Heiko K. Cammenga

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.

Liquid-helium temperature long-path infrared spectroscopy of molecular clusters and supercooled molecules

Collisional cooling and supersonic jet expansion both allow us to perform infrared spectroscopy of supercooled molecules and atomic and molecular clusters. Collisional cooling has the advantage of higher sensitivity per molecule and enables working in thermal equilibrium. A new powerful method of collisional cooling is presented in this article. It is based on a cooling cell with integrated temperature-invariant White optics and pulsed or continuous sample-gas inlet. The system can be cooled with liquid nitrogen or liquid helium and operated at gas pressures between <10−5 and 13 bar. Temperatures range from 4.2 to 400 K and can be adjusted to an accuracy of ±0.2 K over most of the useable range. A three-zone heating design allows homogeneous or inhomogeneous temperature distributions. Optical path lengths can be selected up to values of 20 m for Fourier transform infrared (FTIR) and 40 m for laser operation. The cell axis is vertical, so optical windows are at room temperature. Diffusive trapping shields ...

Temperature, heat and heat flow rate calibration of scanning calorimeters in the cooling mode

Abstract The current publication continues the series of recommendations of the ‘Calibration’ Working Group of the German Society for Thermal Analysis (GEFTA) for temperature, heat and heat flow rate calibration of scanning calorimeters. It deals with calibration in the cooling mode. The procedures to be applied are essentially identical to those applied in the heating mode. Due to the general occurrence of supercooling for first-order phase transitions during cooling, liquid crystals and substances with higher-order phase transitions are recommended for temperature calibration. Substances with weak supercooling or substances for which the temperature dependence of the transformation enthalpy is known are recommended for heat calibration. The thermodynamic fundamentals relevant to the temperature dependence of phase transition enthalpies and phase transition temperatures are discussed. Detailed examples make it easy to follow the recommendations.

Composition, properties, stability and thermal dehydration of crystalline caffeine hydrate

Abstract It has been shown by gravimetric measurements that caffeine hydrate does not exist as a monohydrate but has a water content corresponding to a 5 6 or 4 5 hydrate. This result is supported by density determinations of crystals. the stability-point is found to be D = 51.5 ± 0.7 °C. The enthalpy of dehydration is determined as well directly by DSC (6.8 ± 0.5 kJ mol−1) as from the difference of integral enthalpies of solution of anhydrous caffeine and caffeine hydrate (7.06 ± 0.14 kJ mol−1). The results and their consequences are discussed.

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.

Thermal behaviour of isomalt

Abstract The technological properties of isomalt and its enthalpy of solution and glass transition are described. The transformation temperature (glassy state → subcooled liquid) T g is usually readily obtainable by differential scanning calorimetry (DSC), which detects a characteristic change in the specific heat capacity at T g .

Thermochemical analysis of solvate complexes of silver dimesylaminide

Abstract Two new solvate complexes of silver dimesylaminide with water and acetonitrile were analyzed using several thermoanalytic and calorimetric methods (thermogravimetry, differential scanning calorimetry, thermooptical analysis, solution calorimetry). The hydrate AgN (SO 2 CH 3 ) 2 · 1 4 H 2 O loses its crystal water above 120°C. The dehydratation is overlapped by a phase transition which occurs at 174°C. The acetonitrile complex AgN(SO 2 CH 3 ) 2 ·2 CH 3 CN decomposes between −15 and 95°C. The existence of the hydrate complex is remarkable, because silver salts usually do not form hydrates. For this salt, the coordination of the water molecules to silver ions has been proved by X-ray structure determination.