Calibration of calorimeter

J. Coops, R. S. Jessup, K. vanNes. Calibration of Calorimeters for Reactions in a Bomb at Constant Volume. In Experimental Thermochemistry, vol. 1 R D. Rossini, Ed. Interscience New York, 1956 chapter 3. 

It is shown that irreversible condensation of calibration gases due to trace amounts of Q+ hydrocarbons can be important to the calibration of calorimeters but is not likely to be significant in the calibration of devices... 

The International Union of Pure and Applied Chemistry publication Experimental Thermochemistry Measurement of Heats of Reaction is written by experts for experts, and deals chiefly with organic compounds. The volume opens with a chapter entitled General Principles of Modem Thermochemistry , by Rossini, taken mostly from a book by that author (1950) entitled Chemical Thermodynamics . Other chapters discuss the calibration of calorimeters for flame and bomb reactions, and the combustion of oxygen, nitrogen, sulphur, chlorine, bromine, and iodine compounds. Fifty pages are taken in the discussion of microcalorimetry of slow reactions. 

Electric heaters are usually applied for calibration of calorimeters, especially for heat-flow instruments equipped with thermopiles or Peltier sensors. But often a chemical calibration is more appropriate and matching the experimental conditions more closely. For this end Wadso and coworkers recommended the hydrolysis of triacetin in imidazole/acetic acid buffer with stable, long-lasting heat production rates between 7 and 90 pW/mL at 37 °C [142,143]. Some other possible reactions were cited and discussed in connection with the most important types of calonmetric vessels, batch forms as well as flow-through containers. 

There are two steps in the calculation. First, calibrate the calorimeter by calculating its heat capacity from the information on the first reaction, Cca) = qc, /AT. Second, use that value of Cc-1 to find the energy change of the neutralization reaction. For the second step, use the same equation rearranged to gcal = Cca AT, but with AT now the change in temperature observed during the reaction. Note that the calorimeter contains the same volume of liquid in both cases. Because dilute aqueous solutions have approximately the same heat capacities as pure water, assume that the heat capacity is the... 

No theory can possibly take into account the arrangement of a real heat-flow calorimeter in all its details. Theoretical models of heat-flow calorimeters, which are necessarily simplified versions of the actual instruments, will therefore be used in the following calculations. It must be remarked that because of the limitations of the theory, no absolute measurements can be made with a heat-flow calorimeter, nor with any calorimeter. It is possible, however, to compare successive measurements with precision. A calorimetric study necessarily involves the calibration of the calorimeter and, upon this operation, depends the accuracy of the whole series of measurements. 

Now, it is necessary to calibrate the calorimeter in order to analyze quantitatively the recorded thermograms and determine the amount of heat evolved by the interaction of a dose of gas with the adsorbent surface. The use of a standard substance or of a standard reaction is certainly the most simple and reliable method, though indirect, for calibrating a calorimeter, since it does not require any modification of the inner cell arrangement. [For a recent review on calibration procedures, see 72).3 No standard adsorbent-adsorbate system has been defined, however, and the direct electrical calibration must therefore be used. It should be remarked, moreover, that the comparison of the experimental heat of a catalytic reaction with the known change of enthalpy associated with the reaction at the same temperature provides, in some favorable cases, a direct control of the electrical calibration (see Section VII.C). 

It is clear that the calibration of a calorimeter and the preliminary experiments which have been described are operations of paramount importance. In the case of the apparatus that we use, they have shown that corrections are often not necessary, and that the area of the thermogram is in many cases directly proportional to the amount of heat evolved during a adsorption phenomenon, provided that the gas pressure is maintained below 2 Torr. It may not be so with all adsorption calorimeters, especially if a large sensitivity is needed or if the symmetry of the twin system is not perfect. However, the calibration tests and preliminary experiments which have been described can be used to determine eventually the necessary corrections. Moreover, it should not be forgotten that the... 

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... 

The mean and standard deviation of the mean of the massic standard energy of combustion from the results in table 7.3 is Acm° (4-CNPyNO) = —25781.3 3.4 Jg-1. The corresponding standard molar energy of combustion is AcC/° (4-CNPyNO) = —3096.6 1.1 kJ mol -1, where the error quoted is twice the overall uncertainty (o-overaii), which includes contributions from the calibration of the calorimeter with benzoic acid and the combustion of n-hexadecane. The value of er0veraii is derived from [56,57] ... 

Ideally, the energy equivalents e and f should be measured over the same temperature range of the reaction ran, to avoid errors from their variation with temperature and to achieve maximum compensation for errors in the calibration of the temperature sensor [26,128,129], These errors are, however, frequently negligible in the temperature ranges involved, and the measurement of or f is normally performed outside the Jj -> Tf interval. This procedure saves time because there is no need to readjust the initial temperature of the calorimeter between the calibration and main experiment runs. It is therefore a common practice, even when an exothermic reaction is studied, to measure before the reaction and ef after the reaction and adjust the experimental conditions so that Jr is the midpoint between J] and Tf. In this case, the temperature of the thermostatic... 

The heat flux and energy calibrations are usually performed using electrically generated heat or reference substances with well-established heat capacities (in the case of k ) or enthalpies of phase transition (in the case of kg). Because kd, and kg are complex and generally unknown functions of various parameters, such as the heating rate, the calibration experiment should be as similar as possible to the main experiment. Very detailed recommendations for a correct calibration of differential scanning calorimeters in terms of heat flow and energy have been published in the literature [254,258-260,269]. 

Analogously to the dynamic method, the energy equivalent of the calorimeter, k.Q, can be obtained by performing calibration experiments in the isothermal mode of operation, using electrically generated heat or the fusion of substances with well-known A us//. Recommendations for the calibration of the temperature scale of DSC instruments for isothermal operation have also been published [254,270]. 

C. Mosselman, K. L. Chumey. Calibration of Combustion Calorimeters. In Experimental Chemical Thermodynamics, vol. 1 Combustion Calorimetry S. Sunner, M. Mansson, Eds. IUPAC-Pergamon Press Oxford, 1979 chapter 3. 

G. W. H. Holme, H. K. Cammenga, W. Eysel, E. Gmelin, W. Hemminger. The Temperature Calibration of Differential Scanning Calorimeters. Thermochim. Acta 1990, 160, 1-12. 

E. Gmelin, S. M. Sarge. Calibration of Differential Scanning Calorimeters. Pure Appl. Chem. 1995, 67, 1789-1800. 

Plewinsky, B. et al., Thermochim. Acta, 1985, 94, 33-43 Safety aspects of the combustion of various materials in an amosphere of pine oxygen under the conditions prevailing in oxygen bomb calorimetry were investigated experimentally. The combustion of a stable substance (benzoic acid, used to calibrate bomb calorimeters) in oxygen gives a relatively slow combustion, with a low rate of pressure increase of 17 bar/s to a maximum of 64 bar in 2.3 s, for... 

ASTM E 967-92, Standard Practice for Temperature Calibration of Differential Scanning Calorimeters and Differential Thermal Analyzers, 1992. 

ASTM E 968-83, Standard Practice for Heat Flow Calibration of Differential Scanning Calorimeters, 1983. 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

---