Measurement and correlation of solubility of thiourea in two solvent mixtures from T = (283.15 to 313.15) K
Introduction
Thiourea (CAS Registration No. 62-56-6, figure 1) is a very important industrial raw material, which is widely used in producing sulfonamide, dyestuff and resin [1], [2], [3]. Also, it can serve as vulkacit for rubber and flotation agent for metallic mineral, etc. It is usually obtained by mixing sulfide (H2S) and calcium cyanamide (CaCN2) together at low temperature to get calcium hydrosulfide firstly. Then calcium hydrosulfide is reacted again with calcium cyanamide (CaCN2) at a comparatively higher temperature to get thiourea. The final product is acquired by cooling crystallization.
During the manufacturing process of thiourea, cooling crystallization is the key step that will directly determine the quality of final product, such as purity, crystal habit and size distribution. Many methods can be adopted to control and optimize the cooling crystallization process. Among them, selection of proper solvent is one of the most important factors that will greatly influence cooling crystallization process. And basic physicochemical information about its solubility in different solvents is essential for the selection of solvents. Therefore, it would be of great significance to determine the solubility of thiourea in different solvents and investigate the effects of temperature and solvent composition on solubility systematically. Methanol, ethanol and n-propanol were the common solvents used in industrial crystallization. In this paper, solubility data of thiourea in methanol + ethanol mixtures and methanol + n-propanol mixtures were determined from T = (283.15 to 313.15) K by gravimetric method under atmospheric pressure. Three thermodynamic models were adopted to correlate the experimental solubility data of thiourea in two solvent mixtures. In addition, by analyzing the temperature dependence of solubility data, thermodynamic properties of dissolving and mixing process of thiourea were obtained. To further understand the dissolving process, molecular simulation was adopted to investigate the dissolving behaviors of thiourea from molecular level. These data can not only conduct us in operation of the crystallization process but also help us to understand the dissolving mechanisms and driving force of the dissolving process of the thiourea.
Section snippets
Materials
Thiourea was supplied by Yifeng Co. Ltd (Shandong, China) and was further purified by cooling crystallization. Purity of the final product was above 99.5% in mass fraction. Methanol, ethanol and n-propanol, which are all analytical grade (mass fraction ⩾ 0.995), were purchased from Jiangtian Chemical Co., Ltd (Tianjin, China). Detailed information of the materials is listed in table 1.
Characterization
Powder X-ray diffraction (PXRD) was adopted to identify the polymorphic forms of thiourea, and the measured
The modified Apelblat equation
The modified Apelblat equation, which is deduced from the Clausius–Clapeyron equation [17], shows the relationship between the mole fraction of solute and temperature. A and B stand for the variation of the activity coefficient in solution, and C shows how the enthalpy of fusion is affected by temperature. And the modified Apelblat equation is expressed as equation (5): where x1 stands for mole fraction of solute while T stands for absolute temperature.
λ–h equation
The λ–h equation, with only
Characterization and Identification of thiourea
PXRD was used to characterize the sample before and after the experiments. The result (figure 2) showed that crystal forms of the solute did not change during the experiment.
Melting properties could be obtained from the differential scanning calorimetry (DSC) (figure 3). The instrument was calibrated by using the phase-transition temperature and phase-transition enthalpy reference materials (and the onset temperature was chosen as the melting temperature. indium: ΔfusH = 3266.57 J · mol−1, Tm = 429.75
Conclusions
Solubility data of thiourea in two binary solvent systems were experimentally determined from T = (283.15 to 413.15) K. Effects of solvent composition and temperature on solubility data were analyzed. It is shown that the solubility data of thiourea increases as the temperature and the mole fraction of the methanol increases. And the solubility data in methanol + ethanol mixtures are greater than those in methanol + n-propanol mixtures at the same mole fraction of methanol and temperature. Molecular
Notes
The authors declare no competing financial interest.
Acknowledgments
The authors are very thankful to the National Science Foundation of China (No. 21376165) and Program of International S&T Cooperation from China Ministry of Science and Technology (No. 2013DFE43150) for the financial support.
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