Energetic and kinetic investigation of thermally induced molecular rearrangements of esters of (hydroxymethyl) hydridosilanes by DSC

Silanes of the type R¹R²Si(H)CH₂OEl(O)_xR _y ³ [R=organyl; El=C (x=y=1), S (x=2,y=1), P (x=1y=2)] undergo a thermally induced rearrangement to give silanes of the type R¹R²Si (CH₃)OEl(O)_xR _y ³ . The energetic (reaction enthalpy) and kinetic data (reaction order, enthalpy and entropy of activation) of this reaction were determined by means of differential scanning calorimetry. The results obtained are discussed in terms of mechanistic aspects.

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Zusammenfassung Silane des Typs R¹R²Si(H)CH₂OE10_xR _y ³ mit R=organyl; E1=C (x=y=1), S(x=2, y=1) oder P (x=1,y=2) zeigen eine thermisch induzierte Umlagerung zu Silanen des Typs R¹R²Si(CH₃)OE10_xR _y ³ . Energetische (Reaktionsenthalpie) und kinetische Daten (Reaktionsordnung, Aktivierungsenthalpie und -entropie) dieser Umlagerungsreaktion wurden mittels DSC ermittelt. Die mechanistischen Aspekte der Ergebnisse werden diskutiert.

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R¹R²Si(H)OE1(O)_xR _y ³ , R= , E1=C (x=y=1), S (x=2,y=1), P (x=1,y=2), R¹R²Si(CH₃)OE1(O)_xR _y ³ . ( ) ( , ) . .

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Uncertainty evaluation for the adiabatic temperature rise in isoperibol calorimetry

This article describes a practical approach for evaluating the uncertainty of results for determinations of the adiabatic (corrected) temperature rise in isoperibol calorimetry. The methodology is firmly based on the recommendations of the Guide to the expression of uncertainty in measurement (GUM). Although developed for a specific modification of the Regnault?CPfaundler method, the approach is sufficiently general to make it applicable to virtually any other scheme for the evaluation of temperature?Ctime curves in temperature-rise calorimetry.     

Design and Study of Gas Calorimeter for Absolute Measurements of the Combustion Heat of Natural Gas

A novel burning calorimeter design based on a heat pipe is presented. A circuit for automated control over operation of the proposed device is considered. The stability of the results is assessed. Several accessory parameters affecting the reproducibility of measurement results are evaluated. One of recent designs of the KTT-7 calorimeter, based on a thermosiphon, is analyzed. The uncertainty in heat power measurements on KTT-7 is evaluated.     

The baseline construction and its influence on the measurement of heat with differential scanning calorimeters

In DSC's the shape of the interpolated baseline under a peak is determined by a change in the heat capacity of the sample and the heat transfer characteristics between sample and temperature sensor. The interpolated baseline is constructed according to formal criteria, experimentally or analytically on the basis of physico-chemical assumptions on the change of the heat capacity during transition. By the example of the melting of ice this paper shows analytically on the basis of a simple calorimeter model and a synthetic measuring curve, and experimentally, that the uncertainty of the enthalpy determination depends in general on the type of baseline and is in the order of magnitude of the repeatability of the DSC's (±0.5%).

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Zusammenfassung In Differential-Scanning Kalorimetern wird die Gestalt der interpolierten Basislinie unter dem Peak durch eine Änderung der Wärmekapazität der Probe und die Wärmetransportcharakteristik zwischen Probe und Temperaturfühler bestimmt. Die interpolierte Basislinie wird nach formellen Kriterien konstruiert, experimentell oder analytisch auf der Grundlage von physikalisch-chemischen Annahmen für die Änderung der Wärmekapazität während des Überganges. Anhand des Beispieles schmelzendes Eis zeigt vorliegende Arbeit auf der Grundlage eines einfachen Kalorimetermodelles und einer synthetischen Meßkurve analytisch sowie experimentell, daß die Ungenauigkeit der Enthalpiebestimmung unter anderem von der Art der Basislinie abhängt und daß ihr Wert in die Größenordnung der Reproduzierbarkeit von Differential-Scanning Kalorimetern liegt (0.5%).

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Dedicated to Prof. Dr. H. J. Seifert on the occasion of his 60^th birthday     

Metrologically based procedures for the temperature, heat and heat flow rate calibration of DSC

Metrologically based measuring procedures and evaluation methods are recommended as guidance for practical temperature, heat and heat flow rate calibration of DSC instruments which are largely independent of instrumental, test and sample parameters. The relevant terms are defined, the measuring procedures and evaluation methods described, calibration materials and their characteristic data stated and guidance for the sample handling provided. Reference is made to three extended papers on calibration. The recommendations were developed by the working group Calibration of Scanning Calorimeters of the German Society of Thermal Analysis (GEFTA).Recommendation of the working group Calibration of scanning calorimeters of the Gesellschaft für Thermische Analyse e.V. (GEFTA)     

Method for Measuring the Combustion Heat of Natural Gas with Isothermal Calorimeter

Factors that are to be taken into account in developing a calorimetric technique are considered. The possible deviations of temperatures at the inlet and outlet of gases in the calorimeter from the nominal values are evaluated and an example of such estimate is given for the case of methane burning in batch and continuous operation modes. The necessity for taking into account the humidity of gases fed into the calorimeter burner is demonstrated. The proposed method was used to perform absolute measurements of the combustion heat of methane.     

Determination of an amount of gas by weighing under vacuum conditions and uncertainty analysis

For the determination of the calorific value of gases, a mass determination system for the gas burnt in the calorimeter was developed. Contrary to classical methods, which weigh the gas bottle offline, the method applied here weighs the amount of gas online and continuously. For this purpose, a commercial analytical balance with a readability of 10 µg across the entire weighing range of 205 g was modified for use under vacuum conditions.Since the gas bottle has to be permanently connected to the calorimeter by a steel capillary, an additional force is exerted to the balance. The influence of this force is investigated and determined by a special weighing procedure.In addition, an automatic calibration technique must be developed to ensure the traceability of the results to the national standard of mass. This allows an uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement (GUM) to be performed.This revised version was published online in November 2005 with corrections to the Cover Date.     

The pseudobinary phase diagram cis-/trans-azobenzene and the cis → trans isomerization in various states of aggregation

The pseudobinary phase diagram of trans-azobenzene and the thermally instable cis-isomer was examined by thermomicroscopical and DSC-measurements. Two eutectic temperatures have been determined (at 41.4 C and 36.7 C). The latter occurs only in the presence of a metastable polymorph of cis-azobenzene after quenching a melt, that contained as well cis-as trans-azobenzene, presumable under conditions of heterogeneous nucleation. In the melt and in solution the thermally induced cis-trans-isomerization follows first order kinetics. Below the eutectic temperature the rate is controlled by three-dimensional nuclei growth to reaction extent 0.20. Between the eutectic temperature and the melting points of product respectively educt the kinetics follow the PROUT-TOMPKINS law. The reason is the increasing isomerization rate with increasing fraction of melt.

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Zusammenfassung Das pseudobinäre Phasendiagramm von trans-Azobenzol und des thermisch instabilen cis-Isomeren wurde mit Hilfe thermomikroskopischer und DSC-Messungen untersucht. Es wurden zwei Eutektika bestimmt /bei 41.4 °C und 36.7 °C/, wobei das zweite Eutektikum nur in Anwesenheit einer metastabilen Modifikation von cis-Azobenzol nach Abschrecken einer Schmelze, die gleichzeitig eis- und trans-Azobenzol enthielt, vermutlich bei heterogener Keimbildung entstand. In der Schmelze und in Lösung verläuft die thermische cis-trans-Isomerisierung nach einem Geschwindigkeitsgesetz erster Ordnung. Unterhalb der eutektischen Temperatur ist die Festkörperreaktion bis zu Umsätzen von 0.20 durch dreidimensionales Keimwachstum bestimmt. Zwischen eutektischer Temperatur und Schmelzpunkt lässt sich die Reaktion durch die Gleichung von PROUT-TOMPKINS beschreiben, was auf die zunehmende Isomerisierungsgeschwindigkeit mit zunehmende Anteil Schmelze zurückzuführen ist.

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- -. 41,4 36,7°. - , -, -, , , . - . 0,20. , -. .

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We thank the Deutsche Forschungsgemeinschaft for financial support.