Determination and thermodynamic modelling for 4-nitropyrazole solubility in (methanol + water), (ethanol + water) and (acetonitrile + water) binary solvent mixtures from T = (278.15 to 318.15) K

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Highlights

  • 4-Nitropyrazole solubility in three binary mixed solvents was determined.

  • The determined solubility were correlated and calculated by six co-solvent models.

  • Standard dissolution enthalpy for 4-nitropyrazole in mixed solvents were calculated.

Abstract

The solubility of 4-nitropyrazole in mixed solvents of (methanol + water), (ethanol + water) and (acetonitrile + water) was determined experimentally by using the isothermal dissolution equilibrium method within the temperature range from (278.15 to 318.15) K under atmosphere pressure (101.3 kPa). The solubility of 4-nitropyrazole increased with increasing temperature and mass fraction of methanol for the (methanol + water) system. For the systems of (acetonitrile + water) and (ethanol + water), at a certain composition of acetonitrile or ethanol, the solubility of 4-nitropyrazole increased with the increase in temperature; nevertheless at the same temperature, they increased at first and then decreased with the increase in the mass fraction of acetonitrile or ethanol. At the same temperature and mass fraction of methanol, ethanol or acetonitrile, the solubility of 4-nitropyrazole in (acetonitrile + water) was greater than those in the other mixed solvents. The solubility values obtained were correlated with the Jouyban–Acree model, van’t Hoff–Jouyban–Acree model, modified Apelblat–Jouyban–Acree model, Ma model, Sun model and CNIBS/R-K model. For the first five models, the maximum of the relative average deviation (RAD) and the root-mean-square deviations (RMSD) were 4.51 × 10−2 and 3.37 × 10−3, respectively. The Ma model was proven to provide good representation of the experimental solubility results. On the basis of the measured solubility, the standard dissolution enthalpy of the dissolution process was calculated. Dissolution of 4-nitropyrazole in these mixed solvents is an endothermic process. The experimental solubility and the models in this study could be helpful in purifying the crude 4-nitropyrazole.

Introduction

Nitropyrazoles have drawn renewed attention by explosive chemists due to their positive heat of formation, good thermal stability and higher density [1]. 4-Nitropyrazole (CAS Reg. No. 2075-46-9) is a kind of low detonation velocity and high detonation pressure explosive [2], which has a very promising future in the field of energetic materials. In addition, a number of metal (Cd, Zn, Mn, Co, Ni and Cu) salts of 4-nitropyrazole (HNO2Pz) are identified and characterized by chemical analysis, infrared and ligand field spectra [3]. 4-Nitropyrazole is also used as raw material to synthesize 4-amino-3,5-dinitropyrazole which is a very useful intermediate for the preparation of trinitropyrazole [4], [5]. The production process of metal salts of 4-nitropyrazole or trinitropyrazole generally requires 4-nitropyrazole with higher purity. The purity of 4-nitropyrazole affects not only the yield and purity of metal salts or trinitropyrazole but also the reaction rate. At present, 4-nitropyrazole is prepared by reacting of pyrazoles with nitric acid, nitric acid-sulfuric acid, or nitric acid-trifluoroacetic anhydride mixtures [6], [7], [8], [9]. The reaction product contains a small amount of isomers of 4-nitropyrazole and some unknown by-products. In order to extend the application of 4-nitropyrazole, it is required to isolate and purify the crude product.

It is well known that solvent crystallization is used as an important separation and purification step in the production process. The solubility of a solid in different solvents is an important physicochemical property which plays an important role for understanding the (solid + liquid) equilibrium (SLE) or phase equilibrium in the development of a crystallization process. More particularly, the knowledge of thermodynamic parameters, especially accurate solubility is needed for the design of crystallization processes. The solubility also plays an important role during the separation process and determination of proper solvents via solvent crystallization. In previous publications, solvent crystallization method has been proposed to separate 4-nitropyrazole from the resulting reaction mixture [10], [11], [12]. However, to the best of the authors’ present knowledge, no solubility data are reported in the previous works. In order to acquire high purity of 4-nitropyrazole, the knowledge of 4-nitropyrazole solubility in different solvents at various temperatures and the thermodynamic properties of solution is a necessary procedure.

The purposes of this work are to (1) determine the solubility of 4-nitropyrazole in (methanol + water), (ethanol + water) and (acetonitrile + water) at temperatures ranging from (278.15 to 318.15) K by using the isothermal saturation equilibrium method; (2) correlate the solubility data with different models; (3) calculate the standard dissolution enthalpy for the solution process of 4-nitropyrazole acid in different binary solvents.

Section snippets

Materials and apparatus

4-Nitropyrazole having a mass fraction of 0.975 was purchased from Zhenjiang Jingjing Chemical Co., Ltd., China. It was purified by recrystallization in (methanol + water) mixtures at normal temperature. The final purity of 4-nitropyrazole used for solubility determination was 0.994 mass fraction, which was confirmed by a high-performance liquid chromatography (HPLC). The solvents, including methanol, ethanol and acetonitrile with analytical grade were provided by Sinopharm Chemical Reagent Co.,

Solubility results

Table 2, Table 3, Table 4 contains values of the measured mole fraction solubility of 4-nitropyrazole in the binary solvent mixtures of (methanol + water), (ethanol + water) and (acetonitrile + water), respectively. For comparison with the experimental points, the plots of corresponding mole fraction solubility against temperature and solvent composition are plotted in Fig. 2, Fig. 3, Fig. 4 as well. It is seen from Table 2, Table 3, Table 4 that, for the mixed solvents studied, the solubility of

Conclusions

The solubility of 4-nitropyrazole in three binary solvent mixtures of (methanol + water), (ethanol + water) and (acetonitrile + water) with various composition was obtained experimentally by using the isothermal dissolution equilibrium method at temperatures ranging from (278.15 to 318.15) K under 101.3 kPa. For the three binary solvent mixtures, with the increase in temperature, the solubility of 4-nitropyrazole increased. At fixed temperature, the solubility of 4-nitropyrazole in (methanol + water)

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