Automatic Calorimetry

Automatic Calorimetry.— Before the development of modem electronic techniques, precise measurements of heat capacities were made by manual methods both for control of adiabatic conditions and for measurements of temperatures and electrical energies. Typical examples of such measurements have been described by Southard and Brickwedde and by Ruhrwein and Huffman. To obtain the highest possible accuracy, the measurement required two operators, and, to cover the temperature range (10 to 300 K), a large number of potentiometric observations were necessary. It is now usual to have automatic control of the adiabatic shield. Measurements of temperature and energy with a manually-operated potentiometer can be simplified by the use of constant current supplies for the potentiometer and thermometer circuits. 

10 (xV or less is amplified and displayed on a digital voltmeter. If the potentiometer or bridge is fitted with a set of coding contacts, the readings of the instrument and those of the digital voltmeter can be recorded automatically on tape and in print. 

A fully-automatic Mueller bridge has been described and an instrument of this type has been used at the National Bureau of Standards for heat capacity measurements.  

Hill and A. P. Miller, Proc. Inst. Elec. Engrs., 1963, 110, 453. 

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Accelerating Rate Calorimetry. This is a heat-wait-search technique (see Fig. 5.4-62). A sample is heated by a pre-selected temperature step of, typically, 5 C, and then the temperature of the sample is recorded for some time. If the self-heating rate is less than the calorimeter detectability (typically 0.02 "C) the ARC will proceed automatically to the next step. If the change of the sample temj)erature is greater than 0.02 °C, the sample is no longer heated from outside and an adiabatic process starts. The adiabatic run is continued until the process has been completed. ARC is usually carried out at elevated pressure. 

Characterization. Differential scanning calorimetry and thermal mechanical analysis data were obtained on a DuPont 990 thermal analyzer coupled with a DuPont DSC or TMA cell. Isothermal aging studies were carried out with an automatic multisample apparatus. 

D. R.Stull, Anal ChimActa 17, 133(157)(Automatic calorimeter) aa)C.Napoly et al, MP 41, 155-69 (Contribution to the study of operating conditions for detn of calorimetric potential in calorimetric bombs) bb)S.S.Wise, "The Heats of Formation of Some Inorganic Compounds by Fluorine Bomb Calorimetry , Argonne National Laboratory ANL -6472, Jan 1962, Contract W- 31- 109- eng- 38... 

Finally, significant advances in the techniques of both thermal and thermochemical measurements have come to fruition in the last decade, notably aneroid rotating-bomb calorimetry and automatic adiabatic shield control, so that enhanced calorimetric precision is possible, and the tedium is greatly reduced by high speed digital computation. Non-calorimetric experimental approaches as well as theoretical ones, e.g., calculation of electronic heat capacity contributions to di- and trivalent lanthanides by Dennison and Gschneidner (33), are also adding to definitive thermodynamic functions. 

The calorimetric studies of the surface heterogeneity of oxides were initiated half a century ago, and experimental findings as well as their theoretical interpretation have been recently reviewed by Rudzinski and Everett [2]. The last two decades have brought a true Renaissance of adsorption calorimetry. A new generation of fully automatized and computerized microcalorimeters has been developed, far more accurate and easy to manipulate. This was stimulated by the still better recognized fact that calorimetric data are much more sensitive to the nature of an adsorption system than adsorption isotherm for instance. It is related to the fact that calorimetric effects are related to temperature derivatives of appropriate thermodynamic functions, and tempearture appears generally... 

A modem adiabatic calorimeter is described by Gmelin E, Rodhammer P (1981) Automatic Low Temperature Calorimetry for the Range 0.3- 320 K. J Phys E, Instrument. 14 223-238. 

In Fig. 4.53, a brief look is taken at the history of the DSC. Both, heating curves and calorimetry had their beginning in the middle of the nineteenth century. Progress toward a DSC became possible as soon as continuous temperature monitoring with thermocouples was possible (see Fig. 4.8), and automatic temperature recording was invented (see also Sect. 4.1). These developments led to the invention of differential thermal analysis, DTA. In Sect. 2.1.3 an introduction to thermal analysis and its instrumentations is given. 

There is no doubt that automatic data processing has found an increasing application in calorimetry. Microprocessor technology permits automatic control of calorimeters with a continuous adjustment of such parameters as heating power, heating rate, temperature, and so on in accordance with the pertinent data of the caloric process in the sample. There are already calorimeters in which the results of the measurement are given automatically in a corrected, calibrated, desmeared form. Such a computer-controlled calorimeter may be called a smart calorimeter. 

Detailed reviews have been published on calorimetric design, on isothermal calorimetry,2 and on adiabatic calorimetry. The emphasis in the present account is on automatic methods for measurements of heat capacity and enthalpies of phase changes by adiabatic calorimetry. 

Automatic Measurement and Data Collection.—X description of an automatic data acquisition system for calorimetry has recently been published. The instrumentation for low-temperature calorimetry now used in the Division of Chemical Standards, N.P.L., is illustrated by Figure 2. It is used in conjunction with cryostats and sample containers similar to published designs (Figures 2 and 9, respectively, of ref. 3). 

D. W. Scott, ed., "Experimental Thermodynamics," Vol. 1. Butter-worths, London, 1968. G. K. White, "Experimental Techniques in Low Temperature Physics." Clarendon Press, Oxford, 1979 (third edition). A. C. Rose-Innes, "Low Temperature Techniques." van Nostran Princeton, NJ, 1964. E. Gmelin, Thermochim. Acta, 29, 1 (1979). E. Gmelin and P. Rddhammer, "Automatic low temperature calorimetry for the range 0.3-320 K,"/. Phys. E. Instrument., 14, 223 (1981). 

As it has been already mentioned, isothermal microcalorimeters are those calorimeters designed to work in the microwatt range under essentially isothermal conditions. Isothermal titration microcalorimetry (IT xC) is designed to connect extremely sensitive thermal measurement equipment (approx. 20-100 nanowatts) with an automatic syringe able to add reactants in successive injections to the solution with a precision of few nanoliters [32, 33]. Each injection produces specific heat effect, as shown in Fig. 10.5. The determination of heats evolved as a result of interaction between molecules is a main application of this variation of calorimetry. Consequently, isothermal titration calorimetry is a suitable method for studying degradations and biodegradations of versatile pollutants. 

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