Volume 87, Issue 5

For the spontaneous crystallization of highly soluble salts, a sufficiently high concentration of certain ionic species (complexes) or clusters has to be created in the solution, so that their grouping could yield a suitable crystal nucleus in a reasonably short time. The lowest critical supersaturation needed for nucleation and the highest rate of crystallization are displayed by those salts whose complexes in the solution have analogues in the crystal structure of the crystallizing salt, i.e., when the structure and the composition of the complexes enable their incorporation into the crystal lattice of the crystallizing salt with minimum changes. From this point of view a crystallochemical nucleation mechanism for explaining the Ostwald step rule is advanced. Concerning the rate of crystallization this concept was confirmed by studies on the system Na + , Mg 2+ /Cl – , SO 4 2– //H 2 O and by parallel Raman spectroscopic studies of the microstructure of these solutions. It was established that upon increasing the concentration of Mg 2+ ions and respective lowering of the water activity in the solution, the variety of the SO 4 2– complexes increases. A direct correlation was found between the presence of various SO 4 2– associations and the rate of crystallization of the corresponding salts in these systems.

In the present work, the solid–liquid–liquid equilibrium in the binary system of diethylamine (1) and ionic liquid (2) 1-methyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide and solid–liquid equilibrium in system 1-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide was studied. Phase equilibrium was determined experimentally by means of a polythermic method. These data were then used to determine the activity coefficients for both ionic liquids. For the pure diethylamine the enthalpy of fusion was determined by differential scanning calorimetry, because to the best of our knowledge, this data is not yet reported in the open literature, a contrario of pure ionic liquids tested during this work.

Standard molar quantities of molybdate ion entropy, Sm0,$S_{\rm{m}}^0,$ enthalpy of formation, ΔfHmo,${\Delta _{\rm{f}}}H_m^{\rm{o}},$ and Gibbs energy of formation, ΔfGmo,${\Delta _{\rm{f}}}G_{\rm{m}}^{\rm{o}},$ are key data for the thermodynamic properties of molybdenum compounds and complexes, which are at present investigated by an OECD NEA review project. The most reliable method to determine ΔfHmo${\Delta _{\rm{f}}}H_{\rm{m}}^{\rm{o}}$ of molybdate ion and alkali molybdates directly consists in measuring calorimetrically the enthalpy of dissolution of crystallized molybdenum trioxide and anhydrous alkali molybdates in corresponding aqueous alkali metal hydroxide solutions. Solubility equilibria of sparingly soluble alkaline earth molybdates and silver molybdate lead to trustworthy data for ΔfGmo${\Delta _{\rm{f}}}G_{\rm{m}}^{\rm{o}}$ of molybdate ion. Thereby the Gibbs energies of the metal molybdates and the corresponding metal ions are combined with the Gibbs energies of dissolution. As reliable values are available for ΔfGmo${\Delta _{\rm{f}}}G_{\rm{m}}^{\rm{o}}$ of the relevant metal ions the problem reduces to select the best values of solubility constants and ΔfGmo${\Delta _{\rm{f}}}G_{\rm{m}}^{\rm{o}}$ of alkaline earth molybdates and silver molybdate. There are two independent possibilities to achieve the latter task. (1) ΔfHmo${\Delta _{\rm{f}}}H_{\rm{m}}^{\rm{o}}$ for alkaline earth molybdates and silver molybdate have been determined by solution calorimetry. Entropy data of molybdenum have been compiled and evaluated recently. CODATA key values are available for Smo$S_{\rm{m}}^{\rm{o}}$ of the other elements involved. Whereas Smo(CaMoO4, cr)$S_{\rm{m}}^{\rm{o}}({\rm{CaMo}}{{\rm{O}}_4},{\rm{ cr}})$ is well known since decades, low-temperature heat capacity measurements had to be performed recently, but now reliable values for Smo$S_{\rm{m}}^{\rm{o}}$ of Ag 2 MoO 4 (cr), BaMoO 4 (cr) and SrMoO 4 (cr) are available. (2) ΔfHmo(BaMoO4, cr),${\Delta _{\rm{f}}}H_{\rm{m}}^{\rm{o}}({\rm{BaMo}}{{\rm{O}}_4},{\rm{ cr}}),$ for example, can be obtained from high temperature equilibria also, but the result is less accurate than that of the first method. Once Gibbs energy of formation, ΔfGmo,${\Delta _{\rm{f}}}G_{\rm{m}}^{\rm{o}},$ and enthalpy of formation, ΔfHmo,${\Delta _{\rm{f}}}H_{\rm{m}}^{\rm{o}},$ of molybdate ion are known its standard entropy, Smo,$S_{\rm{m}}^{\rm{o}},$ can be calculated.

Interesting general tendencies of changes of solubilities of elements and groups of compounds may be observed when the corresponding solubility data are arrayed according to the increasing atomic number of the elements. Such trends are exemplified with the data of various systems (metallic and salt-water type) evaluated in several volumes of the IUPAC-NIST Solubility Data Series. The solubilities of elements in mercury as well as in liquid alkali metals, when ordered according their atomic numbers, change roughly in a corresponding way as the temperatures and energies of melting or boiling points of the elements. However, majority of transition metals dissolved in alkali metals are subject to some side reactions with nonmetallic impurities that may drastically elevate their concentration levels. The solubilities of intermetallic compounds in mercury depend primarily on the energies of formation of these intermetallics in the binary alloys and then on the dissolution energies of the component metals in mercury. It has been observed that the experimental solubilities of metal halates in water show quite well defined periodical changes. The arrayed solubility data of rare earth metal fluorides and chlorides in water display quite smooth changes with the increasing atomic numbers if the solutes are isomorphic. Some exceptions from the smooth changes for rare earth metal bromides and iodides are explained. These general observations are useful in evaluating and predicting solubilities in experimentally unknown systems.

The complexation of An III /Ln III with nitrate was investigated in dilute to concentrated Na–Mg–Ca–Cl–NO 3 solutions at 2.94 ≤ pH m ≤ 13.2 and T = 22 ± 2 °C. Comprehensive solubility experiments with Nd(OH) 3 (s) were performed from undersaturation conditions and complemented by extensive spectroscopic studies using Cm(III)–TRLFS (time resolved laser fluorescence spectroscopy) and Nd–L III EXAFS (extended X-ray absorption fine structure) under analogous pH m , I m and m NO 3 − conditions. Solid phases recovered from batch solubility experiments were characterized by XRD and SEM-EDS. A significant influence of nitrate on the solubility of Nd(OH) 3 (s) is observed in concentrated weakly alkaline MgCl 2 –Mg(NO 3 ) 2 solutions with total salt concentration ≥ 2.83 m and m NO 3 − ≥ 1.13 m. Nitrate has no effect in any of the studied NaCl–NaNO 3 and CaCl 2 –Ca(NO 3 ) 2 mixtures, thus highlighting the key role of Mg 2+ in the Nd(III)–NO 3 interaction. The formation of inner-sphere complexes with the participation of Mg 2+ is further confirmed by Cm(III)–TRLFS and Nd–L III EXAFS. Based on these experimental evidences, the proposed chemical model includes the definition of two new aqueous species Mg[An III /Ln III NO 3 OH] 3+ and Mg[An III /Ln III NO 3 (OH) 2 ] 2+ in equilibrium with solid Nd(OH) 3 (s) and allows to derive the thermodynamic and activity models (Pitzer) for the system Nd 3+ /Cm 3+ –H + –Mg 2+ –OH − –Cl − –NO 3 − –H 2 O.

Given the solubilities of non-ionic organic solutes in water, their solubilities in seawater are obtained by correlation expressions involving two descriptors for the constituent ions (or salts) of seawater and two descriptors of the solutes. The former are the standard partial molar volumes and the intrinsic molar volumes and the latter are the molar volume (the Le Bas variant of it) and either the Hildebrand solubility parameter δ H or the Kamlet–Taft solvatochromic π *.