History repeats itself: Progress in a.c. calorimetry

Periodic heating has been applied more than a century ago to study thermophysical properties of materials. The measurement of heat capacity using a.c. calorimetry was first performed by Corbino in 1910. In connection with the technological development and the progress in science and technology, new and sophisticated apparatus have been constructed in an a.c. calorimetric heat capacity measurements. In this measurement, the noise level of a.c. temperature can be reduced markedly as opposed to the other nonperiodic methods and, therefore, high precision determination can be attained. Furthermore, not only the amplitude but also the phase in a.c. temperature is a useful parameter in constructing much advanced apparatus.     

Enthalpies of mixing in the iron-manganese system by direct reaction calorimetry

Integral enthalpies of mixing of solid iron—manganese alloys have been obtained at 1443K, 1073K and 873K using a modified Dench-type calorimeter. The modifications in calorimetric technique required to cope with the high volatility of manganese are described. Results are presented and discussed.Dedicated to Prof. Dr.Kurt L. Komarek on the occasion of his 60th birthday on 23rd June 1986.     

Combined use of adiabatic calorimetry and heat conduction calorimetry for quantifying propellant cook-off hazards

Recent work performed at DERA (now QinetiQ) has shown how accelerating rate calorimetry (ARC) can be used to obtain time to maximum rate curves using larger samples of energetic materials. The use of larger samples reduces the influence of thermal inertia, permitting experimental data to be gathered at temperatures closer to those likely to be encountered during manufacture, transportation or storage of an explosive device. However, in many cases, extrapolation of the time to maximum rate curve will still be necessary. Because of its low detection limit compared to the ARC, heat conduction calorimetry can be used to obtain data points at, or below, the region where an explosive system might exceed its temperature of no return and undergo a thermal explosion.Paired ARC and heat conduction calorimetry experiments have been conducted on some energetic material samples to explore this possibility further. Examples of where both agreement and disagreement are found between the two techniques are reported and the significance of these discussed. Ways in which combining ARC and heat conduction calorimetry experiments can enhance, complement and validate the results obtained from each technique are examined.     

Heteroatom influence in substituted benzenes and pyridine-like heterocycles in terms of direct and indirect intramolecular interactions

The general expression for the one-electron density matrix obtained previously for saturated organic molecules (V. Gineityte, J. Mol. Struct. (Theochem), 343 (1995) 183) is shown to be applicable also to substituted benzenes and pyridine-like heterocycles. On this basis, a new interpretation of the influence of a heteroatom (substituent) upon the remaining fragment of an aromatic molecule is suggested. To this end, the occupation number of a 2p_z AO of the aromatic ring has been expressed as a sum of five terms, two of them describing the intramolecular charge transfer and the remaining ones representing the secondary (induced) dipole moments arising within the ring under the influence of heteroatom, viz. the so-called ipso–ortho (para–meta), para–ipso and ortho–meta dipole moments. Just the latter two moments proved to play the principal role in the formation of the observed picture of the electron density distribution, viz. of an increase (reduction) of occupation numbers of 2p_z AOs in the ortho and para positions after introducing an electron-donating (accepting) substituent. For pyridine-like heterocycles and substituted benzenes, these dipole moments are determined mostly by the direct and the indirect interactions, respectively, between the highest occupied and the lowest vacant MO of benzene. Orbitals of the heteroatom (substituent) play the role of mediators in the above-mentioned indirect interaction.     

The art of calorimetry

An introduction to the various types of calorimeters is given. The requirements for precise temperature measurements with thermistors are derived. Methods and equations for accurate heat exchange corrections for isoperibol temperature-change calorimetry under various conditions are derived. The characteristics and design principles for constant temperature baths are discussed. The construction of devices for addition of reagents, of stirrers, and of calibration heaters is described.     

Capillary zone electrophoretic analysis of carbohydrates by direct and indirect UV detection

Summary The analysis of carbohydrates has been always hampered by their lack of UV absorbance above 200 nm, which is an especially challenging problem in capillary electrophoresis due to the very small (nl) sample volumes injected. The introduction of 2-aminopyridine as derivatizing agent allows sensitive direct UV detection of saccharides in the fmol range. However, due to the requirement of the presence of a free aldehyde group only aldoses and uronic acids can be determined. This limitation was recently overcome by means of precolumn derivatization withp-aminobenzoic acid or ethylp-aminobenzoate, which permits the analysis of fructose with a lower mass detection limit of 0.3 and 0.14 pmol, respectively. The detection limits for aldoses were even as low as 15 and 7 fmol. A more universal approach is the use of indirect UV detection, which permits the analysis of carbohydrates, including (1–2)-linked disaccharides and aldonic acids, at the lower pmol level without the need for derivatization.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

On-line calorimetry as a technique for process monitoring and control in biotechnology

This contribution reviews laboratory-scale investigations carried out on the usefulness of biological heat release measurements, as a means for monitoring and controlling the metabolic state of microbial cultures. Such studies are carried out in high-quality bench-scale calorimeters, but measuring heat generation rates by establishing energy balances ought to be applicable to large-scale bioreactors without resorting to sophisticated instrumentation. The signal received can either be interpreted by more qualitative correlation with the evolution of the culture, or it may be quantitatively exploited - together with other on-line measurements - in order to assess the rates at which various types of metabolisms proceed in the culture. The work described shows how this can be used to keep a culture in a desired metabolic state during fed-batch and transient continuous cultures of the yeast, S. cerevisae, and how a bacterial fed-batch culture can be controlled in order to optimize biosynthesis of an antibiotic.     

Closer relation between thermal analysis and calorimetry

Several DTA experiments followed by calorimetric works are reviewed here to emphasise the importance of complementary role of both techniques. The thermal analysis is advantageous in the sense that it gives quickly the overall view of thermal behaviour of a material under various conditions. Calorimetric work provides accurate heat capacity data which enable to derive thermodynamic functions including the enthalpy and entropy. The latter quantity is especially important in judging whether the material obeys the third law of thermodynamics. However, calorimetric work leads occasionally to an erroneous conclusion if the work is not preceded by thermal analysis performed under various conditions. Sometimes, quality of information obtained by DTA exceeds that obtained by laborious calorimetry.     

Pseudo-adiabatic calorimetry

A procedure is described for dealing with the error sources inherently present in any real calorimeter: work of powerP _s input from stirrer and possibly temperature sensor, and heat exchange at a rate ?G(T?T _e ) whereT andT _c are the temperatures of calorimeter and surroundings respectively. The constantsP _s andG are calculated from a period of thermal decay, and afterwards are used to correct the entire run. A calorimeter was designed with high thermal homogeneity and used in a test. The curve of calculated temperature exactly traces the heater energy, even after 5 h, with a standard deviation of about 1 mK. The relative error inC _p is less than 1/1000.     

Modulated differential scanning calorimetry

The Modulated Differential Calorimetry (MDSC) is applied to the determination of the reversibility in the cholesteryl chloride, which presents a cholesteric monotropic phase between the isotropic and crystalline states. The experimental modulation parameters that govern this method i.e. frequency, amplitude and heating/cooling rate, are determined. MDSC curves and complementary thermomicroscopical observations assign melting, crystallization and liquid cholesteric transition as non reversing, and clarification as reversing.     

Electrolysis in calorimetry

The procedure used by many electrochemists in calculating enthalpy in calorimetric measurements of electrolysis reactions is compared to a purely thermodynamic approach, using the data published by Fleischmannet al. [J. Electroanal. Chem., 287 (1990) 293.] as a case study.The set of excess values dH _ex/dt=dH _obs/dt -dH_calo/dt obtained with the former procedure was neither correlated to any of the experimental parameters nor to the set of values found using thermodynamics. The latter, smaller by factors of up to two orders of magnitude, are shown to follow an expression of the form dH _ex/dt=–kI exp (–E _a/RT) with an activation enthalpy of about 85 kJ·mol^–1. It is suggested that recombination of electrolysis gases may account for this.

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Zusammenfassung Das von vielen Elektrochemikern verwendete Verfahren zur Berechnung der Enthalpie in kalorimetrischen Messungen an Elektrolysereaktionen wurde unter Anwendung der von Fleischmann et.al. in einer Fallstudie [J. Electroanal. Chem., 287 (1990) 293.] veröffentlichten Angaben mit einer rein thermodynamischen Näherung verglichen.Eine Reihe von mit der ersten Methode erhaltenen überschu\werten dH _ex/dt=dH _obs/dt-dH _calc/dt korrelierte weder mit den experimentellen Parametern noch mit den entsprechenden, thermodynamisch gefundenen Werten. Letztere, um etwa zwei Grö\enordnungen kleinere Werte konnten durch die Gleichung dH _ex/dt= -kI exp (-E _a/RT) mit einer Aktivierungsenthalpie von etwa 85 kJ·mol^–1 beschrieben werden. Es wird deshalb nahegelegt, da\ dies einer Rekombinierung der Elektrolysegase zugeschrieben werden kann.

     

A feasibility analysis for alkaline membrane direct methanol fuel cell: thermodynamic disadvantages versus kinetic advantages

As the proton exchange membrane direct methanol fuel cell (PEMDMFC) faces sustaining obstacles, alkaline membrane direct methanol fuel cell (AMDMFC) is attracting increasing attention. Although some advantages may be expected, the feasibility of AMDMFC does not seem well verified. In this paper, thermodynamic disadvantages and kinetic advantages of AMDMFC are elucidated. In thermodynamic aspect, a large voltage loss due to the pH difference across the membrane is predicted by theoretical calculation; in kinetic aspect, besides the well-known superiority of alkaline media for oxygen reduction, experimental data show much higher anodic performance in carbonate/bicarbonate than in acid. In-situ FTIR measurements indicate that methanol can be fully oxidized to carbon dioxide in carbonate/bicarbonate as in sulfuric acid. Taking into account all the foreseeable advantageous and disadvantageous factors, AMDMFC is worth study, and an alkaline membrane stable at elevated temperatures is the prerequisite for a successful AMDMFC.     

Deformation calorimetry of polyurethane block-copolymers

A thermodynamic analysis of the uniaxial stretching of polyurethanes of various compositions and mechanical histories was carried out by using deformation calorimetry. The initial small strain deformations were found to result from the volume elasticity of the hard phase. The intramolecular energy contributions of the soft blocks were estimated. The hard block contributions were shown to depend on their content and on the degree of sample stretching. The predominant role of the soft component is proved to be manifested only in softened samples with a hard block content not exceeding 30%. The thermodynamics of the softening and hysteresis phenomena were studied. The dependence of the deformation mechanism on the hard block content and mechanical history is discussed.The authors express their thanks to Dr. A. R. Korigodsky and Dr. M. P. Letunovsky for the PU samples.     

Learning about calorimetry

Calorimetry deals with the energetics of atoms, molecules, and phases and can be used to gather experimental details about one of the two roots of our knowledge about matter. The other root is structural science. Both are understood from the microscopic to the macroscopic scale, but the effort to learn about calorimetry has lagged behind structural science. Although equilibrium thermodynamics is well known, one has learned in the past little about metastable and unstable states. Similarly, Dalton made early progress to describe phases as aggregates of molecules. The existence of macromolecules that consist of as many atoms as are needed to establish a phase have led, however, to confusion between colloids (collections of microphases) and macromolecules which may participate in several micro- or nanophases. This fact that macromolecules can be as large or larger than phases was first established by Staudinger as late as 1920. Both fields, calorimetry and macromolecular science, found many solutions for the understanding of metastable and unstable states. The learning of modern solutions to the problems of materials characterization by calorimetry is the topic of this paper.This work was financially supported by the Div. of Materials Res., NSF, Polymers Program, Grant # DMR 90-00520 and Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U. S. Department of Energy, under contract number DE-AC05-96OR22464. Support for instrumentation came from TA Instruments, Inc. Research support was also given by ICI Paints, and Toray Industries, Inc.     

The application of the reaction calorimetry to investigate reactions involving unstable compounds

The RC1 calorimeter revealed itself a suitable instrument to obtain information about safety and mechanisms involved in the reaction between cyclohexanecarboxylic acid (AEB) and oleum. A previous hypothesis about the existence of an unstable intermediate was confirmed and its heat of formation was calculated. The heat of sulphonation related to undesirable by-products production and the heat of protonation of AEB with H₂SO₄ were also evaluated. Therefore, it was possible to distinguish the reactions involved in the process and, through their thermal behaviour, to determine the limit conditions to avoid the by-products formation.

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Zusammenfassung Das RC1 Kalorimeter erwies sich als ein geeignetes Instrument, um Informationen über Sicherheit und Mechanismen bei der Reaktion von Cyclohexancarbonsäure (AEB) und Oleum zu gewinnen. Eine bereits bestehende Hypothese über die Existenz eines instabilen Zwischenproduktes konnte bestätigt und dessen Bildungswärme berechnet werden. Weiterhin wurde auch die Sulphonierungswärme bezogen auf die Entstehung von unerwünschten Nebenprodukten und die Protonierungswärme von AEB durch H₂SO₄ ermittelt. Es ist deshalb möglich, die einzelnen Reaktionen innerhalb dieses Prozesses über ihr thermisches Verhalten voneinander zu unterscheiden, um die Grenzbedingungen zur Vermei-dung der Bildung von Nebenprodukten zu ermitteln.

     

Time-domain and frequency-domain differential calorimetry

The heat capacityC _p of a sample can be considered as a frequency dependent quantity; its behaviour can reflect the dynamics of enthalpy fluctuations. In order to take into account the dynamic nature of the measured quantity, calorimetry can mimic experimental methods as those of dielectrometry, performing experiments in time domain or in frequency domain.In this paper, an instrument is presented which is based on a calorimetric method meeting these requirements, and thus allowing to study sample dynamics of very viscous systems as glasses and some supercooled liquids. Moreover, experimental procedures permitting investigation of samples undergoing chemical and/or physical transformations by simultaneous measurements of enthalpy variation, heat capacity and, under certain conditions, thermal conductivity, are discussed.     

Plant calorimetry: A window to plant physiology and ecology

The use of calorespirometry in developing models for biological growth is discussed. The relationships between respiratory heat, CO₂, and O₂ rates and physiological parameters such as substrate carbon conversion efficiency, and substrate and biomass carbon oxidation states are presented and discussed. A biochemical model relating ratios of the respiratory rates to the physiological parameters and biochemical pathways in plants is presented. Examples of applications of these models and equations to plant physiology and ecology are given.     

Determination of the heats of gasification of polymers using differential scanning calorimetry

The amount of heat that is required to gasify unit mass of material is one of the key properties that define its ignition resistance and fire response. Knowledge of this property is necessary to assess a material's fire hazard in a particular fire scenario. Nevertheless, even for the most common polymers the values of this property are not well established. Here we present a method for determining the heat of gasification using differential scanning calorimetry and apply this method to a set of ten common plastics and engineering polymers.     

Chip reactor for microfluid calorimetry

A microchip module with integrated flow channel and highly sensitive thin-film thermoelements of Bi_0.87Sb_0.13 and Sb was developed in order to realize a microcalorimeter for small volumes and flows in the μl and μl/min range, respectively. It was designed according to the principle of flow-injection analysis (FIA) and prepared using means of micromachining technology. The flow channel comprises two inlets, a mixing region, a measurement region and one outlet. In this way, the initiation of chemical reactions by interdiffusion takes place in the closest possible contact with the sensing elements. Three thermopiles (i.e. thermoelements connected in series) are arranged on a thin-film membrane covering the fluid channel. A thin-film heater of an NiCr alloy is integrated for internal electrical calibration. The microreactor was first tested in the neutralization reaction between sodium hydroxide and sulphuric acid for different NaOH concentrations, flows, and sample volumes, investigating for the present the integral signal of the three thermopiles connected in series. From the collected data, the minimum detectable power, minimum detectable NaOH concentration, and the reaction enthalpy were calculated. The results characterize the module as a sensitive chip calorimeter, showing the suitability of the micromachining technology for calorimetry.