Solubility of RbCl and CsCl in pure water at subzero temperatures, heat capacity of RbCl(aq) and CsCl(aq) at T = 298.15 K, and thermodynamic modeling of RbCl + H2O and CsCl + H2O systems
Graphical abstract
Introduction
Rubidium (Rb) and cesium (Cs) are rare alkali elements. For many applications, e.g. DNA separation, fiber optics, lamps, night version devices, standards for atomic absorption analysis and atomic clocks, Rb and Cs are irreplaceable [1]. Carnallite deposits are the major source for the production of Rb and Cs. Isomorphic, or isodimorphic co-crystallization (i.e. formation of mixed crystals) of isostructural RbCl, CsCl and KCl, and RbBr, CsBr and KBr, and of isostructural carnallite and brom-carnallite double salts of K, Rb and Cs (MX·MgX2·6H2O; M = K,Rb,Cs; X = Cl,Br) is the only reason why Rb and Cs rich deposits are always enriched with potassium [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Thermodynamic properties of Rb and Cs containing aqueous solutions and information on phase behavior of these chloride salt-water systems, especially containing Na, K and Mg simultaneously, are essential for the development of Rb and Cs enriching process from natural brines and salt evaporates. Temperature is one of the most significant factor on the formation of solid solution and the solid solubility. Generally, the solid solubility would decrease when reduce the crystallization temperature, and fractional crystallization at low temperature may be an effective method for concentrating of Rb and Cs in aqueous phase. Therefore, phase diagrams and thermodynamic models of NaCl + H2O, KCl + H2O, RbCl + H2O, CsCl + H2O, MgCl2 + H2O and their mixing systems at various temperatures are essential for understanding the crystallization behavior of these chloride salts and their solid solutions during evaporation crystallization and freezing crystallization processes.
Extensive experimental and thermodynamic modeling studies have been carried out by Balarew, Barkov and Christov et al. [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] based on the Pitzer approach for accurate prediction of activities and solid liquid equilibria in binary, ternary and multicomponent Rb and Cs systems. Pitzer binary and mixed ion-interaction parameters for Rb and Cs are determined for more than 40 systems within multicomponent K + NH4 + Mg + Co + Ni + Mn + Cu + Zn + Cl + Br + I + SO4 + SeO4 + H2O system at T = 298.15 K [16]. Holmes and Mesmer [18] have reported a Pitzer type model for representing the temperature dependent thermodynamic properties of CsCl(aq), however, the solubility data were not included in their model. Then the modeling work of the binary system CsCl + H2O was extended by Monnin and Dubois [19] by introducing the solubility of CsCl(cr). Unfortunately, up to now, the Pitzer models simultaneously incorporating activity and solubility properties for the binary system CsCl + H2O is only valid below 298.15 K. For the system RbCl + H2O thermodynamic model has never been extended to temperature other than 298.15 K [16]. Nevertheless, these available work provided importance basis for new thermodynamic model development and verification. The phase diagram and comprehensive thermodynamic models of the NaCl + H2O, KCl + H2O and MgCl2 + H2O systems for temperature below 450 K have been reported in our previous study [20]. Cohen-Adad et al. [21] have published the evaluated phase diagram of RbCl + H2O system for temperature below 388 K. Due to the lack of RbCl solubilities at subzero temperatures, they can only evaluate these low temperature data by fitting and extrapolation, and they finally said these extrapolation values are need to be verified by further experimental work. Similarly, the solubilities of CsCl in pure water at subzero temperatures have never been reported by any researcher up to now. To obtain the phase diagram of CsCl + H2O system, especially for subzero temperatures, Monnin and Dubois [19] and Dubois et al. [22] proposed two predictive approaches based on the well known solubility data and thermodynamic data of CsCl(aq) at temperature higher than 273.15 K, respectively. These predictions for the phase diagram of CsCl + H2O system at subzero temperatures still need to be verified by experimental work, although the two individual predictions agreed with each other well.
In this study, the solubilities of RbCl(cr) and CsCl(cr) in pure water at subzero temperatures are elaborately determined with isothermal dissolution method. To develop comprehensive temperature dependent thermodynamic models for RbCl + H2O and CsCl + H2O systems, solubility data and thermodynamic data (solvent and solute activity, enthalpy of dilution and heat capacity) to near saturation at various temperatures are necessary. Except for the solubility and activity data, however, calorimetric data are only available for dilute solutions. To fill this gap, we determined the heat capacities of RbCl(aq) and CsCl(aq) at 298.15 K to near saturation with a flow calorimetry. These solubility and heat capacity data determined in this work together with other thermodynamic and solubility data reported in open literature are used to regress the parameters of the thermodynamic models for RbCl + H2O and CsCl + H2O systems within the Pitzer-Simonson-Clegg model framework in accordance with our previous reported models for other binary systems [20], [23]. Further these binary models will be applied for phase diagram prediction on complex multi-component systems containing RbCl and CsCl at various temperatures.
Section snippets
Materials
Sodium chloride (Aladdin Industrial Inc., 0.9999), rubidium chloride (Aladdin Industrial Inc., 0.9995) and cesium chloride (Aladdin Industrial Inc., 0.9999) were used without further purification and the impurities in the salts were also not analyzed (see Table 1). Doubly distilled water (electrical conductivity <1.8 × 10−4 S m−1) was used in all the experiments.
Experimental apparatus
A thermostat (Lauda Proline Command, Germany) with a refrigerator and temperature stability of 0.05 K was used to solubility measurement.
Solubility
The solubility of RbCl(cr) and CsCl(cr) in pure water determined in this work are listed in Table 2, Table 3 respectively. The solubility both of RbCl(cr) and CsCl(cr) in pure water decreases with temperature. There are no hydrous solids found in both systems.
For the RbCl + H2O system, the solubility of RbCl(cr) at 298.15 K was determined in the present work and compared with literature values. According to the literature compilation of Cohen-Adad [21], the solubilities of RbCl(cr) at 298.15 K
Thermodynamic framework
To make the thermodynamic models of the binary systems RbCl + H2O and CsCl + H2O compatible to the available developed models of the binary systems NaCl + H2O, KCl + H2O and MgCl2 + H2O by the authors, the thermodynamic framework based on the Pitzer-Simonson-Clegg (PSC) model [27], [28], parameter regression and data weighting method were performed according to previous study [23].
For any species in an aqueous solution and any solid phase, its standard Gibbs energy as a function of temperature can be
Conclusions
The solubilities of RbCl(cr) and CsCl(cr) at subzero temperatures were determined firstly for temperature to neat the lowest eutectic points. These data can be used to verify model extrapolation results given in literature. The specific heat capacity of concentrated RbCl(aq) and CsCl(aq) solutions were determined using a flow calorimeter. These data broaden the concentration range of the available thermochemical data for RbCl(aq) and CsCl(aq) solutions. From the subzero temperature solubility
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (U1407131). The authors thank Editor Professor W. E. Acree, Prof C. Christov and a anonymous reviewer for their constructive suggestions and help on literature survey.
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