Isoperibol Flow Calorimeter

Hnedkovsky et al. (2002) constructed an isoperibol flow calorimeter for the determination of heat capacities of liquids at temperatures of up to 700 K and pressures of up to 35 MPa. The sample flowing through the calorimeter system is heated by an electrical heater, and the power needed for a constant temperature rise is measured relative to that for a reference fluid (e.g., water) in a second tube. In other words, this is a differential flow calorimeter that has mainly been used to measure the heat capacity of dilute solutions (with the pure solvent as the reference medium). 

It appears therefore that during the operation of all usual calorimeters, temperature gradients are developed between the inner vessel and its surroundings. The resulting thermal head must be associated, in all cases, to heat flows. In isoperibol calorimeters, heat flows (called thermal leaks in this case) are minimized. Conversely, they must be facilitated in isothermal calorimeters. All heat-measuring devices could therefore be named heat-flow calorimeters. However, it must be noted that in isoperibol or isothermal calorimeters, the consequences of the heat flow are more easily determined than the heat flow itself. The temperature decrease... 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

Figure 6.2 Schematic representation of (a) an adiabatic calorimeter, (b) an isoperibol calorimeter, and (c) a heat conduction (or heat flow) calorimeter. fc and 7] are the temperatures of the calorimeter proper and the external jacket, respectively, and is the heat flow rate between the calorimeter proper and the external jacket.

The isothermal and isoperibol calorimeters are well suited to measuring heat contents from which heat capacities may be subsequently derived, while the adiabatic and heat-flow calorimeters are best suited to the direct measurement of heat capacities and enthalpies of transformation. 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

The thermokinetic parameter as defined above provides semiquantitative information on the kinetics of the processes occurring in a calorimeter. The rigorous mathematical modeling of the thermokinetics for heat-flow calorimeters (2,34,42,130-132) and isoperibol calorimeters (133) has been recently discussed. Using these methods it is possible to obtain quantitatively the energetic as well as the kinetic parameters describing a number of important processes such as adsorption, desorption, consecutive processes involving the formation of adsorption intermediates, and chemical reactions. 

RC measurements can be classified either as devices using jacketed vessels with control of the jacket temperature (heat balance calorimeters, heat flow calorimeters and temperature oscillation calorimeters) or as devices using a constant surrounding temperature, e.g., jacketed vessels with a constant jacket temperature, (isoperibolic calorimeters and power compensation calorimeters) such instruments may also feature single or double cells. 

Figure 7.17 Measured function of an isoperibol heat flow calorimeter after starting a constant heat production in the calorimeter vessel.

Heat-flow Calorimeters.— In calorimeters of the adiabatic or isoperibol types, heat-exchange between the calorimeter and its surroundings is either eliminated or is restricted to a small, accurately determined amount. An alternative method is to transfer the heat of reaction completely to a heatsink, so that both the calorimeter and the heat-sink remain essentially isothermal and the calorimetric determination consists of measuring the heat transferred. Two main types have been employed. 

Names have been given to calorimeters (usually having these three components) that are operated in certain modes. In the isothermal calorimeter the temperature Tc of the calorimeter is kept equal to the temperature Ts of the surroundings and both are held constant. In the adiabatic calorimeter Tg is kept equal to T although both may change. In the isoperibol calorimeter 7 is kept constant and 7i, usually initially near 7, undergoes an excursion. In the constant heat-flow calorimeter (7 — 7i) is kept constant. 

A liquid serves as the calorimetric medium in which the reaction vessel is placed and facilitates the transfer of energy from the reaction. The liquid is part of the calorimeter (vessel) proper. The vessel may be isolated from the jacket (isoperibole or adiabatic), or may be in good themial contact (lieat-flow type) depending upon the principle of operation used in the calorimeter design. 

Albert H J and Archer D G 1994 Mass-flow isoperibole calorimeters Solution Calorimetry, Experimental Thermodynamics vol IV, ed K N Marsh and PAG O Hare (Oxford Blackwell)... 

It is, of course, not necessary to use a heat-flow microcalorimeter in order to determine the heat released by rapid adsorption phenomena. Dell and Stone (74), for instance, using an isoperibol calorimeter of the Garner-Veal type, found an initial heat of 54 4 kcal mole-1 for the adsorption of oxygen on nickel oxide at 20°C. The agreement with the value (60 2 kcal mole-1) in Fig. 19 is remarkably good, particularly if it is considered that very different methods were used for the preparation of the nickel-oxide samples (19, 74)-... 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

The thermochemical study of photochemical or photochemically activated processes is not amenable to most of the calorimeters described in this book, simply because they do not include a suitable radiation source or the necessary auxiliary equipment to monitor the electromagnetic energy absorbed by the reaction mixture. However, it is not hard to conceive how a calorimeter from any of the classes mentioned in chapter 6 (adiabatic, isoperibol, or heat flow) could be modified to accommodate the necessary hardware and be transformed into a photocalorimeter. 

Titration calorimetry is a method in which one reactant inside a calorimetric vessel is titrated with another delivered from a burette at a controlled rate. This technique has been adapted to a variety of calorimeters, notably of the isoperibol and heat flow types [194-198]. The output of a titration calorimetric experiment is usually a plot of the temperature change or the heat flow associated with the reaction or physical interaction under study as a function of time or the amount of titrant added. 

The historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

The principles of titration calorimetry will now be introduced using isoperibol continuous titration calorimetry as an example. These principles, with slight modifications, can be adapted to the incremental method and to techniques based on other types of calorimeters, such as heat flow isothermal titration calorimetry. This method, which has gained increasing importance, is covered in section 11.2. 

Solution calorimetry allows us to investigate processes that involve enthalpy changes. Adiabatic microcalorimeters and isoperibol calorimeters used in batch modes or flow modes allow for the precise determination of the heat of solution. Mixing the reactants is accomplished by breaking a bulk allowing reactants to mix or by special chambers where the reactants are mixed together. 

Isoperibolic calorimeters also comprise calorimeters based on the measurement of heat flow, since they fulfill the condition Tg = constant, changes. With these calorimeters, the temperature difference (Tc - Tg) is not measured but directly the heat flow between the calorimetric vessel and the cover. 

Solution calorimetry involves the measurement of heat flow when a compotmd dissolves into a solvent. There are two types of solution calorimeters, that is, isoperibol and isothermal. In the isoperibol technique, the heat change caused by the dissolution of the solute gives rise to a change in the temperature of the solution. This results in a temperature-time plot from which the heat of the solution is calculated. In contrast, in isothermal solution calorimetry (where, by definition, the temperature is maintained constant), any heat change is compensated by an equal, but opposite, energy change, which is then the heat of solution. The latest microsolution calorimeter can be used with 3-5 mg of compound. Experimentally, the sample is introduced into the equilibrated solvent system, and the heat flow is measured using a heat conduction calorimeter. 

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