Calorimetric calibration methods

Calorimetric methods are infrequently used for routine quality control purposes because of their non-specific nature and relatively slow speed. However, data from calorimetry experiments are commonly presented in applications for new product licenses and in support of patent applications. To ensure the integrity of all calorimetry data, normal procedures for good laboratory practices, standard operating procedures, appropriate calibration methods, and regular instrument servicing are necessary. The use of DSC for the measurement of transition temperatures and sample purity is described in the United States Pharmacopoeia, and standard procedures for DSC analyses are also suggested by the ASTM (100 Barr Harbor Dr., West Conshohocken, Pennsylvania 19428). 

The use of the Joule heat effect from a resistance element on passage of electric charge is the preferable method for an absolute calorimetric calibration. It certainly requires a special setup of the measuring head enabling... 

A heat flow calorimeter and the drop calorimetric method were used by Connor, Skinner, and Virmani to investigate the thermal decomposition of Cr(CO)6 at 514 K (the calibration was made with iodine as described above) [164], The only peak observed corresponded to an endothermic process ... 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

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. 

The idea of calorimetry is based on the chemical reaction characteristic of molecules. The calorimetry method does not allow absolute measurements, as is the case, for example, with volumetric methods. The results given by unknown compounds must be compared with the calibration curve prepared from known amounts of pure standard compounds under the same conditions. In practical laboratory work there are very different applications of this method, because there is no general rule for reporting results of calorimetric determinations. A conventional spectrophotometry is used with a calorimeter. The limitations of many calometric procedures lie in the chemical reactions upon which these procedures are based rather than upon the instruments available . This method was first adapted for quinolizidine alkaloid analysis in 1940 by Prudhomme, and subsequently used and developed by many authors. In particular, a calorimetric microdetermination of lupine and sparteine was developed in 1957. The micromethod depends upon the reaction between the alkaloid bases and methyl range in chloroform. 

The reliability of the calorimetric results for these reactive organometallic compounds was further substantiated by the observations that the results were all quite reproducible ( 0.1 kcal/mole) and that the results obtained do not depend on (1) the source or method of purification of the base or the solvent (2) the source or method of purification of the alkyllithium and (3) the sodium content of the lithium metal used to prepare the alkyllithiums. Furthermore, the calorimetric equipment was regularly calibrated with internationally accepted standards for calorimetry. 

We must therefore rule out the simplifying asumption (initially made by the pioneers of the method, Chessick et al [6] and Taylor [7]) that the areal enthalpy of immersion is somewhat independent from the chemical nature of the adsorbent. This means that a calibration with a non-porous sample is needed for each type of surface. This is not too much of a problem since the duration of a complete calorimetric experiment (after preliminary weighing and outgassing of the sample) is ca 2 hours. 

Calorimetric methods and ionization chambers were used for calibration... 

Blood and urine pH can be measured easily by means of a calibrated glass electrode, whereas pH measurement inside the metabolizing cells is not easily accomplished. Techniques for estimating intracellular pH include glass electrode measurements on homogenates, calorimetric or fluorometric analysis of intracellular distribution of indicator dyes, and microelectrode methods. 

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

These authors report the enthalpies of formation of RX3 compounds (R = Y, Th or U X = Ga, In, n, Sn or Pb) as determined by dynamic differential calorimetry (integration of DTA peaks, with a calibration from elements and compounds of known heats of fusion). These studies follow previous studies by the same group on rare earth intermetal-lic compounds having the same general formula. The calorimetric method used is described in one of these earher publications [1973PAL]. 

The most common physical methods applied in the dosimetry practice of ionizing radiation are calorimetry and ionization methods. Calorimetry is a primary standard method in dosimetry used both to measure dose rate in various radiation fields and to calibrate standard and routine dosimeters. Ionization chambers are discussed in O Chap. 48, Vol. 5, thus only calorimetric systems wfll be shortly described here, with particular respect to those applied mainly in radiation processing. 

Calorimetry is an absolute method of dosimetry, since almost all absorbed radiation energy is converted into heat that can be readily measured as a temperature rise of the calorimetric body. Calorimeters that are used as primary dosimeters do not require calibration and, ideally, their response is independent of dose rate, radiation characteristics, and environmental factors (Domen 1987). The calorimeters that are used in radiation processing for the measurement of absorbed dose are relatively simple and need calibration (ISO/ASTM 2003b). The use of calorimeters as primary standard dosimeters for electron beam irradiation is described by McEwen and Dusatuoy (2009) and for gamma irradiation by Seuntjens and Duane (2009). 

Calorimeters measure the heat produced by a sample of nuclear material, whereby the heat originates primarily from the a-decay of the isotopes making up the nuclear material. Calorimeters have traditionally been fabricated using a sensor of nickel wire wound around a measurement chamber (Bracken et al. 2002). The nickel wire provides a temperature-sensitive resistance leading to highly accurate and precise electrical measurements of the power produced by a sample. Calorimeters are calibrated with Pu heat somces or plutonium samples with known mass and isotopic composition. Calorimetric assay is the most precise and accmate NDA measmement method for plutonium products (>100 grams). 

The acoustic pressure can be measured through the effects produced in the propagation liquid, namely (1) heating, measured calorimetrically and (2) the action of radiation pressure, determined by measuring the force exerted on a surface (the radiometric method). While the former allows the measurement of the mechanical power of the vibrating source, the latter is associated with the acoustic power transmitted by the acoustic radiation. The acoustic pressure can also be determined using calibrated hydrophones (the acoustic method). 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

Obviously, the experimental description of the device should also contain information such as 1) the purpose of the instmment (combustion, heat of mixing, heat capacity, sublimation, etc.) 2) the principles and design of the calorimeter proper, including the ranges of temperature and pressure in which measurements can be performed 3) the measured quantity and measuring device 4) the static and dynamic properties of the calorimeter the calibration mode and the methods of measurement and determination of heat effects 5) the operational characteristics of the calorimetric device, the sensitivity noise level, the method of calibration, the accuracy, etc. 6) a description of the experimental procedure used in the calibration and the actual measurements. 

When the corrected temperature rise method was applied for the calibration of the calorimetric bomb, it was established [22-24, 284] that the calculated heat capacity of the device was not equal to the sum of the heat capacities of the calorimeter parts. 

A Channel Calibration of the power level monitoring channels by the calorimetric method was last performed on July 31, 2000. Which one of the following dates is the latest the maintenance may be performed again without exceeding a Technical Specifications requirement ... 

In order to establish the correct functioning of the microcalorimeter, which is then connected to the volumetric adsorption unit, the sensitivity is evaluated determining the calorimeter constant. The calibration constant reports the voltage generated by the calorimeter when a heat flow is emitted from inside the micro-calorimetric cell. There are two methods to determine the calibration constant K by application of electric power and by the stationary method [9, 10]. 

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