Zur mikrochromatographischen Bestimmung des Vitamin-C-Gehaltes

Zusammenfassung Eine Mikromethodik zur Bestimmung der Ascorbinsäure in Früchten und Gemüse wurde beschrieben, bei der dieses Vitamin papierchromatographisch isoliert wird. Als Laufmittel hat sich ein Gemisch von n-Butanol/Wasser (2 3) mit 2% Oxalsäure und einer Spur KCN zur Maskierung jener Schwermetalle bewährt, die die Oxydation der Ascorbinsäure katalysieren.Die Herstellung der Extrakte bzw. Preßsäfte unter schonenden Bedingungen sowie die Anreicherung des auf mehreren Streifen isolierten Vitamins werden beschrieben. Für die photometrische Bestimmung wird eine eigene Anreicherungsapparatur verwendet, die faserfreie Konzentrate liefert. Für die titrimetrische Methodik kann die Anreicherung des Vitamins durch einfache Extraktion der Papierabschnitte im Schliffspitzbecher mit 2%iger Oxalsäure erzielt werden, da die verbleibenden Filterpapierstückchen die Titration mit dem Tillmannsreagens eher erleichtern als stören. Eichkurven für die photometrische und die titrimetrische Mikromethodik zeigen den jeweiligen Anwendungsbereich.

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Summary A microprocedure is described for determining the ascorbic acid in fruits and vegetables, in which this vitamin is isolated chromatographically on several strips. The mobile phase consists of a mixture ofn-butanol/water (2 3) with 2% oxalic acid and a trace of KCN to mask those heavy metals that catalyze the oxidation of ascorbic acid.The preparation of the extracts or expressed juices under protective conditions and also the accumulation of the vitamin that has been isolated on several strips are described. An apparatus developed by us is employed for the photometric determination; it delivers fiber-free concentrates. For the titrimetric procedure, the accumulation of the vitamin can be accomplished through simple extraction of the paper sections with 2% oxalic acid in the conical beaker, because the residual filter paper fragments facilitate rather than hinder the titration with the Tillmann reagent. Calibration curves for the photometric and the titrimetric micro procedure show the particular application range.

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Herrn Prof. Dr.Hans Lieb zum 80. Geburtstag gewidmet.     

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.     

Light scattering by a finite obstacle and fano resonances

The conditions for observing Fano resonances at elastic light scattering by a single finite-size obstacle are discussed. General arguments are illustrated by consideration of the scattering by a small (relative to the incident light wavelength) spherical obstacle based upon the exact Mie solution of the diffraction problem. The most attention is paid to recently discovered anomalous scattering. An exactly solvable one-dimentional discrete model with nonlocal coupling for simulating diffraction in wave scattering in systems with reduced spatial dimensionality is also introduced and analyzed. Deep connections between the resonances in the continuous and discrete systems are revealed.     

Moving near-gap solitons in the nonlinear optical medium

For a one-dimensional nonlinear optical medium with a periodic refraction index, new two-parameter soliton solutions of electrodynamics equations have been found. These solutions represent two interacting waves that propagate in two opposite directions. The oscillation frequency of each wave may fall either into the forbidden gap in the linear spectrum or outside it, and the group velocity may vary from zero to a maximal value that is determined by the parameters of the medium. Algebraic soliton solutions have been found as the limit of the nonlinear solutions, when the nonlinear wave frequency tends to the frequency of one of the linear-spectrum branches.     

Polycondensation kinetics of orthosilicic acid in a hydrothermal solution

The polycondensation kinetics of orthosilicic acid in hydrothermal solutions was studied experimentally at different temperatures, pH, and ionic strengths. As a result of the mathematical processing of experimental data, equations approximating the time dependence of the polycondensation process and the dependence of the polycondensation rate constant on the solution ionic strength were obtained. Experimental data on the kinetics of the reverse process of dissolving colloidal silica were obtained at different temperatures and pH.