Stefan M. Sarge

The caloric calibration of scanning calorimeters

Abstract The present recommendation of the GEFTA working group “Calibration of Scanning Calorimeters” allows a precise heat and heat flow rate calibration of scanning calorimeters, largely independent of instrumental, sample-related and experimental parameters. Electric energy, electric power, heats of transition and heat capacities of suitable calibration substances are used for calibration. The measuring method, measuring and evaluation procedure, calibration materials, significant influencing factors, sources of error and detailed examples are presented for these calibration methods. Besides specific problems of heat measurement (interpolation of the baseline for the peak area determination) and heat capacity measurement (interpolation between initial and final isotherms, determination of the true heating rate of the sample, thermal lag of the sample), general aspects (thermodynamic fundamentals, difference between heat and heat flow rate calibration factor, weighing procedure) are also discussed.

The temperature calibration of scanning calorimeters: Part 2. Calibration substances

The recommendation for temperature calibration consists of two parts. Part 1 (see section 2.3) presented a correct method for the calibration of scanning calorimeters (DSC) and of instruments for differential thermal analysis (DTA), irrespective of the instrument type. The present paper recommends calibration substances for the range 120-1350 K. Sections 2 and 3 indicate the documents to be taken into account and define the most important terms used. Section 4 consists of general requirements to be met by the calibration substances, a list of the substances and explanations with respect to their use. Basically, such materials have been selected as calibration substances which define fixed points of the International Temperature Scale of 1990 (ITS-go). Added to these are fixed point materials from the previously valid International Practical Temperature Scale of 1968 (IPTS-68, see References in Table 1). Moreover, two substances are recommended for temperature ranges for which no suitable fixed point material exists, and the respective uncertainty of measurement is stated.

Calibration of differential scanning calorimeters using ΔH-/T- and Cp -standards: extended application to two instruments

Abstract This paper introduces some organic substances which might be suitable for simultaneous temperature and enthalpic calibration of differential heat-flux-calorimeters. The results are compared with values obtained with other well-accepted reference materials and methods. The influences of sample and instrumental properties are discussed.

The phase transformation of caffeine : investigation by dynamic X-ray diffraction and emanation thermal analysis

Abstract The solid-solid phase transformation of caffeine has been studied with time- and temperature-resolved X-ray powder diffractometry (TXRD) and with emanation thermal analysis (ETA). The phase transformation of caffeine at 141°C has been followed by heating a sample of the low-temperature phase. After cooling again, the high-temperature phase converts only slowly back into the low-temperature phase. This transformation needs weeks or months at room temperature to complete. With TXRD, it was possible to study this slow transformation in situ at elevated temperatures. A nucleation-controlled transformation mechanism appears likely.

Calorimetric determination of purity by simulation of dsc curves

This paper presents further results of the purity determinations by simulated DSC curves. The instrumental parameters of a heat-flux DSC are now determined at two temperatures whereby the accuracy of the results is improved. In addition the calorimeter model is adapted to another calorimeter. The method also allows a more exact determination of fusion temperature.

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

Silanes of the type R1R2Si(H)CH2OEl(O)xRy3 [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)xRy3. 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.ZusammenfassungSilane des Typs R1R2Si(H)CH2OE10xRy3 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)OE10xRy3. Energetische (Reaktionsenthalpie) und kinetische Daten (Reaktionsordnung, Aktivierungsenthalpie und -entropie) dieser Umlagerungsreaktion wurden mittels DSC ermittelt. Die mechanistischen Aspekte der Ergebnisse werden diskutiert.РЕжУМЕсИлАНы тИпА R1R2Si(H)OE1(O)xRy3, гДЕ R=ОРгАНИЧЕскИИ РАДИкАл, E1=C (x=y=1), S (x=2,y=1), P (x=1,y=2), пОДВЕРгАУтсь т ЕРМИЧЕскИ НАВЕДЕННОМУ пРЕВРАЩ ЕНИУ с ОБРАжОВАНИЕМ сИлАНОВ тИпА R1R2Si(CH3)OE1(O)xRy3. с п ОМОЩьУ Дск БылИ ОпРЕДЕлЕНы ЁНЕРгЕтИ ЧЕскИЕ (ЁНтАльпИь РЕАкцИИ) И кИНЕтИЧЕск ИЕ пАРАМЕтРы (пОРьДОк РЕАкцИИ, ЁНтАльпИь И ЁНтРОпИь АктИВАцИИ) ЁтОИ РЕАкцИИ. пОлУЧЕН НыЕ РЕжУльтАты ОБсУж ДЕНы НА ОсНОВЕ МЕхАНИстИЧЕс кИх пРЕДстАВлЕНИИ.

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.ZusammenfassungDas 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.РезюмеМетодой термомикрос крпии е ДСК изучена псевдобинарная фазовая диаграмма тр анс-азобензола и терм ически нестабильного цис-изомера. Определе ны две температурные эвтектики при 41,4 и 36,7°. Последняя наблюд ается только в присут ствии метастабильно го полиморфа цис-азобен зола после быстрого охлаждения расплава, содержащег о как цис-, так и транс-азобензол, в образующегося, веро ятно, в условиях гетер огенного образования центров кристаллиза ции. В расплаве в в раст воре термически наведенная цис-тране изомеризация подчин яется реакции первого порядка. Ниже температуры эвтекти ки скорость реакции контролируется трех размерным ростом зар одышей кристаллов со степенью превращени я равной 0,20. В интервале между температурой эвтектики и точками п лавления реакция сле дует кинетике, подчиняющейся закон у Праут-Томпкинса. Это является причиной увеличения скорости изомеризации с увели чением доли расплава.

A detailed study of the mechanism of an organic solid state reaction

Abstract The thermal cis → trans-isomerization of azobenzene was examined in various states of aggregation. Below the eutectic temperature of educt and product the reaction rate is controlled by two-dimensional nuclei growth up to a reaction extent 0.15. Around an extent between 0.15 and 0.20 a distinct change in mechanism occurs. The following region can be described by a three-dimesional diffusion law. The evaluation of the data was done by fitting master curves and by a modified Weibull distribution. Above the eutectic temperature the reaction kinetics follow the Prout-Tompkins law caused by liquid eutectic layers. In solution and in the melt the kinetics are first order. In addition the phase diagram of the pseudobinary system cis-/trans-azobenzene was determined.