Determination and correlation of solubility of N-methyl-3,4,5-trinitropyrazole (MTNP) in ten pure solvents from 283.15 K to 323.15 K
Introduction
Crystallization [1] refers to the process of coagulation from a saturated solution, or the formation of a solid (crystal) with a certain geometry from the gas. In the natural environment, the role of temperature dropped will cause crystallization. Crystals are easy to wash when they are larger and regular. The MTNP particles obtained by synthetic reaction [2], [3] in our own laboratory have low purity and irregular morphology, so the performance of MTNP can not be shown when used directly. For industrial production, crystallization is a very effective means to purify the prepared product. However, the determination of solubility not only it is valuable to select the proper solvent and determine reasonable time for crystallization but also it is convenient and time consuming. So the data is significant to crystallize as well. Besides, it can provide a crucial theoretical basis for industrial production. So far, because of few data of the solubility of MTNP, this work has become more critical and meaningful.
N-methyl-3,4,5-trinitropyrazole (MTNP, C4H3O6N5, Fig. 1), a light yellow crystal, is one of the new insensitive and high energy single explosive. The crystal density, corresponding detonation velocity and the detonation pressure of MTNP is 1.83 g cm−3, 8690 m s−1 and 33.54 GPa, respectively [4]. The study showed that the corresponding detonation velocity is 1800 m s−1 higher than 2-methyl-1,3,5-trinitrobenzene (TNT) and the detonation pressure is 13 GPa higher than TNT. Moreover, MTNP is similar to cyclotrimethylenetrinitramine (RDX) in detonation properties and it is close to Composition B (the mixed explosives of TNT and RDX) in sensitivity. TNT, RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) were replaced by MTNP in many explosive formulations and it is a good substitute as a cast explosive [5]. Dalinger et al. [6] have studied the single crystal structure and spatial structure showed that MTNP is an orthorhombic crystal, space group is Pn2(1)/c and the unit cell parameter is a = 11.919(5) Å, b = 8.352(3) Å, c = 8.477(3) Å, V = 843.9(6) Å3, Z = 4, D = 1.709 g cm−3. Because of a great overall performance, it is worth to vigorously study.
At present, many studies have focused on the synthesis of N-methyl-3,4,5-trinitropyrazole, but only a few have attempted to establish purification methods to obtain products with high purity and yields. In this study, the solubility of MTNP in ten pure solvents were measured at temperatures from (283.15 K–313.15 K) by a gravimetric method, which is the essential information needed for operations and optimization of crystallization procedures [1]. In order to extend the application of the solubility, data of the experimental solubility of MTNP were correlated by the modified Apelblat equation, λh equation, the ideal model and the polynomial empirical equation. The decomposition temperature Tm was determined using differential scanning calorimetry (DSC). From the experimental values, it is easy to select the most suitable crystallization solvent for MTNP.
Section snippets
Materials
A light yellow powder of MTNP in this experiment was obtained [9] with N-methyl pyrazole nitrated at 90 °C for 6 h in our own laboratory. The reaction equation is as follows:
The purity of MTNP is 0.990 (mass fraction) and determined by HPLC (Fig. 2) was purified by benzene. Benzene, methanol, ethanol, n-propanol, isopropyl alcohol, n-butanol, isobutyl alcohol, n-pentanol, isoamyl alcohol were of analytical grade with mass percentage purity higher than 99.5% and purchased from a local reagent
The modified Apelblat equation
The modified Apelblat equation depicted the relationships between temperature and solubility is deduced from the Clausius-Clapeyron equation and is used to semiempirical model extensively. The equation can be expressed as [10], [11]:where A, B, C are the empirical constants and T is the absolute temperature. The values of A and B reflect the variation of the activity coefficient, C represents the effect of temperature on enthalpy of fusion [12]. Table 2 contains data of the
Discussion
Combining the data in Table 6 and in Fig. 4, it could be found clearly that the experimental data are close to the calculated values and the solubility of MTNP is a function of temperature and increases with increasing temperature in each pure solvent. It can be described that as the activation energy of the solute molecules increasing with increasing temperature, larger solute molecules are more likely to become smaller solute molecules, and it is easy to combine with solvent molecules.
Calculation of thermodynamic properties
In this work, we can calculate some thermodynamic properties such as the standard dissolution enthalpy, standard dissolution entropy and the standard Gibbs free energy. And the equation can be expressed as [23], [24]:Here R represents the gas constant (8.3145 J·K−1·mol−1), and represent the standard dissolution enthalpy and standard dissolution entropy, respectively. By comparison with the ideal equation, we can observe that and can be
Conclusion
The solubility of MTNP in ten pure solvents were firstly determined by the gravimetric method with temperature ranging from 283.15 K to 323.15 K under atmospheric pressure. For all solvents, the solubility of MTNP increases with the increasing temperature. The solubility fits the following order from high to low: benzene > methanol > ethanol > n-propanol > n-butanol > n-pentanol > isopropyl alcohol > isoamyl alcohol > isobutyl alcohol > water. As the temperature ranges from 283.15 K to
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