Electric calibration

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

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). 

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 reaction curves, together with a conventional electrical calibration, also yielded an enthalpy. From this and the initial concentration of monomer, ra0, and the quantity of unreacted monomer determined by GLC (always less than ca 5%), the enthalpy of polymerisation, AHp, was calculated. 

If a polymerisation is too fast for the reaction mixture to be kept isothermal, then the rate of temperature rise under adiabatic conditions can be used with advantage as a measure of the rate of the reaction. By means of a conventional electrical calibration and the experimentally established relation between Am and AT, the rate of temperature change, dT/dt, is converted to -dm/dt, and the enthalpy of the polymerisation, AHp, can be found as well. A version of the technique suitable for cationic polymerisations was developed by Biddulph and Plesch (1959) and subsequently used by many workers substantial improvements were made by Sigwalt s group (Cheradame etal, 1968 Favier etal., 1974) and by Pask and Plesch (1989). The method is most useful for reactions which have half-lives in the range of 5 to 300 s, and... 

In the case of an electrical calibration, at the beginning of the main period a potential V is applied to a resistance inside the calorimeter proper, causing a current of intensity / to flow over a period t. As a result, an amount of heat Q = Vlt is dissipated in the calorimeter proper, causing the observed temperature rise. If the calibration is carried out on the reference calorimeter proper (without contents ), then eci = ecf = 0 and the internal energy change of the calorimetric system during the main period is... 

Electrical calibration has the advantage of being more flexible. It can afford s0 through equation 7.23 ifitisdone on the reference calorimeter proper. Flowever, it can also be performed on the initial or final state of the actual experiment leading to (e0 + ecl) or (e0 + ecf), respectively. Twenty or 30 years ago the electrical calibration required very expensive instrumentation that was not readily available except in very specialized places, such as the national standards laboratories. Although the very accurate electronic instrumentation that is available today at moderate prices may change the situation, most users of combustion calorimetry still prefer to calibrate their apparatus with benzoic acid. 

The calorimeter in figure 7.10 was electrically calibrated [54,99] by using the heater O. Flame calorimeters are, however, most frequently calibrated on the basis of the reaction of hydrogen with oxygen, which has been recommended for this purpose by IUPAC [39]. 

The obtained A 7 a() value and the energy equivalent of the calorimeter, e, are then used to calculate the energy change associated with the isothermal bomb process, AE/mp. Conversion of AE/ibp to the standard state, and subtraction from A f/jgp of the thermal corrections due to secondary reactions, finally yield Ac f/°(298.15 K). The energy equivalent of the calorimeter, e, is obtained by electrical calibration or, most commonly, by combustion of benzoic acid in oxygen [110,111,113]. The reduction of fluorine bomb calorimetric data to the standard state was discussed by Hubbard and co-workers [110,111]. 

The value of s (e, or sr) is usually determined by electrical calibration (note that contrary to combustion calorimetry, it is not common practice to separate the initial and final energy equivalents of the calorimeter into the contribution of the reference calorimeter, e0, and those of the contents present in the initial, C1, and, final, ecf, states see section 7.1). In the case of the calorimeter in figure 8.1, a current I is passed trough the resistance F for a known period of time t and the potential change V across F is measured. Then ... 

The experimental procedure adopted in the thermochemical study of reaction 10.17 was fairly simple. First, an electrical calibration was made. Then, after balancing the light input to the cells, 2.7 cm3 of a 7 x 10-3 mol dm-3 solution of hrms-azobenzene in heptane was added to the photochemical reactor. This solution was irradiated for a certain period (1.5-3.8 h) with 436 nm light, and the thermogram was recorded. The area of this thermogram multiplied by the calibration constant (e) gives A0b H. 

Figure 11.1a shows a scheme of a widely used reaction vessel for isoperibol titration calorimetry [211]. It consists of a silvered glass Dewar A, which can be adjusted to a lid B supporting a stirrer C, a resistance D for electrical calibration, a thermistor E for temperature measurement, and a Teflon tube F for titrant delivery. The assembled Dewar and lid set-up is immersed in a constant... 

The value of e0 is only constant for a fixed volume V of solution inside the calorimetric vessel. The change of e0 with V is primarily due to an increase of the reaction vessel wall in contact with the liquid as the liquid volume increases [ 197,200]. This change, de0/dV, which is constant for well-designed calorimeters [197,200], can be determined by measuring e0 as a function of V. Because it has been found that as expected, e0 and d 0/dV are independent of the nature of the liquid used in the calorimeter, they are normally determined by performing electrical calibrations with the calorimeter filled with different volumes of water [200]. The energy equivalent of the calorimeter at any point during a titration can therefore be calculated from... 

The dependence of k on the volume of liquid inside the calorimetric vessel can be determined by making electrical calibrations for different values of V. For each calibration, /t, = kf and m, = Mf, and equation... 

Careful heat-flow calibrations have to be performed. Chemical calibrations present many disadvantages they rely on prior results, with no general agreement and no control of rate, and are generally available only at a single temperature. On the contrary, electrical calibrations (Joule effect) provide many advantages and they are easy to perform at any temperature [103],... 

The reaction is initiated by the addition of a reactant, which must be exactly at the same temperature as the Dewar contents, in order to avoid the sensitive heat effects. Then the temperature is recorded as a function of time. The obtained curve must be corrected for the heat capacity of the Dewar flask and its inserts, respective of their wetted parts, which are also heated by the heat of reaction to be measured. The temperature increase results from the heat of reaction (to be measured), the heat input by the stirrer and the heat losses. These terms are determined by calibration, which may be a chemical calibration using a known reaction or an electrical calibration using a resistor heated by a known current under a known voltage (Figure 4.2). The Dewar flask is often placed into thermostated surroundings as a liquid bath or an oven. In certain laboratories, the temperature of the surroundings is varied in order to track the contents temperature and to avoid heat loss. This requires an effective temperature control system. 

CALORIMETRIC MEASOREMEMTS Solution calorimetry was performed at 298.2 0.1 K by using a C-80 differential flux calorimeter manufactured by Setaram. The energy equivalent of the calorimetric signal was determined by electric calibration. The reliability of the equipment was checked by the dissolution of tris-(hydroxymethyl) aminomethane (THAM). Agreement within 0.4% with the published value of +17.75 kJ. mol-1 ( 21) was obtained. 

One general feature of the calorimetric technique which reduces ihe likdiliood of a systematic error is that the energy equivalent of C e calorimeter Is not found by direct electrical calibration but by... 

Demas JN, Bowman WD, Zalewski EF, Velapoldi RA. Determination of the quantum yield of the ferrioxalate actinometer with electrically calibrated radiometers. J Phys Chem 1981 85 2766-2771. 

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). 

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