Construction, calibration and testing of a micro-combustion calorimeter

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Abstract

An isoperibolic micro-combustion calorimeter was designed, built and set up in our laboratory, taking as base a 1107 Parr combustion bomb of 22 cm3 of volume. Taken into account the geometrical form of the bomb, it was designed and constructed a vessel and a submarine chamber in brass. All of the pieces of the calorimeter were chromium-plated to reduce heat loss by radiation. The calorimeter was calibrated by using pellets of standard benzoic acid (mass approximate of 40 mg) leading to the energy equivalent of ε(calor) = (1283.8 ± 0.6) J · K−1. In order to test the calorimeter, combustion experiments of salicylic acid were performed leading to a value of combustion energy of Δcu = −(21,888.8 ± 10.9) J · g−1, which agrees with the reported literature values. The combustion of piperonylic acid was carried out as a further test leading to a value of combustion energy of Δcu = −(20,215.9 ± 10.4) J · g−1 in accordance with the reported literature value. The uncertainty of the calibration and the combustion of salicylic acid and piperonylic acid was 0.05%.

Introduction

The knowledge of the formation enthalpies and energies of any molecule is very useful in the determination of molecular structure and energetic relations. Today, the organic synthesis leads to obtain very small amounts of substance, which with conventional calorimetric techniques such as macro-combustion calorimetry is practically impossible to carry out a systematic thermochemical study of this kind of compounds.

During the last years, attention was paid to the miniaturization of the calorimetric techniques well-tested until today, with the objective of carrying out combustion experiments with small amounts of samples without precision loss [1], [2], [3], [4], [5], [6], [7]. In the micro-calorimeter the samples are small; thus the energy quantity will be small too. Parallel to the design and building of the calorimeter, the devices of measurements have a high sensitivity in order to register small changes of temperature.

In order to reach accuracy of the measurements when the quantity of sample is reduced, it is necessary to reduce the energy equivalent of the calorimeter. Last condition is achieved by using a minimum of water necessary to cover the combustion bomb inside the calorimetric vessel.

The present work describes the design, construction, and testing of a micro-combustion calorimeter suitable for determining combustion energy of about 40 mg solid samples. As in our previous work about the designing of a combustion calorimeter [8], the present instrument has a precision similar to those well-tested ones.

Section snippets

Construction of calorimeter

Figure 1 shows the main parts that constitute the instrument developed in our laboratory.

The construction of the calorimeter started from a modified 1107 Parr combustion bomb A, which has an internal volume of 22 cm3. The heights of the gas filling and the terminal nut for ignition in the combustion bomb were shorten in order to reduce the amount of water in the calorimetric vessel.

Taken into account the size and the geometrical form of the combustion bomb, the calorimetric vessel B and the

Combustion experiments

For calibration of the calorimeter, benzoic acid (NIST SMR 39j, Δcu = −(26,434 ± 3) J · g−1 under certificate conditions) was used. Due to in the micro-combustion calorimeter there are significantly high departures from the certification conditions, it was necessary to use the combustion standard massic energy of benzoic acid, Δcu = −(26,414 ± 3) J · g−1 [9], and the appropriate Washburn corrections, in order to determine the energy equivalent.

Salicylic acid, used to test the instrument, was supplied by

Results and discussion

Table 2 shows the results obtained for the energy equivalent of the calorimeter, where the uncertainty is the standard deviation of the mean.

From nine combustion experiments of benzoic acid, the calorimetric equivalent was determined as ε(calor) = (1283.8 ± 0.6) J · K−1, by using the following equations:ε(calor)(-ΔTc)=ΔUIBP-ε(cont)(-ΔTc)-ΔUign,ΔUIBP=mΔcu(benzoic acid)+mΔcu(cotton)-ΔUΣ.All the uncertainties of the present work (except for the calorimetric equivalent) are the final overall standard

Conclusions

The design and construction of an isoperibolic micro-combustion calorimeter (vessel, jacket, lid, and heater) were made starting from commercial materials and pieces easily machined. The constructed calorimetric system with a sensitivity of (1283.8 ± 0.6) J · K−1 works with a mass of 40 mg of solid sample.

The values of energy of combustion of salicylic acid and piperonylic acid obtained from determinations with the present calorimeter agree with previous reported determinations.

The reproducibility of

Acknowledgements

Thanks are to CONACyT for financial support throughout the Project No. G3221-E. Thanks to Dr. Aarón Rojas (CINVESTAV México) for making the modifications of the combustion bomb and the platinum crucible. Thanks to Dr. M. Victoria Roux (CSIC Spain) for comments and suggestions to the present work and for the revision of the manuscript.

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