Calibration and testing of an isoperibolic micro-combustion calorimeter developed to measure the enthalpy of combustion of organic compounds containing C, H, O and N

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Abstract

In order to obtain reliable data of the standard enthalpy of combustion of compounds containing carbon, hydrogen, oxygen, and nitrogen atoms, an isoperibolic micro-combustion calorimeter has been developed from a 22 cm3 1109A Parr semi-micro oxygen bomb. The calorimeter was calibrated with standard benzoic acid and the resulting energy equivalent was ε(calor) = (1497.39 ± 0.37) J · K−1, which means an uncertainty of 0.027%. Combustion measurements using salicylic acid and 1,2,4-triazole were made in order to verify the accuracy of the device. The values of −Δcu° at T = 298.15 K for the compounds were (21877.2 ± 4.6) J · g−1, and (19217.7 ± 1.9) J · g−1, respectively, in agreement with the literature values.

Highlights

► Calibration and testing of a isoperibolic micro-combustion calorimeter. ► A micro-combustion calorimeter with a high sensibility. ► A micro-combustión calorimeter suitable for determining combustión energy of 30 mg solid samples. ► Determination of ΔcUmo, ΔcHmo and ΔfHmo of the salicylic acid and 1,2,4-triazole.

Introduction

Calorimeters have been widely used to determine the combustion enthalpy of compounds containing carbon, hydrogen, oxygen, and nitrogen and rotating-bomb combustion calorimeter for compounds containing sulfur or halogens [1], [2], [3], [4], [5], [6], [7], [8]. However, this technology requires the use of tablets of about 1 g, restricting the thermochemical studies to relatively large amounts of substances. Thus, traditional calorimeters, as they are considered, belong to the calorimetry of macro–combustion.

Nevertheless, with the recent technological advances, semi-micro combustion calorimeters have able to carry out experiments of combustion with high precision; this allows the thermochemical study of small amounts of substances of recent synthesis, compounds that are very difficult to obtain and purify in large quantities.

In a micro-calorimeter, the size of the bomb is diminished. The calorimetric vessel and the water that covers the bomb are necessary to measure the small temperature increase during combustion. The reduction of water volume makes it possible to use milligrams of a given substance, described by several authors [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. In the calorimeter scaling down process, the reduction of the calorimetric equivalent does not have to affect the accuracy of the combustion energy measurement; it is comparable to the measurements obtained in macro-bomb combustion calorimeters.

Herein, we present an isoperibolic micro-bomb combustion calorimeter, which was designed and assembled from commercially available materials. This device will be used to measure of the energy of combustion of new organic compounds containing C, H, O and N. The calorimeter was calibrated with benzoic acid, recommended as a primary standard. Later, to verify the combustion and the accuracy in the energy of combustion of compounds containing C, H, O, the calorimeter was tested with salicylic acid. Finally, the energy of combustion of 1,2,4-triazole, a secondary standard for compounds containing nitrogen, was measured. The combustion energy results provided by our calorimeter, using a mass of substance of about 30 mg per experiment, showed it to be as accurate as the conventional method and well tested macro-combustion calorimeters.

Section snippets

Calorimeter assembly

The main parts that constitute the isoperibolic micro-bomb combustion calorimeter are shown in figure 1.

The base of the design is the size and the geometrical form of a semi-micro oxygen bomb Parr 1109A A, the vessel B (diameter = 60 mm, height = 120 mm) and the jacket C (diameter = 75 mm, height = 150 mm) constructed in chromium-plated brass. At the bottom of the jacket, three supports of Nylamid D avoid direct contact between the jacket and the vessel. The external jacket was constructed over a brass

Experimental procedure

The energy equivalent of our micro-bomb combustion calorimeter was determined using benzoic acid NIST Standard Material Reference 39j, with a massic energy of combustion under certified conditions of Δcu = −(26434 ± 3) J · g−1. Nevertheless, we use the value of Δcu = −(26414 ± 3) J · g−1 because there are significantly high departures from the certification conditions in the micro-combustion calorimeter [24]. The best conditions in mass of substance and temperature of the calorimetric system were determined

Results and discussion

The calorimeter micro-combustion was calibrated through eleven combustion experiments with benzoic acid. The energy equivalent of our calorimeter was determined as ε(calor) = (1497.39 ± 0.37) J · K−1. The uncertainty associated with the average value of the energy equivalent is the standard deviation of mean. The equation (1) and (2) were used in the calculation of the calorimetric equivalent and the internal energy change for the isothermal bomb process, respectively, where ΔTc is the corrected

Conclusions

An isoperibolic micro-combustion calorimeter with a highly competitive sensibility compared with other similar calorimeters has been constructed. As the standard deviation of the mean shows in the measurements of combustion standard benzoic acid, the reproducibility of the calibration experiments attain a value within 0.027%. The value of energy of combustion from salicylic acid and 1,2,4-triazole obtained with the present calorimeter agrees with the values reported in the literature. These

Acknowledgements

Thanks to PROMEP for financial support throughout the project 103.5/10/7342. Thanks to Dr. Henoc Flores Segura for advices and support in my studies of doctorate and thanks to Dr. Aitor Aizpuru for the revision of the manuscript.

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