Calvet calorimeter

Calvet and Persoz (29) have discussed at length the question of the sensitivity of the Calvet calorimeter in terms of the number of thermocouples used, the cross section and the length of the wires, and the thermoelectric power of the couples. On the basis of this analysis, the micro-calorimetric elements are designed to operate near maximum sensitivity. The present-day version of a Tian-Calvet microcalorimetric element, which has been presented in Fig. 2, contains approximately 500 chromel-to-constantan thermocouples. The microcalorimeter, now commercially available, in which two of these elements are placed (Fig. 3) may be used from room temperature up to 200°C. 

Fig. 4. Vertical cross section of a high-temperature Calvet calorimeter (16) cell guides (A) thermal insulation (B) top (C) and bottom (N) electrical heaters thermostat consisting of several metal canisters (D, G, and H) switch (E) electrical heater (F) thermometers (I, J, and K) microcalorimetric element (L) and heat sink (M).

Calvet microcalorimeters are particularly convenient for such studies. Figure 19 show s, for instance, the evolution of the differential heat of adsorption of oxygen, measured at 30°C with a Calvet calorimeter, as a function of the total amount of oxygen adsorbed on the surface of a sample (100 mg) of nickel oxide, NiO(200) (19, 73). The volume of the first... 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

A. Rojas-Aguilar, M. Martmez-Herrera. Enthalpies ofCombustion and Formation of Fullerenes by Micro-combustion Calorimetry in a Calvet Calorimeter. Thermochim. Acta 2005, 437, 126-133. 

The adsorption experiment is conducted until a relatively high pressure is reached without significant evolution of heat and the adsorbed amount becomes negligible. Owing to the high sensitivity of the method, only small quantities of sample are required. The error may be around 1%, as for the Tian-Calvet calorimeter [129]. 

FIGURE 6.5 Schematic of Tian-Calvet calorimeter at the Canadian National Research Council. (Reproduced from Handa, Y.P., Calorimetric Studies of Laboratory Synthesized and Naturally Occurring Gas Hydrates, paper presented at AIChE 1986 Annual Meeting Miami Beach, November 2-7, 28 (1986b). With permission.)... 

In this category of calorimeters, we find the isothermal calorimeters and the dynamic calorimeters where the temperature is scanned using a constant temperature scan rate. The instrument must be designed in such a way that any departure from the set temperature is avoided and the heat of reaction must flow to the heat exchange system where it can be measured. The instrument acts as a heat sink. In this family we find the reaction calorimeters, the Calvet calorimeters [7], and the Differential scanning calorimeter (DSC) [8],... 

The Calvet calorimeters have their roots in the work of Tian [26] and the later modifications by Calvet [7]. Presently this calorimeter type is commercially available from Setaram and the models C80 and BT215 are particularly well adapted for safety studies. It is a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC in the temperature range from room temperature to 300°C for the C80 and -196 to 275°C for the BT215. They show a high... 

Figure 6.15 Determination of thermal data for the addition reaction example at 30°C in a Calvet calorimeter and followed by a temperature scan at 0.5 Kmin-l to 300°C, in order to determine the heat of decomposition.

A reaction should be stopped by flooding with a cold solvent. The amount of solvent needs to be sufficient to cool the reaction mass to a thermally stable level. To test this theory, flooding was tested in a Calvet calorimeter (Figure 10.4). The experiment showed that the dilution is endothermal with a heat release of—ffikjkg"1 of mixture (reaction mass and solvent). The reaction mass (2230 kg) has a specific heat capacity of 1.7kJ kg 1 K 1 and a temperature of 100 °C. The dilution is with 1000 kg of a solvent at 30 °C, with a specific heat capacity of 2.6kJ kg"1 K"1. The resulting mixing temperature (Tm) can be calculated from a heat balance ... 

For cases where the secondary reaction plays a role (class 5), or if the gas release rate must be checked (classes 2 or 4), the heat release rate can be calculated from the thermal stability tests (DSC or Calvet calorimeter). Secondary reactions are often characterized using the concept of Time to Maximum Rate under adiabatic conditions (TMRad). A long time to maximum rate means that the time available to take risk-reducing measures is sufficient. However, a short time means that the... 

Figu re 11.13 Thermogram in a Calvet calorimeter showing the main reaction performed under isothermal conditions and the secondary reaction triggered during the temperature ramp (solid line and baseline). The dashed line is the temperature and the dash-point line the pressure. 

Calorimetric measuring techniques give additional information on thermodynamic data. The measuring technique is sophisticated and specialised instruments are not yet on the market. In general Tian-Calvet calorimeters are used by which the heat of adsorption is measured (Fig.l). From this the adsorption enthalpy and the adsorbed mass in principle can be calculated. 

To eliminate the effects of external temperature fluctuations in the calorimetric block, the calorimeter has two heat-flow meters, which are connected in opposition. The process under investigation is carried out in one of two identical calorimeter vessels, the other serving as the tare or reference element. This differential arrangement permits the compensation of parasitic phenomena such as external connections and reagent introduction heat, and it provides a good stability of the baseline. (From the development in Section II,A on thermodynamics it follows that for adsorption of gas in a Calvet calorimeter the heat measured corresponds to a differential molar enthalpy of adsorption because all other effects are compensated.)... 

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

A quantitative approach of the basic properties is presented here using the adsorption of CO2 by microcalorimetry at room temperature. The instrument was a Calvet calorimeter coupled to a volumetric ramp [18]. The samples (0.1 g) were first evacuated at 673 K for 3 h, and then contacted with small doses of... 

For the determination of the enthalpies of formation of the two isomorphs, a custom-built Tian-Calvet calorimeter was used, operating at 1080 K. The solvent, contained in a platinum cmcible, was 2Pb0-B203. The reported results are derived from experiments in which samples at room temperature (25°C) were dropped into the solvent at the calorimeter temperature. 

Conduction calorimeters typically, the Tian-Calvet calorimeter. They comment that, in spite of its good external thermal insulation, this calorimeter is not adiabatic, because the calories produced are continuously eliminated from the calorimetric vessel. They also consider that, in spite of the very small temperature variations of the sample cell, this calorimeter is not strictly isothermal , which justifies a separate group, except when a Peltier compensation totally cancels the temperature variations in the sample. 

These authors were aware of the difficulty of establishing a comprehensive classification of calorimeters In every classification there are certain calorimeters which do not clearly belong to a particular category.The Calvet calorimeter, for instance, can be used eidier isothermally with electric compensation... or in an isoperibol manner involving the measurement of a local temperature difference. Moreover, a number of existing calorimeters remain outside our classification. One example is a calorimeter involving a compensation of the thermal effect other than by thermoelectric means or by phase transition. But such devices can be easily included in our classification by analogy. ... 

The most used conduction instruments are Calvet calorimeters [73,74], in which the calorimeter has two Knudsen cells, one for the sample and the other as reference. The whole system is thermo-regulated and the heat flow to the reference cell is measured with two thermopiles that surround both cells. 

Fio. 7. Reaction cell of a Calvet calorimeter (for measuring very small heats) (taken from Calvet, ref. 28). 

Figure 19. Phase diagram of a narrow fraction of HPC (molar substitution MS = 5.0, M = 209x 10 g/mol) in H2O. Onset of phase separation by tur-bidimetry , transition temperatures with scanning Calvet calorimeter O (adapted from [2] and [72]).

We will hmit our interest in this book to the study of the Tian-Calvet calorimeter. 

In the case of a Tian-Calvet calorimeter, we note that the presence of a temperature difference between the internal and external enclosure will lead to the creation of an electromotive force that is proportional to this difference (Seebeck effect). 

The overall calorimeter equation of the Calvet calorimeter is finally given by Eq. (4). The overall heat effect, AH, is equal to the time integral over the Peltier compensation, the major effect to be measured, corrected for two factors the time-integral over the just-discussed losses, 4>, and, if the temperature does not stay exactly constant during the experiment, a correction term which involves the heat capacity of the calorimeter and the sample, C. All three terms can be evaluated by the measurement of the Peltier current i, the measurement of the emf of the measuring thermocouples, and a measurement of the change of the emf with time. The last term is needed for the calculation of the heat capacity correction which is written in Eq. (4). The last two terms in Eq. (4) are relatively small as long as the operation is close to isothermal. 

For larger temperature differences such calorimeters can also be used as heat-flux calorimeters, using only the last two terms in Eq. (4). Because of the small losses, Tian-Calvet calorimeters have found application for the measurement of slow, biological reactions. 

The heats of complex formation were measured in a Tian-Calvet-Calorimeter (Setaram, Lyon). The electron micrographs were obtained using an electron microscope Hitachi H 500. 

The specific heat of the complex formation (AH), which appears when mixing solutions of PMMA of different tacticities in suitable solvents, was measured in a Calvet calorimeter. it was always negative. The influence of solvent (o-xylene, chloroform, dimethyl-formamide), temperature, and polymer concentration on the value of AH, was extensively studied. 

Figure 17. Tian-Calvet calorimeter a, b) Heat conduction paths between measuring cells and block c, d) Sample and reference containers e) Isoperibolic block f) Thermostatic jacket g) Thermal insulation h) Thermopiles...

For a clear-cut representation of the heat flows in a calorimeter as well as their combined effects and the consequences of their action, it is recommended to draw an analog electric representation diagram that translates the thermal relationships into electric circuits (for an example, see Section 7.9.2.3 on Calvet calorimeter). Thus, the temperature difference, the heat flow rate, and the thermal resistance become analogous to the voltage, the current, and the electric resistance, respectively. These electric circuits are often easier to interpret, and the electric laws are more familiar. 

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