Chemical diffusion and crystalline nucleation during oxidation of ferrous iron-bearing magnesium aluminosilicate glass

Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) have been used to evaluate the mechanism and kinetics of oxidation of a Fe²⁺-doped MgO---Al₂O₃---SiO₂ glass (with nominal composition along the enstatite-cordierite-liquid divariant) which was heat treated in air under the time and temperature ranges 10–150 h and 700–800°C, respectively. The results clearly demonstrate that oxidation occurs by a cation diffusion process: specifically, the divalent cations diffuse from the interior of the glass to the free surface where they subsequently react with environmental oxygen to form a two-phase, MgO---(Mg, Fe)₃O₄ crystalline layer which covers the (divalent cation-depleted) glass. Oxidation of some Fe²⁺ within the glass occurs via the inward flux of electron holes (a counterflux to the divalent cation diffusion required to maintain charge neutrality of the glass); this internal oxidation results in the fine-scale ( 1–5 nm), homogeneous nucleation of crystalline (Mg, Fe)₃O₄ within the divalent cation-depleted layer of the glass. Chemical diffusion of an oxygen species is thus demonstrated to be a slower, parallel kinetic process which is not required for oxidation to occur in this material. A first-order analysis of oxidation kinetics in the glass is presented.     

Implicit Scaling of Linear Least Squares Problems

We consider the solution of weighted linear least squares problems by Householder transformations with implicit scaling, that is, with the weights stored separately. By holding inverse weights, the constrained case can be accommodated. The error analysis of the weighted and unconstrained case is readily extended and we show that iterative refinement may be applied.     

Micro-potentiometric determination of periodate by arsenite-iodine titration

Summary TheFleury-Lange method for the estimation of periodate can be extended to samples containing less than 0.05 micromoles by potentiometric titration of the excess arsenite with dilute iodine solution. Iodine solutions down to 0.0001N can be titrated with 0.4% accuracy.

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Zusammenfassung Die Methode vonFleury undLange zur Bestimmung von Perjodat kann auf Proben, die weniger als 0,05 Mikromol enthalten, durch potentiometrische Titration des überschüssigen Arsenits mit einer verd. Jodlösung erweitert werden. Bis 0,0001-n Jodlösungen lassen sich mit 0,4% Genauigkeit titrieren.

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Résumé La méthode deFleury etLange pour l'estimation du periodate peut être étendue aux échantillons contenant moins de 0,05 micromoles par le titrage potentiométrique de l'excès d'arsénite avec une solution d'iode diluée. Des solutions d'iode jusqu'à 0,0001N peuvent être titrées avec une précision de 0,4%.

     

Iron(III) coordination chemistry of alterobactin A: a siderophore from the marine bacterium Alteromonas luteoviolacea

Alterobactin A is a siderophore produced by the oceanic bacterium Alteromonas luteoviolacea. The thermodynamic stability constant of the ferric alterobactin A (Alt-A) complex was estimated from electrochemical measurements on the basis of a previously reported linear relationship between the reduction potentials and the pH-independent stability constants for known iron(III) complexes. The reduction potential of the ferric alterobactin A complex determined by square wave voltammetry is -0.972 V vs SCE and reversible, corresponding to a thermodynamic stability constant of 10(51+/-2). Potentiometric titration of Fe(III)-Alt-A shows the release of six protons on complexation of Fe(III) to Alt-A. The 1H NMR resonances of the Ga(III)-Alt-A complex show that the C-4, C-5, and C-6 catecholate protons and the C(alpha) and C(beta) protons of both beta-hydroxyaspartate moieties are shifted downfield relative to the free ligand, which along with the potentiometric titration data is consistent with a complex in which Fe(III) is coordinated by both catecholate oxygen atoms and both oxygen atoms of each beta-hydroxyaspartate. The UV-vis spectrum of Fe(III)-Alt-A is invariant over the pH range 4-9, indicating the coordination does not change over a wide pH range. In addition, in the absence of a coordinated metal ion, the serine ester of Alt-A hydrolyzes forming Alt-B.