mixing occurs. Poor agreement with measured consolute solution temperature and solvus is attributed to neglect of: (1) ordering of magnetic moments of cations in the tetrahedral sublattice antiparallel to the moments of those in the octahedral sublattice and (2) pair-wise electron hopping between octahedral site Fe{sup 3+} and Fe{sup 2+} ions. Disordering free energies ({Delta}G{sub D}), from which free energies of mixing are calculated, are modeled by {Delta}G{sub D} = {alpha}{chi} + {beta}{chi}{sup 2} - T(S{sub c} + {chi}{sigma}{sub el} + {gamma}{chi}{sigma}{sup mag}) where the previously-neglected effects are accommodated by: (1) adding a non-configurational entropy term to provide coupling between cation disordering and magnetic ordering and (2) revising the configurational entropy (S{sub c}) analysis. Applying the constraint {alpha} = -(2/3){beta} and regressing the existing database for Fe{sup 2+} ion disorder in Fe{sub 3}O{sub 4} gives: {beta} = -31,020 {+-} 1050 J mol{sup -1}, {sigma}{sub el}/R = -0.730 {+-} 0.081 and {gamma}, the coupling parameter between cation disordering and magnetic ordering, = -0.664 {+-} 0.075. The revised mixing model predicts a consolute solution temperature (T{sub cs}) = 600 C and a solvus at 500 C of n = 0.05 and 0.70 for the Fe(Fe{sub 1-n}Cr{sub n}){sub 2}O{sub 4} spinel binary