Heat capacity and standard molar enthalpy of formation of crystalline 2,6-dicarboxypyridine (C7H5NO4)

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Abstract

Low-temperature heat capacity Cp,m of 2,6-dicarboxypyridine (C7H5NO4; CAS 499-83-2) was precisely measured in the temperature range from (80 to 378) K with a high precision automated adiabatic calorimeter. No phase transition or thermal anomaly was observed in this range. The thermodynamic functions [HT  H298.15] and [ST  S298.15] were calculated in the range from (80 to 378) K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, ΔcHm(C7H5NO4,cr)=-(2741.41±0.49)kJ·mol-1 and ΔfHm(C7H5NO4,cr)=-(727.74±1.50)kJ·mol-1, by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The thermodynamic properties of the compound were further investigated through differential scanning calorimeter (DSC) and the thermogravimetric (TG) analysis.

Introduction

2,6-Dicarboxypyridine is present in nature as an oxidative degradation product of vitamins, coenzymes and alkaloids, and it is also a component of fulvic acids [1], [2]. 2,6-Dicarboxypyridine is a necessary raw material for synthesizing 2,6-diacetylpyridine, which has widely been used in organic synthesize as an important intermediate compound [3]. Moreover, 2,6-dicarboxypyridine has also been used as a template molecule for the synthesis of molecular imprinted polymers for use in sample preparation with biological materials [4]. However, few data on thermodynamic properties of the compound are reported till now. In order to improve the process of chemical synthesize and increase understanding of the compound, the thermodynamic properties of the compound have been studied in detail in the present research.

The molecular structure of 2,6-dicarboxypyridine is:

In this paper, the low-temperature heat capacity of the compound over the temperature range (80 to 378) K was measured by automated adiabatic calorimeter and the standard molar enthalpies of combustion of the compound at T = 298.15 K was determined by oxygen-bomb combustion calorimeter. In addition, the thermal stability of the compound was examined by thermogravimetry (TG) and differential scanning calorimetry (DSC).

Section snippets

Sample

The 2,6-dicarboxypyridine is a white crystal. The sample for the present calorimetric study was purchased from ACROS ORGANICS Company. The labeled purity is >0.980 mass fraction. The sample was purified by recrystallizing three times using ethanol with an analytical grade prior to the calorimetric experiments. The structure of the compound was determined by IR, 1H n.m.r. and 13C n.m.r., and the purity of the sample was analyzed by high-performance liquid chromatography (HPLC) to be more than

Heat capacity

The experimental molar heat capacities of 2,6-dicarboxypyridine are shown in figure 1 and tabulated in table 1. The molar heat capacities were fitted to the following polynomial in reduced temperature (X), by means of least-square fitting.

Over the temperature range from (80 to 378) K,Cp(J·K-1·mol-1)=113.18+102.42X+30.749X2-14.967X3where X = (T  229)/149, and T is the absolute temperature. The correlation coefficient of the fitted curve, R2 = 0.9997.229 was obtained from half the sum of the upper

Acknowledgement

The authors gratefully acknowledge the National Natural Science Foundation of China for financial support to this work under the NSFC Grant No. 20373072.

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