Raphaël Sabbah

Reference materials for calorimetry and differential thermal analysis

This publication is the result of the efforts of the ICTAC working group “thermochemistry” 1 during 1997–1998. It deals with reference materials (in abbreviated form: RM) for calorimetry and differential thermal analysis. It represents the updated version of two previous documents produced by the IUPAC Commission “Physicochemical Measurements and Standards”: the first was published in Pure and Applied Chemistry in 1974 [1] and the second in the book entitled “Recommended Reference Materials for the Realization of Physicochemical Properties” [2]. Calorimetry and differential thermal analysis are applicable to a wide range of scientific and technological research fields involving physical, chemical and biological processes. Calorimetry usually yields highly reproducible results which may, however, be inaccurate because of faulty calibration of the measurement system. Calibration is a fundamental requirement for every thermoanalytical study. It requires the establishment of a quantitatively defined relationship between the value indicated by the measuring instrument and the correct value. The calibration of a modern calorimeter is achieved by the quantification of the produced

Thermodynamique de composes azotes III. Etude Thermochimique de la glycine et de la l-α-alanine

Abstract The enthalpies of formation of glycine and L-α-alanine are determined by two complementary techniques: combustion calorimetry and change of state (sublimation) calorimetry. The values of the experimental and theoretical energy of conjugation of these two molecules (partially conjugated) and the energetical value of the CN bond in Laidlers scheme, derived from the experimental enthalpy of atomization, are also given.

Quelques reflexions a propos de la mesure calorimetrique de l'enthalpie de sublimation ou vaporisation

Abstract In order to determine the enthalpy of sublimation or vaporization of a compound with a vapour pressure between 10−5 and some tens of Torr by calorimetry in the range 298–498 K, an experimental set-up associated with a Tian-Calvet calorimeter is described. With regard to a previously described apparatus, this set-up is of differential type and presents many advantages, especially in the practical field. The reproducibility and accuracy of the first results obtained are satisfying. A careful study of the influence of the conditions on the evaluation of enthalpy of sublimation or vaporization is also done. The results obtained with some classical compounds illustrated the influence of vaporization rate on the value of Δb or vapH(T).

Mesures calorimétriques des enthalpies de vaporisation et de sublimation par effusion; mise au point de la technique

Abstract The association of an effusion cell to a Tian-Calvet microcalorimeter offers the possibility of measuring, at 298.15 K, energies of vaporisation (or sublimation) of compounds, the vapour pressures of which range from 10−4 to 100 torr, with a reproducibility of 1%. The following results are obtained for enthalpies of vaporisation [(ΔHvap)m] of water (vapour pressure, 23.7 torr at 298.15 K), 43.84±0.29 kJ.mol−1; cyclohexane (vapour pressure, 97.5 torr at 298.15 K), 32.89±0.29 kJ.mol−1; hexamethylphosphotriamide (vapour pressure, 51.10−3 torr at 298.15 K), 61.1± 1.7 kJ.mol−1. In the present work, a critical study of the technique is also made.

Utilisation du microcalorimetre CRMT en calorimetrie de combustion

Abstract From several series of expriments using, successively, benzoic, salicylic and succinic acids, the authors study the possibility of using the CRMT calorimeter for combustion calorimetry. Based on experimental results, the discussion emphasizes possible errors in microcombustion calorimetry in general, and more particularly in the case of using a CRMT calorimeter. Means of avoiding such errors are proposed.

Thermodynamique de substances soufrees. IV. Etude thermochimique de la thiouree, de la thiosemicarbazide et de la thiocarbohydrazide

Abstract The thermochemistry of thiourea, thiosemicarbazide and thiocarbohydrazide has been studied by combustion and sublimation calorimetry, using a rotative bomb isoperibol calorimeter in the first case, and a Tian—Calvet calorimeter equipped with a Knudsen effusion cell in the second. These investigations enabled the determination of the corresponding enthalpies. Substance − △H°c(c, 298.15 K) (kJ mole−1) △H°sub(c, 298.15 K) (kJ mole−1) Thiourea 1478.03±0.45 112.0±1.5 Thiosemicarbazide 1712.43±0.54 125.8±1.5 Thiocarbohydrazide 1961.05±0.28 152.1±3.0 The experimental values of the conjugation of these molecules are also given, and a comparison with the theoretical values is discussed. The CS bond energy was calculated from the experimental enthalpy of atomization of thiourea. The use of these value to determine △H°a,calc (298.15 K) for the thioamides studied resulted in excellent agreement with △H°a,exp (298.15 K).

Thermodynamique de composes azotes. X. Etude thermochimique de quelques acides ω-amines

Abstract In order to determine energetical values for intra- and intermolecular bonds in ω-amino-acids which have the zwitterion configuration within the crystal lattice H 3 N + -(CH 2 ) n -COO − , five compounds of this family for which n = 2, 3, 4, 5 and 7 were studied by calorimetry. The energies for Van der Waals and hydrogen bonds which link together the molecules in the crystal were determined from the values of the enthalpy of sublimation. A comparison of the experimental values for the enthalpy of formation of these compounds in the gaseous state with the values calculated using Bensons scheme, showed that ω-aminoacids take the molecular structure in the vapour phase, resulting from a proton transfer which occurs in the crystalline state between two molecules linked together with a hydrogen bond ( N + -H…Ō → N…H-O) with rearrangement of the electronic charges in these molecules.

Thermodynamique de substances soufrees. II. Etude thermochimique de la L-cysteine et de la L-methionine

Abstract The enthalpies of combustion in the condensed state of L -cysteine and L -methionine have been determined using an isoperibol calorimeter equipped with a rotating bomb. The values of these enthalpies are, respectively, −2248.84 ± 0.55 kJ mole −1 and −3564.11 ± 0.61 kJ mole −1 . The determination of the enthalpy of sublimation of L -cysteine has not been possible because this substance sublimes with difficulty, even under a residual pressure of about 10 −5 Torr. Moreover, its temperature of decomposition, which is situated around 450 K under atmospheric pressure, is lowered when the pressure is reduced. On the contrary, with some precautionary measures indicated in the text, the determination of the enthalpy of sublimation of L -methionine at 298.15 K has been possible, and is equal to Δ H 0 sub (298.15 K) = 164 ± 4 kJ mole −1 . The L -methionine molecule contains CS bonds. From its experimental enthalpy of atomization, it has been possible to propose a value for the enthalpy of these bonds.

New reference materials for the calibration (temperature and energy) of differential thermal analysers and scanning calorimeters

Several organic compounds like phenanthrene, benzanilide, anisic acid, triphenylene, 2-chloroanthraquinone, hexachlorobenzene, carbazole, 4-iodobenzoic acid, perylene, anthraquinone, as new reference materials for temperature and energy calibration of DTA and DSC apparatuses were studied in the range 300—600 K.ZusammenfassungAls neue Referenzsubstanzen für Temperatur- und Energiekalibrierung von DTA- und DSC-Geräten wurden im Temperaturbereich 300–600 K einige organische Verbindungen, wie z.B. Phenanthren, Benzanilid, Anissäure, Triphenylen, 2-Chloranthrachinon, Hexachlorbenzol, Karbazol, 4-Jodbenzoesäure, Perylen und Anthrachinon untersucht.

Etude thermodynamique des chlorobenzenes

Abstract The enthalpies of sublimation and fusion of all the tetrachlorobenzene isomers, and of pentachloro- and hexachloro-benzenes, and their triple point temperatures have been determined by sublimation calorimetry and differential thermal analysis. Using these thermodynamic quantities and corresponding values for the other chlorobenzenes (taken from the literature) we could explain, in some cases, the difference between the thermodynamic values of isomers and the molecular interactions in chlorobenzenes.