Adiabatic microcalorimeters

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. 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Twin differential microcalorimeters have been described by Berghausen el al. (S), by Hackerman (8), and by Whalen and Johnson (9). Hacker-man employs thermistors, whereas the other two are based on thermocouples and in addition are run adiabatically. These calorimeters appear to have about 10 times the sensitivity of simpler designs, but for many purposes the large additional diflSculties in design, construction, and operation do not seem to be warranted. Berghausen and coworkers, however, have shown that they can estimate slow heat evolutions, after the first few minutes, due to surface reactions. 

Microcalorimeters are well suited for the determination of differential enthalpies of adsorption, as will be commented on in Sections 3.2.2 and 3.3.3. Nevertheless, one should appreciate that there is a big step between the measurement of a heat of adsorption and the determination of a meaningful energy or enthalpy of adsorption. The measured heat depends on the experimental conditions (e.g. on the extent of reversibility of the process, the dead volume of the calorimetric cell and the isothermal or adiabatic operation of the calorimeter). It is therefore essential to devise the calorimetric experiment in such a way that it is the change of state which is assessed and not the mode of operation of the calorimeter. 

Microcalorimetry is a growing technique complementary to DSC for the characterization of pharmaceuticals. Larger sample volume and high sensitivity means that phenomena of very low energy (unmeasurable by DSC) may be studied. The output of the instrument is measured by the rate of heat change dq/dt) as a function of time with a high sensitivity better than 0.1 pW. Microcalorimery can be applied to isolated systems in specific atmospheres or for batch mode where reactants are mixed in the calorimeter. Solution calorimetry can be used in adiabatic or isoperibol modes in microcalorimeters at constant temperature. (See the corresponding article about calorimetry of this edition.)... 

One of the first scarming adiabatic calorimeters designed in the past was the DASM IM microcalorimeter [107], used to determine the apparent molar heat capacity and conformational changes of proteins and nucleic acids. Measurements are performed in the temperature interval from 10 to 100°C, the shield heating rate can vary from 0.1 deg-min to 2 deg-min", and the sensitivity of the instmment is 4-10 cal-deg". A... 

The isoperibol calorimeter is also termed an enviromnent constant-temperature calorimeter. The famous Nemst-type calorimeter is a typical low-temperature isoperibol calorimeter. Before the 1940s, the enthalpies of phase transition and heat capacity of hundreds of organic compounds were determined with an inaccuracy of between +0.5 to 0.2 percent by Nemst-type calorimeters. Low temperature adiabatic calorimeters were developed based on the Nemst-type calorimeter, and, at present, adiabatic calorimeters have replaced Nemst-type for most low temperature heat capacity and phase change measurements on organic compounds. Besides the Nemst-type calorimeters, the term isoperibol calorimeter also refers to other types of environment constant-temperature calorimeters such as the commercial LKB-microcalorimeter. 

Mosselmann C, Mouiik J, Dekker H (1974) Enthalpies of phase change and heat capacities of some long-chain alcohols. Adiabatic semi-microcalorimeter for studies of polymorphism. J Chem Thermodynamics 6 477 87... 

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