Thermogravimetrische Untersuchung der Mangan(II)-oxid-Pyrolyse

Zusammenfassung Es wurde dieg —t Kurve bei allmählich steigender Temperatur der thermischen Mangan(II)-nitrat-Zersetzung und der darauffolgenden Mangan(IV)-oxid-Dissoziation aufgenommen. Der erste Vorgang spielt sich etwa zwischen 70 und 200, der zweite zwischen 480 und 560° C ab. Auf Grund thermogravimetrischer Aufnahmen unter isothermen Bedingungen konnten die Arrheniusschen Geraden, und daraus für die MnO₂ MnO_1,5-Dissoziation die Aktivierungsenergie von 101 kcal/Mol für ein stabileres und 86 für ein aktiveres MnO₂ errechnet werden. Die thormogravimetrisch ermittelten Anfangstemperaturen der thermischen Dioxiddissoziation sind Gleichgewichtstemperaturen, weil sie mit befriedigender Genauigkeit auf die theoretisch errechnete log p_o2–1/T-Gerade fallen. Es gibt Mn(NO₃)₂-Lösungen, die am Thermogramm anormale MnO₂Stufen ergeben, infolge gleichzeitiger Bildung von MnO_1,5. Vorhergehendes Zufügen von HNO₃ + H₂O₂ oder NH₄NO₃ heben diese Regelwidrigkeit auf, die Gegenwart von Oxalsäure, Ascorbinsäure oder NH₄OH rufen sie hervor und können die Bildung von MnO₂ völlig verhindern.

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Summary Theg -t curve was recorded for the thermal decomposition of manganese(II) carbonate with gradual rise of the temperature and of the subsequent manganese(IV) oxide dissociation. The former reaction takes place between 70 and 200° C, the latter between 480 and 560° C. On the basis of thermogravimetric recordings under isothermal conditions, it was possible to calculate the Arrhenius straight lines, and from these the activation energy (101 kcal/mol) of the MnO₂-MnO_1.5 dissociation for a more stable and 86 kcal/mol for a more active MnO₂. The thermogravimetric initial temperatures of the thermal dioxide dissociation are equilibrium temperatures, because they fall, with adequate accuracy on the theoretically calculated log p_o2–1/T line. There are Mn(NO₃)₂ solutions that yield abnormal MnO₂. stages because of the simultaneous formation of MnO_1.5. Preliminary addition of HNO₃ + H₂O₂ or NH₄NO₃ counteract this departure from the rule, while the presence of oxalic acid, ascorbic acid or NH₄OH occasion it and may completely prevent the formation of MnO₂.

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Résumé On a étudié, pour des températures régulièrement croissantes, la courbe de décomposition thermique poids/t du nitrate de manganèse-II et celle de la dissociation de l'oxyde de manganèse-IV qui s'ensuit. La première étape se produit entre 70 et 200° environ, la seconde entre 480 et 560° C. D'après les données thermogravimétriques en conditions isothermes, les droites d'Arrhénius ont pu être tracées et on en a déduit l'énergie d'activation pour la dissociation MnO₂—MnO_1,5, 101 kcal/mol dans le cas d'un MnO₂ plus stable et 86 pour un MnO₂ plus actif. Les températures du début de la dissociation thermique du bioxyde déduites des données thermogravimétriques sont des températures d'équilibre car elles tombent avec une précision satisfaisante sur les droites log p_O2–1/T, calculées théoriquement. Certaines solutions de Mn(NO₃)₂ donnent sur le thermogramme des paliers anormaux pour MnO₂, par suite de la formation simultanée de MnO_1,5. L'addition préalable de HNO₃ + H₂O₂ ou de NH₄NO₃ supprime cette anomalie; la présence d'acide oxalique, d'acide ascorbique ou d'ammoniaque la provoque et peut masquer complètement la formation de MnO₂.

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Vorgetragen an einem gemeinsamen Seminar der Institute für physikalische bzw. anorganische und analytische Chemie der Universität in Innsbruck, im Rahmen des Chemikertreffens des Vereins Österreichischer Chemiker, 1965.

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Wir danken auch an dieser Stelle Herrn DirektorF. Kömüves für die Genehmigung der Publikation.     

Link formation in cooperative situations

In this paper we study the endogenous formation of cooperation structures or communication graphs between players in a superadditive TU game. For each cooperation structure that is formed, the payoffs to the players are determined by an exogenously given solution. We model the process of cooperation structure formation as a game in strategic form. It is shown that several equilibrium refinements predict the formation of the complete cooperation structure or some structure which is payoff-equivalent to the complete structure. These results are obtained for a large class of solutions for cooperative games with cooperation structures. Received September 1995/Revised version I October 1996/Revised version II April 1997/Final version September 1997     

Comparison of sample preparation methods based on proteolytic enzymatic processes for Se-speciation of edible mushroom (Agaricus bisporus) samples

A sequential sample preparation process was developed for the speciation analysis for Se-enriched edible mushroom, Agaricus bisporus containing 110.2 mug of total Se/g sample. Five different sample extraction methods were compared and the most efficient method (a three-step process involving the use of water extraction and two proteolytic enzymes - pepsin and trypsin) proved to be the most suitable for extracting selenium, with an extraction efficiency of 75%. As the analogues of these enzymes play an important role in human digestion the bioavailability of the selenium present in the sample was estimated. Selenocystine (SeCys(2)) and Se(IV) were detected in considerable amounts (27.7 mug Se/g sample and 46.4 mug Se/g sample, respectively). For the quality control of peak identification a spiking procedure was developed, using a low selenium mushroom containing 4.3 mug of total Se/g sample. During the analysis with HPLC-HHPN (Hydraulic High Pressure Nebuliser)-AFS complicated background effects and matrix problems were observed: stable and reproducible signals generated by the low selenium mushroom and the compounds used in sample preparation were detected. The three-step sample preparation method connected with the HPLC-HHPN-AFS system proved to be applicable for the speciation analysis of the Se-enriched Agaricus bisporus.     

Preparation,homogeneity and stability studies of a candidate LRM for Se speciation

A laboratory reference material (LRM) was prepared from Brazil nuts (Bertholletia excelsa) for quality control (QC) purposes of selenium speciation. The preparation of this LRM led through the usual operation steps applied during routine reference material production from biota samples-preparation of the raw material, homogenisation, storage design, checking of homogeneity, microbiological status and possible irradiation effects, and monitoring the species stability vs time at different storage temperatures. The selenium speciation studies to check species stability were carried out on a HPLC-UV-HG-AFS measurement set-up. Special attention was paid to the correct identification of selenium species by applying independent HPLC separation techniques (ion-pairing and anion-exchange chromatography). The concentration of selenomethionine (SeMet) and total Se content were quantified (79.9 microg g(-1) (calculated as Se) and 82.9 microg g(-1), respectively). The homogeneity and stability of this candidate reference material passed the relevant tests recommended by Bureau Communautaire de Référence (BCR).     

Influence of stereochemistry on the thermal properties of partially cycloaliphatic polyamides

The effects of the partial substitution of 1,4‐disubstituted cyclohexane monomers for linear aliphatic monomers in polyamides are discussed. More specifically, the relation between the stereochemistry of the cycloaliphatic residues and the thermal properties [melting temperature (T_m) and crystallization temperature (T_cr)] was investigated. For this purpose, two different types of copolyamides were synthesized: in polyamides 12.6, the adipic acid residues were partially replaced by cis/trans‐1,4‐cyclohexanedicarboxylic acid (1,4‐CHDA), whereas in polyamides 4.14, the 1,4‐diaminobutane residues were partially substituted with cis/trans‐1,4‐diaminocyclohexane (1,4‐DACH). For both systems, increasing the degree of substitution of cycloaliphatic residues for linear aliphatic residues resulted in a rise of both T_m and T_cr. This points to the isomorphous crystallization of the linear and cycloaliphatic residues. In contrast to the use of 1,4‐DACH as a comonomer, 1,4‐CHDA residues showed isomerization upon thermal treatment of the polyamides. This isomerization of the cyclohexane residues influenced the thermal properties of the copolyamides. The use of a nonisomerizing cis–trans mixture of 1,4‐DACH exhibited the large influence of the stereochemistry of the cycloaliphatic residues on the T_m of the copolyamides. For both the 1,4‐CHDA‐ and 1,4‐DACH‐based copolyamides, differential scanning calorimetry analysis revealed that recrystallization occurs during melting. This exothermal effect becomes less pronounced with an increasing content of rigid cycloaliphatic residues. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1962–1971, 2002     

Hexakis(urea‐κO)zinc(II) dinitrate at 110 and 250 K: uniaxial negative thermal expansion

The crystal structure of the title compound, [Zn{CO(NH₂)₂}₆](NO₃)₂, has been determined at 110 and 250 K. The structure is stabilized by 12 individual hydrogen bonds, both intra‐ and intermolecular. Analysis of the thermal expansion tensor, based on unit cells determined over a temperature range of 180 K, shows uniaxial compression in the direction of the b axis during warming. The hydrogen bonds form layers perpendicular to this axis and these layers are connected by coordinative bonds parallel to the axis. As expected, the intermolecular hydrogen bonds expand during warming. Surprisingly, the coordinative bonds contract, accompanied by changes in the O—Zn—O angles. Overall, this behaviour can be described as an accordion‐like effect.     

Synthesis of 2-pyridone-fused 2,2'-bipyridine derivatives. An unexpectedly complex solid state structure of 3,6-dimethyl-9H-4,5,9-triazaphenanthren-10-one

2-Pyridone-fused 2,2'-bipyridine derivatives and were synthesised. X-Ray diffraction analysis of revealed a highly complex solid state structure with a disordered molecule imbedded in a channel structure formed by a centrosymmetric lattice of hexagonally packed, hydrogen bonded columns. The columns are assembled from three symmetry independent molecules. Dimerisation of the self-complementary cis-amide hydrogen bond motif is overridden by the fulfilment of the proton coordination ability of the phenanthroline nitrogens in accordance with Etter's rules of hydrogen bond priorities.