Equipment enthalpy calibration

Instrumental. The Mettler TA2000B thermal analysis system is equipped with an interface system, and Hewlett-Packard 9815 desk top calculator and 7225 plotter. Samples, weighing 5-10 mg, sealed in aluminum pans and under a nitrogen blanket, were heated in the calorimeter at a rate of 10 deg/min from -35 or 10 deg C to 180 deg C respectively, for specific heat and kinetic scans. Specific heat measurements were calibrated with alumina to an accuracy of 3%. Temperatures and enthalpies were calibrated with an Indium sample. The accuracies were .02 deg C and 2% (Indium 28.5 J/g), respectively. 

The disadvantage of the first approach is the requirement to know the quantities involved in the energy balance (e.g. molar reaction enthalpy, fluid heat capacity etc.). This disadvantage is eliminated by approaches based on calibration of the micro calorimeter. Moreover, calibration can compensate for small systematic errors produced by the micro calorimetric equipment, as well. In the second approach, however, the post-column analysis of reactant concentrations by an... 

Selivanova and Pakhorukov measured the integral heat of dissolution of H2Se03(cr) in a conventional calorimeter. The heat equivalent of the equipment was obtained by electrical calibration. The accuracy of the instrumentation was checked by a measurement of the enthalpy of dissolution of KCI to KCI(aq, 1 200). 

A total of five oxygen combustion experiments, using two calibrated equipments were carried out, with concordant results and led to a value of -(1224.2 + 5.8) kJ moT for the enthalpy of formation of Th02(cr) at 293 K. 

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. 

So far the comments concerning experimental aspects have been in general terms - this is the part of the chapter where they become more specific. Although calibration is primarily concerned with temperature and energy (or enthalpy) it is also the stage at which the practitioner gets to know the equipment. It is a good idea to start with preliminary experiments as a precursor to the detailed calibration. 

With computer equipped DSC instrviments it is possible to measure temperatures and enthalpies with one and the same run. For the practical application of this possibility, standards are needed which allow the simultaneous calibration for both types of measurements. Based on numerous T and AH measurements a set of 12 standards is proposed in this paper. 

Differential scanning calorimetry (DSC) investigations were performed on a Perkin-Elmer DSC-2 apparatus equipped with scanning autozero. Polymer samples and inserts (8-10 mg) and equivalent amounts of pilocarpine salts (ca. 1.0 mg) were analyzed at heating-cooling rates of 20 /min under dry nitrogen flow. Indium standards were employed for temperature calibration and enthalpy change evaluation. 

Basically, there are three approaches to peak area measurement (a) the cut and weigh method, which is accurate to within 5% (b) the planimeter method, which is accurate to within 1% (c) the integrator method, which is more accurate than method (b). It should be noted that when computer-equipped apparatus is used, enthalpy calculations are carried out automatically if the apparatus is correctly calibrated using calibration reference materials. 

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. 

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