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Thermochimica Acta 331 (1999) 93–204 Reference materials for calorimetry and differential thermal analysis R. Sabbah (France, Chairman) ∗, An Xu-wu (China), J.S. Chickos (USA), M.L. Planas Leitão (Portugal), M.V. Roux (Spain), L.A. Torres (México) Centre de Thermodynamique et de Microcalorimétric du CNRS, 26, rue du 141ème R.I.A., 13331 Marseille Cedex 03, France 1. Introduction signal when a known quantity of energy is generated within the system. Experimental conditions of calibra- This publication is the result of the efforts of the tion and measurement should be matched as closely ICTAC working group “thermochemistry” 1 during as possible: not only the quantity of energy to be mea- 1997–1998. It deals with reference materials (in abbre- sured must be similar but the site and kinetics of ge- viated form: RM) for calorimetry and differential ther- neration and the temperature range (or the temperature mal analysis. It represents the updated version of two of an isothermal system) should be as close as previous documents produced by the IUPAC Commis- possible in both calibration and measurement sion “Physicochemical Measurements and Standards”: experiments. the first was published in Pure and Applied Chemistry The energy of calibration should ideally be gene- in 1974 [1] and the second in the book entitled “Re- rated electrically but when the nature of the calorime- commended Reference Materials for the Realization tric system makes this difficult to achieve, then a re- of Physicochemical Properties” [2]. ference material with a well established value of the Calorimetry and differential thermal analysis are ap- property under study, preferably with a knowledge of plicable to a wide range of scientific and technological the uncertainties of measurement, may be used as a research fields involving physical, chemical and bio- calibrant. The word “material” is intended to include logical processes. Calorimetry usually yields highly single substances, mixtures and devices. Whichever reproducible results which may, however, be inaccu- method of calibration is employed, reference mate- rate because of faulty calibration of the measurement rials are needed to check the results and to ensure system. that the calibrated calorimeter is applicable to the Calibration is a fundamental requirement for every particular type of material, reaction, or process for thermoanalytical study. It requires the establishment which it is intended and is not subject to systematic of a quantitatively defined relationship between the errors. value indicated by the measuring instrument and the During the past 50 years reference materials for correct value. The calibration of a modern calorime- calorimetry (mainly bomb, reaction and heat capa- ter is achieved by the quantification of the produced city) and for differential thermal analysis have been developed by national standardizing laboratories such ∗ Tel.: +33-4-91-28-20-50; fax: +33-4-91-50-38-29; e-mail: as the National Institute of Science and Technology [email protected] in the USA and through projects sponsored by bodies 1 Membership of the ICTAC working group “thermochemistry” such as IUPAC, ICTAC, the Calorimetry Conference for the period 1996–1998 was as follows: R. Sabbah (France, chairman); An Xu-wu (China); J.S. Chickos (USA); M.L.P. Leitão (USA), the Association Française de Calorimétrie (Portugal); M.V. Roux (Spain); L.A. Torres (Mexico)´ et d’Analyse Thermique (AFCAT, France), the 0040-6031/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S0040-6031(99)00009-X 94 R. Sabbah et al. / Thermochimica Acta 331 (1999) 93–204 Gesellschaft für Thermische Analyse (GEFTA, Ger- The past three decades have seen a decline in acti- many) and others. Ideally, a reference material should vity in the traditional areas of precise calorimetry. Few have been studied in several specialized laboratories additional measurements have become available with and its properties established by measurements in- the high accuracy necessary for reference materials, volving several techniques and critical evaluation of although some new data have been published on exis- the corresponding results. ting substances. There are still gaps in the availabi- Reference materials recommended in this publica- lity of reference materials in such important fields tion concern heat-capacity measurements, enthalpies of calorimetry as the measurement of heat capacities of phase changes (fusion, transition, sublimation and of liquids and gases at high pressures, the measure- vaporization), enthalpies of reaction and related pro- ment of enthalpies of solid + solid, solid + liquid and cesses (solid + solid, solid + liquid, liquid + liquid and liquid + liquid processes especially those concerned gas + gas processes) and enthalpies of combustion with binary systems at high temperatures and pres- (combustion of solids in oxygen, combustion of li- sures and ternary systems. quids in oxygen, combustion of solids in fluorine). The introduction of each section is not intended to In each of these sections the materials are classified be comprehensive but reflects the present state of the as primary, secondary and tertiary reference materi- art for the measurement under consideration. Many als and a definition for each of them is given. Some of the recommendations include a brief outline of the materials have their properties certified by, or their experimental methods available and the advantages certification is traceable to, an authoritative national and disadvantages of each method. Most of the re- or international organization, agency or laboratory. commendations include the following information for These materials are called certified reference mate- each recommended reference material: physical prop- rials. When available, these materials have advantages erty, units, range of variables, physical state(s) within over uncertified reference materials because source, the range, apparatus used, recommended value, the lot, purity and uncertainties in the property value are names of contributors (to the previous and to this defined. version), intended usage, source of supply, methods A reference material should satisfy the following of preparation, pertinent physicochemical data and requirements: it must be easily obtainable in a pure references. state, quite stable, non-hygroscopic, non-volatile, ea- The recommendations given in this publication are sily brought into a suitable form for the measurement based on information from the literature up to the and physiologically harmless. In addition, reference end of 1997, from correspondence with scientists in materials must not react with the instrument material, each discipline and from comments solicited from the surrounding atmosphere and photoreactions must metrological bodies, national standards laborato- not occur. ries, manufacturers and suppliers. The recommended In addition to updating existing reference materials values in this document are the result of a careful found in previous editions on this subject, this publi- evaluation by the contributors. The selected com- cation identifies new reference materials. A total of 58 pounds and measurements reflect their best judge- pure substances are proposed as reference materials ment. The temperatures in this work are reported for the measurement of enthalpies of phase change in terms of the ITS-90 [3] unless otherwise stated. (29 for solid-solid or solid-liquid phase changes, 11 Molar masses of the elements are taken from the for solid-vapor phase changes and 18 for liquid-vapor 1993 report of the IUPAC Commission on Atomic phase changes) in comparison to the fourteen recom- Weights and Isotopic Abundances [4]. Uncertainties mended in the previous issue [2]. As reference mate- associated with each measurement represent twice the rials for the measurement of enthalpies of reaction and overall standard deviation of the mean unless stated related processes, the ethanol + water system is pro- otherwise. posed in addition to the materials previously published It should be observed that: [2]. Four new pure substances and three candidates are 1. the recommended materials, in most instances, proposed as reference materials for the measurement have not been checked independently by ICTAC; of enthalpies of solid combustion in oxygen. 2. the quality of material may change with time; R. Sabbah et al. / Thermochimica Acta 331 (1999) 93–204 95 3. the quoted sources of supply may not be exclusive at this moment, that tungsten be suggested as a refe- sources because no attempt has been made to seek rence material. out all possible alternatives; To perform heat-capacity measurements, ␣- 4. ICTAC does not guarantee any material that is aluminum oxide remains as the reference material recommended. having the widest range of application for calibration Finally, I want to thank IUPAC for the permission and testing of calorimeters in the temperature range given for the use in this publication of material from 10–2250 K. This compound is non-hygroscopic, non- the previous issue [2] and all my colleagues who have volatile, chemically stable, relatively cheap, available contributed to this work. Their names are reported in in large quantities in a very high purity state, and the preliminary pages of this publication in the section resistant under oxidizing, neutral or weakly-reducing “Contributors”. I am also indebted to Marie-Thérèse atmosphere. Being a thermal insulator, it is well suited Vialle for secretarial assistance. for use with
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