Enthalpy calibration material

Proper calibration of the DSC instruments is crucial. The basis of the enthalpy calibration is generally the enthalpy of fusion of a standard material [21,22], but electrical calibration is an alternative. A resistor is placed in or attached to the calorimeter cell and heat peaks are produced by electrical means just before and after a comparable effect caused by the sample. The different heat transfer conditions during calibration and measurement put limits on the improvement. DSCs are usually limited to temperatures from liquid nitrogen to 873 K, but recent instrumentation with maximum temperatures close to 1800 K is now commercially available. The accuracy of these instruments depends heavily on the instrumentation, on the calibration procedures, on the type of measurements to be performed, on the temperature regime and on the... 

With this possibility, however, the need arises for standards which are suitable for simultaneous calibration of enthalpy and temperature measurements. In the present paper a set of 12 calibration materials is proposed covering most of the temperature range of the employed DSC cell. They were selected from 25 inorganic compounds and metals with 32 polymorphic transitions or melting points. Various sources of error were investigated. To get a good statistical mean every experiment was repeated 3 to 9 times, always with newly prepared samples. 

In the previous paper a number of potential calibration materials were investigated and various types of error (sample size, heating rate, background correction, enthalpy size) were considered. As a result the materials in Table 1 were proposed for calorimetric calibration. 

Reference material sets which are certified by the International Confederation for Thermal Analysis and Calorimetry (ICTAC) are available through the US National Institute of Standards and Testing (NIST), and are listed in Appendix 2.2. High-purity metals and organic compounds including polymers have been certified. If the standard reference material must be dispensed with a syringe into the sample vessel (for example cyclohexane), care must be taken to ensure that only one droplet is formed in the sample vessel. Multiple transition peaks will be observed if there is more than one droplet present. The transition temperatures listed in Appendix 2.2 are the statistical mean values of measurements made in a number of laboratories and institutes. The ICTAC reference materials are certified for temperature calibration only and not for enthalpy calibration. The reference temperatures in Appendix 2.1 should be used if very accurate calibration of the instrument is required. In order to determine the heat capacity Cp ) of a sample, sapphire (a-alumina, AI2 O3) is used as a standard reference material. The Cp of... 

Calibration of TA instrumentation and development of standards for calibration continue to be administered by ICTAC in conjunction with ASTM. The standardization Committee of ICTAC has certified a range of materials for temperature calibration of TA systems, and in addition, standards for calibration of mass (known as Class M Standards ) are available for this purpose. A range of certified reference materials are available for enthalpy calibration in DSC. Temperature calibration for TMA and dynamic mechanical analysis (DMA) is effected by using disks of pure metals (silver, aluminum, and tin) separated by alumina disks. Load or force calibration for DMA is a complex process involving the use of calibrated weights. Temperature calibration for DETA is effected by measuring the melting transition of benzoic acid and dielectric calibration is... 

Table 1.5 Melting points and enthalpies enthalpy calibration reference materials [53]... 

Example Determination of the Heat of Fusion by Means of DSC Let us assume that the task is to determine the heat of fusion of an unknown sample by means of differential scanning calorimetry. The calorimeter is first calibrated by using a certified sample of indium of mass mdb = 10.12 mg as the calibration material. The certificate states an enthalpy of fusion of Afush<-ib = (28.64 0.06) Jg. Four measurements are performed and give the following results (peak areas 2 <-ii,) 284.15, 281.39, 263.49, and 276.04m). The mean value is Adb = 276.27 mj. The calibration factor Kq of the instrument is calculated according to the following equation ... 

Most modern equipment has the temperature measuring device located very close to the samples. The instrument manufacturers will supply suitable reference materials on request with appropriate certification. Table IV sets out some suitable materials that can and have been used as temperature standards for calibration purposes. Directly these are most useful for calibrating DTA or DSC units. In DSC units knowledge of the enthalphy changes may also be required. Table V sets out the enthalpy of fusion for selected materials. Again, most DSC instrument manufacturers provide materials with their equipment for this purpose. Indium is used as a calibration material in many DSC units, and other systems undergoing phase changes can be used in a similar way. 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

Tables 1 and 2 are examples of calibration of the temperature and of the calorimetric response of PE-DSC-7 instruments by using different materials. Very important for pharmaceutical industry is the confidence of the laboratory that delivers the reference. Since the heating rate may have an influence on the data, it is recommended to compare the melting point and the melting enthalpy of organic standards, additionally to indium, at different heating rates covering the measurement range. For very accurate determinations, it is recommended to use standards with a melting point in the range of the considered temperature in a series of measurements.

We adopt 652 3 k as the transition temperature of stoichiometric NiS from rhombohedral (B) to hexagonal (a) form base on the phase diagram of Kullerud and Yund (8). The temperature of this transition is very dependent on the exact stoichiometry of the material (8 ). The transition enthalpy has been measured via a DTA technique by Conard et al. (7) and we adopt their value of 1.54 0.1 kcal mol". This is considerably higher than an older value of 0.63 kcal mol measured by Biltz et al. (9, 5) but should be much more accurate due to the calibration technique used. Mah and Pankrantz ( 4) estimated 0.7 kcal mol. This transition enthalpy cannot be measured by drop calorimetry since the a form does not revert to the B form on cooling. 

DSC) is a method that measures the difference in energy (heat flux or heat flow) between a reference and a sample. The result of a DSC analysis is a thermogram, a plot of temperature difference versus temperature and represents the enthalpies of various processes occurring in the heated sample, such as solvent loss, crystallization, polymorphism, and chemical reactions. Additionally, DSC is an absolute method and with proper calibration can be used to accurately measure the melting point and purity of the reference material.52,53... 

TGA has been used with some success to determine the extent of sublimation of volatile solid materials. TGA-DSC can be applied to determine the corresponding sublimation enthalpies, which are usually difficult to determine by conventional methods. The determination of sublimation enthalpies is notoriously difficult since with most volatile solids, the effusion process is rarely smooth and continuous and thus the use of STA for this purpose is a most welcome adjunct to many less reliable analytical techniques which rely on an ideal sublimation process. For sublimation studies, the sample is sealed in a container with a small pin-hole in the lid. The TGA curve shows a progressive mass loss until the sample is exhausted. The corresponding DSC curve shows a deviation from the base-line equivalent to the sublimation enthalpy which can be correlated with the rate of mass loss. Calibration of the... 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

From Equation 1.1, it follows that the temperature difference between sample and reference is a measure of the difference in heat flow due to the presence of the sample in one of the crucibles, provided that the furnace and heat paths are truly symmetrical. Consequently, this differential heat flow is a measure of the properties of the sample, with all other influences (heat adsorption by the crucible, heat losses through convection, etc.) having been eliminated by use of the comparison with the reference. The AT signal requires calibration to provide a heat flow as a function of temperature, and this is usually carried out by use of standards that are usually pure metals with known enthalpies of melting and materials with known heat capacities (see Section 2.4 in Chapter 2). 

Heat capacities as function of temp can be determined directly from the output serial from weighed specimens without the need of a separate calibration (see Sect 5.2.3). Enthalpies are obtained from the integrated areas under the DSC curve, as can the fraction of sample reacted which is proportional to the fraction of the area generated with time or temp. Likewise, reaction temps, initiation temps and the role of catalysts on mixts can be observed directly. The use of DSC for kinetic studies of reactive materials is limited by the tendency of such materials to undergo sublimation or melting. Suitable corrections can be applied to the data by the simultaneous use of TG. The role of impurities and of decompn products in promoting autocatalysis can be investigated by the use of reactive atms or reduced pressures as illustrated by the use of open (perforated) and closed sample pans (Ref 51)... 

The DSC peak area must be calibrated for enthalpy measurements. The same types of high purity metals and salts from NIST discussed for calibration of DTA equipment are also used to calibrate DSC instmments. As an example, NIST SRM 2232 is a 1 g piece of high purity indium metal for calibration of DSC and DTA equipment. The indium SRM is certified to have a temperature of fusion equal to 156.5985°C + 0.00034°C and a certihed enthalpy of fusion equal to 28.51 + 0.19 J/g. NIST offers a range of similar standards. These materials and their certified values can be found on the NIST website at www.nist.gov. Government standards organizations in other countries offer similar reference materials. 

It is necessary to know the heat capacity of the system to calculate the enthalpy of the reaction. This can be determined by measurement of the temperature increment produced by the reaction of a reference material, or by electrical calibration supplying a determined quantity of electrical power from a heater during a known time. There are other types of reaction calorimeters [50], such as adiabatic calorimeters, where the jacket is maintained at the same temperature as the reaction vessel during the whole experiment, and no corrections need to be applied to the observed temperature rise [51]. 

Temperature calibration is achieved using standard reference materials whose transition temperatures are well characterized (Appendices 2.1 and 2.2) and in the same temperature range as the transition in the sample. The transition temperature can be determined by DTA, but the enthalpy of transition is difficult to measure because of non-uniform temperature gradients in the sample due to the strueture of the sample holder, which are difficult to quantify. This type of DTA instrument is rarely used as an independent apparatus and is generally coupled to another analytical instrument for simultaneous measurement of the phase transitions of metals and inorganic substances at temperatures greater than 1300 K. 

There are two principal methods for determining A//. The most common method is to calibrate the energy scale of the instrument with standard reference materials whose enthalpies of melting are well characterized and whose melting... 

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