Certification of Ga, In, Sn and Bi as temperature and heat calibration materials for differential scanning calorimetry (DSC)
Stefan M. Sarge, Hans-Walter Krupke
Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany
To ensure uniformity in metrology, modern quality management systems – e.g. systems according to international standards of the ISO 9000 series (Basic standards on quality management and quality assurance), ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories) or European standards of the EN 45000 series (general criteria for the operation of testing laboratories) – require, as far as possible, the traceability of all measurements to national or international measurement standards (primary standards).
Differential calorimeters must be calibrated by means of certified reference materials to realize measurement traceability. The growing demand for certified (and thus traceable) reference materials is covered by existing commercially available materials which can be purchased from PTB (Physikalisch-Technische Bundesanstalt, Germany), NIST (National Institute of Standards and Technology, USA) and LGC (Laboratory of the Government Chemist, UK). Reference materials are, however, only one of the requirements needed to meet international standards in quality assurance. Accepted procedures to realize the certified values (especially with dynamic instruments such as differential scanning calorimeters) are just as important. The Deutsche Gesellschaft für Thermische Analyse (German Society of Thermal Analysis, GEFTA) has therefore elaborated three recommendations for the temperature, heat and heat flow calibration of dynamic calorimeters which not only contain recommended values for calibration materials, but also detailed specifications on how to perform the procedure.
Four metals (Ga, In, Sn, Bi) are offered by PTB as calibration materials for temperature and heat calibration in the medium temperature range (0 °C to 300 °C). The recommended procedure ensures that the influence of the heat transfer properties of these metals is negligible if the calibration results are used for measurements with materials such as polymers.
The materials used have a nominal purity of 99.9999 % and are available in the form of granulate. Certification was carried out under similar conditions to differential scanning calorimetry (DSC), i.e. in air and with sample weights of a few 100 mg.
To determine the melting temperature, platinum resistance thermometers are used which are directly traceable to the currently valid temperature scale (International Temperature Scale of 1990, ITS-90).
The heat of fusion is determined with a commercial Calvet-type calorimeter which is equipped with a special compensation heater. The energy necessary for the melting of the sample is mainly provided by Joule heat. Heat flows resulting from incomplete compensation are measured by means of the thermopile of the calorimeter. The heat of fusion of the sample is the sum of the electric energy (which can be measured with high accuracy) and the integrated residual heat flow rate.
If all relevant influence properties are taken into account, the relative uncertainty of the heat of fusion determined in this way amounts to approx. 4 × 10-3.
References [1] G. W. H. Höhne et al., Thermochim. Acta 160 (1990) 1 - 12 [2] H. K. Cammenga et al., Thermochim. Acta 219 (1993) 333 - 342 [3] S. M. Sarge et al., Thermochim. Acta 247 (1994) 129 - 168
(Paper presented at 11th ICTAC, Philadelphia, USA, 12.-16.08.96)