Precision calorimetry

Gilman and G. D. Lichtenwalter, J. Amer. Chem. Soc., 80, 608 (1958). P. Gross, C. Hayman and D. L. Levi, Trans. Faraday Soc., 51, 626 (1955). P. Gross, C. Hayman and D. L. Levi, Trans. Faraday Soc., 53,1285 (1957a). P. Gross, C. Hayman and D. L. Levi, Trans. Faraday Soc., 53,1601 (1957b). KPG, Keele Polymer Group, unpublished. 

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Instruction Manual ofLKB 8700-1 Precision Calorimetry System for Reaction and Solution Calorimetry. LKB-Produkter AB, Sweden. 

All enthalpy of solution measurements were carried out with an LKB 8700-1 precision calorimetry system. The experimental procedure and tests of the calorimeter have been reported previously (3, 4, 5). The purification of the solvent DMF (Baker Analyzed Reagent) and of all solutes used has been described in the same papers. The solvent mixtures were prepared by weighing and the mole fraction of water in the DMF-water mixtures was corrected for the original water content of the amide as measured by Karl Fischer titration. 

W. N. Hubbard, C. Katz, and G. Waddington, A rotating combustion bomb for precision calorimetry. Heats of combustion of some sulfur-containing compounds,. Phys. Chem. 58 142-152 (1954). 

The enthalpies of solution were measured with a LKB 8700-1 precision calorimetry system. The experimental procedure and test of the instrument have been given before (6,7). EC (Fluka, purissimum) was distilled under reduced pressure and the middle fraction was stored over molecular sieves (4 A) for at least 48 hr. ACN (Merck, pro analysis) was dried over molecular sieves and used without further purification. The purity of both solvents (determined shortly before use), as deduced from GLC, was always better than 99.8%. The volume fraction of water, determined by K. Fischer titration (8) was always less than 3.10-4. The mixed solvents were prepared by weight as shortly as possible before the measurements. AH° of Bu4NBr in W-ACN mixtures have been measured at 25°C while those in W-EC are at 45°C, which is above the melting point of pure EC. 

Devising a series of reactions from which an accurate value of the heat of formation of a particular compound can be obtained can be a challenging problem. A calorimetric reaction must take place quickly (that is, be completed within a few minutes at most), with as few side reactions as possible and preferably none at all. Very few chemical reactions take place without concomitant side reactions, but their effects can be minimized by controlling the reaction conditions so as to favor the main reaction as much as possible. The final product mixture must be carefully analyzed, and the thermal effect of the side reactions must be subtracted from the measured value. Precision calorimetry is demanding work. 

The name calorimeter is used for the combination of sample and measuring system, kept in well-defined surroundings, the thermostat or furnace. To describe the next layer of equipment, which may be the housing, or even the laboratory room, one uses the term environment. For precision calorimetry the environment should always be kept from fluctuating. The temperature should be controlled to +0.5 K and the room should be free of drafts and sources of radiating heat. 

Dale, A., Lythall, C., Aucott, )., and Sayer, C. (2002) High precision calorimetry to determine the enthalpy of combustion of methane. Thermochim. Acta, 382, 47-54. 

Cryostatic arrangements range from simple, thermally rather isolated objects, through immersion baths, to those for precision calorimetry. Electric controls range from manual through simple proportional, to combinations of proportional, rate and reset, employing thermocouples or resistance thermometers. 

A different, but important application of equations (1) through (3) is to the control of adiabatic shields in precision calorimetry. Here H = 0. The shields must follow the rising temperature of the sample container, to which their temperature is referred by thermocouples. At the inception of an experiment all temperatures are equal and = 0. The reference or balance-point temperature, T, then increases during an... 

The unit of heat used throughout this article is the calorie. Historically the calorie is defined as the specific heat of water. As soon as more accurate measurements were possible, it was found that the specific heat of water varies by as much as Va% between 0 and 100° C. For a more precise definition the temperature of the water had to be specified. Soon after 1900 practically all precision calorimetry was ultimately linked to the more convenient and accurate electrical measurement of oiergy. Out of habit most measurements were, however, still converted to calories, so that finally an artificial calorie was defined. The present value of this defined calorie is ... 

Figure 6.11 shows a famous example of the application of isothermal calorimetry. Gordon (1955) deformed high-purity copper and annealed samples in his precision calorimeter and measured heat output as a function of time. In this metal, the heat output is strictly proportional to the fraction of metal recrystallised. 

In differential scanning calorimetry (DSC), higher precision can be obtained and heat capacities can be measured. The apparatus is similar to that for a DTA analysis, with the primary difference being that the sample and reference are in separate heat sinks that are heated by individual heaters (see the following illustration). The temperatures of the two samples are kept the same by differential heating. Even slight... 

We use differential scanning calorimetry - which we invariably shorten to DSC - to analyze the thermal properties of polymer samples as a function of temperature. We encapsulate a small sample of polymer, typically weighing a few milligrams, in an aluminum pan that we place on top of a small heater within an insulated cell. We place an empty sample pan atop the heater of an identical reference cell. The temperature of the two cells is ramped at a precise rate and the difference in heat required to maintain the two cells at the same temperature is recorded. A computer provides the results as a thermogram, in which heat flow is plotted as a function of temperature, a schematic example of which is shown in Fig. 7.13. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

In general, although the results of microcalorimetric studies do not pretend to provide enthalpy values of the very highest accuracy presently attainable by macroscale combustion calorimetry, they do offer a basis for application on a wide scale and are sufficiently precise for most purposes. The conflicting claims of accuracy and usefulness are particularly acute in the area of transition metal organometallic chemistry this review will attempt to follow a middle way between them. 

The first reported attempt to use fluorine in calorimetric measurements is probably Berthelot and Moissan s study of the reaction between K.2SC>3(aq) and F2(g), in 1891 [ 19,120]. Modem fluorine bomb calorimetry, however, was started in the 1960s by Hubbard and co-workers [110,111,121], while in the same period Jessup and Armstrong and their colleagues [ 109,115-117] developed the method of fluorine flame calorimetry to a high degree of accuracy and precision. 

As illustrated in this section, the problems associated with using fluorine in combustion calorimetry seem to have been largely overcome. The fluorine bomb and flame calorimetry methods have been perfected to such an extent that, provided the chemistry of the process under study is well characterized, results of very good accuracy and precision can be obtained routinely. 

Use of medium-scale heat flow calorimeter for separate measurement of reaction heat removed via reaction vessel walls and via reflux condenser system, under fully realistic processing conditions, with data processing of the results is reported [2], More details are given elsewhere [3], A new computer controlled reaction calorimeter is described which has been developed for the laboratory study of all process aspects on 0.5-2 1 scale. It provides precise data on reaction kinetics, thermochemistry, and heat transfer. Its features are exemplified by a study of the (exothermic) nitration of benzaldehyde [4], A more recent review of reaction safety calorimetry gives some comment on possibly deceptive results. [5],... 

More advanced techniques are now available and section 4.2.1.2 described differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DTA, in particular, is widely used for determination of liquidus and solidus points and an excellent case of its application is in the In-Pb system studied by Evans and Prince (1978) who used a DTA technique after Smith (1940). In this method the rate of heat transfer between specimen and furnace is maintained at a constant value and cooling curves determined during solidification. During the solidification process itself cooling rates of the order of 1.25°C min" were used. This particular paper is of great interest in that it shows a very precise determination of the liquidus, but clearly demonstrates the problems associated widi determining solidus temperatures. 

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