Reaction kinetics at linearly increased temperature. IV. Relationships between dta curves,rate curves and adiabatic calorimetry

A general concept is presented for the kinetic interpretation of DTA curves. This is based on the limiting conditions of a DTA measurement: either the kinetic cell constant is zero (adiabatic conditions), or infinite (rate curve). On the other hand, the self-heating effect (thermal feedback), based on the product of the reaction enthalpy with the reactant feed, may be absent (“ideal” kinetic DTA curve) or infinite (impulse reaction). Our recent formulae for the correction of the kinetic classification parameters, shape index and reaction type index, as well as other relationships and their utility, are successfully tested by application to ca. 2000 experimental DTA curves obtained in stirred solutions.The expressions reveal the influence of the activation parameters, heating rate, maximum signal height and cell constant and, therefore, allow a general discussion of the kinetics, independent of the experimental conditions.     

Röntgenspektrometrische bestimmung von blei in automatenstahl : Teil II. Sulfat-verfahren

A method is described for the x-ray spectrometric determination of 0.02–0.5% Pb in free-cutting steels. Lead in the sample solution is precipitated as sulphate with barium sulphate as collector, and the dried fixed precipitate is used for the X-ray Spectrometric measurement. The method shows a reproducibility (s) of ±0.002 % Pb. The time required is ca. 32 min for one determination and ca. 120 min for a series of 20 samples.     

Neue Ergebnisse bei der Bestimmung von Stickstoff und Sauerstoff in Stahl durch Schmelzextraktion im Graphittiegel unter Inertgas

Zusammenfassung Vergleichende Stickstoffuntersuchungen mit dem naßchemischen Aufschlußverfahren, dem oxydierenden Aufschmelzverfahren und dem Trägergasverfahren zeigten, daß das Trägergasverfahren auch für die Untersuchung vieler legierter Stähle anwendbar ist. Die Ergebnisse von Stickstoffbestimmungen 81 legierter und 100 bereits früher untersuchter unlegierter Stähle werden miteinander verglichen. Eine Korrelationsrechnung ergab, daß die Untersuchungsergebnisse des Trägergasverfahrens und des oxydierenden Aufschmelzverfahrens gleichwertig sind. Sie sind etwas höher als die des naßchemischen Aufschlußverfahrens. Die Differenzen sind gering.Ferner wurde festgestellt, daß die Reduktion aluminiumoxidhaltiger Stähle bei der Sauerstoffbestimmung in Stahl innerhalb weniger Minuten durchgeführt werden kann, wenn man, wie bei der Stickstoffbestimmung nach dem Schmelzextraktionsverfahren, für jede Untersuchung einen neuen kleinen Graphittiegel benutzt. Die Ergebnisse der alten und neuen Arbeitsweise werden in einer Gegenüberstellung gezeigt.

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Summary Comparative nitrogen determinations using the wet-chemical digestion, the oxidizing fusion, and the carrier-gas techniques showed that the latter one may also be applied for analysing a great number of alloy steels. A comparison was made of the results of nitrogen determinations of 81 alloy and 100 non-alloy steels analysed previously. A correlation calculation proved that the carrier-gas and the oxidizing fusion techniques are equivalent when considering the results. Their values were slightly higher than those obtained by the wet-chemical digestion. There were minor differences only.In addition, it has been found that the reduction of alumina-bearing steels during the determination of oxygen can be effected within a few minutes, provided a new small graphite crucible is used for each new determination as in nitrogen determination by the fusion-extraction technique. A comparison was made of the results of the old and the new methods.

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Herrn E. Voss danken wir für seine stets sorgfältige und wertvolle Mitarbeit.

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Herrn Prof. Dr. C. Mahr zum 65. Geburtstag gewidmet.     

Further developments in the high-precision coulometric titration of uranium

An experimental study of the current efficiency in the coulometric generation of Ti(III), as a function of electrolyte composition, current density and electrode material, has been performed. The cathodes investigated include platinum, mercury and graphite. The first two are suitable for high-precision determination of uranium. The graphite surface is readily poisoned, rendering it useless for high-accuracy work. The use of mercury requires thorough removal of chloride from the system. The precision and error obtained are comparable for both the mercury and platinum cathodes, and are of the order of 50 ppm.     

Application of redox reactions in spectrophotometry-II Detection and spectrophotometric determination of phenolic compounds with the iron(III)/1,10-phenanthroline complex

To test the utility of the iron(III)/1,10-phenanthroline reagent system for the determination of organic compounds, its reaction with phenolic compounds has been characterized. By a redox reaction the reagent forms the chelate [Fe(phen)(3)](2+), which is extractable as ion-association complexes. Determinations based on these complexes are very sensitive.     

Dynamic NMR study of the flexibility of 2-amino-3-aroyl-4,6-diaryl-pyrylium salts

Dynamic NMR investigations of a number of 2-amino-3-aroyl-4,6-diaryl-pyrylium salts were carried out. The barrier to rotation of the partial C, N double bond was determined and proved to be in the range of 62 to 63 kJ/mol. Quantum chemical calculations of bond orders and electron densities of the different atoms in the molecules show the distinct double bond character of the exocyclic C, N bond. This is in agreement with the relatively high barrier to rotation. By quantum chemical ab initio 3-21G calculations, the dynamic behaviour of this kind of compounds was simulated; two pairs (image and mirror image) of ground state conformations, in coincidence with the experiment, were obtained. Received: 10 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Visualization of aerosol particles generated by near infrared nano- and femtosecond laser ablation

The expansion of aerosols generated by near infrared (NIR) nanosecond (ns) and femtosecond (fs) laser ablation (LA) of metals at atmospheric pressures was explored by laser-induced scattering. In order to achieve adequate temporal and spatial resolution a pulsed laser source was utilized for illuminating a 0.5 mm-wide cross section of the expanding aerosol. It could, for instance, be shown that NIR-ns-LA under quiescent argon atmosphere provokes the formation of a dense aerosol confined within a radially propagating vortex ring. The expansion dynamics achieved under these conditions were found to be fairly slow whereas the degree of aerosol dispersion for NIR-ns-LA using helium drastically increased due to its lower viscosity. As a consequence, the maximum diameter of expansion differed by a factor of approximately four. The trajectories of aerosol particles generated by NIR-ns-LA using argon could, furthermore, be simulated on the basis of computational fluid dynamics (CFD). For this purpose, a model inspired by the thermal character of NIR-ns-LA taking into account a sudden temperature build-up of 10,000 K at the position of the laser focus was implemented.     

Über alkylierte Amide bicyclischer Dicarbonsäuren, 5. Mitt.: Die jodometrische Bestimmung des zentralen Analepticums Endomid®

A highly specific iodometric titration method for the estimation of N,N,N′,N′-tetraethyl-bicyclo[2.2.1]hept-5-ene-2.3-trans-dicarboxamide (Endomide^®) has been developed. The reaction products have been identified and a scheme for the reaction mechanism proposed.