Volume 85, Issue 11

The 15th International Symposium on Solubility Phenomena and Related Equilibrium Processes (ISSP-15) was held at the Qinghai Institute of Salt Lakes, in Xining, China, 22-27 July 2012. The meeting was a marked success, with over 100 participants from around the world. Xining, located on the Tibetan plateau, provided an interesting glimpse of China, outside of the very large cities of the Chinese lowlands. The Qinghai Institute was established in recognition of the great potential of China’s upland salt lakes, where thousands of years of evaporation have resulted in huge bodies of brines with great commercial potential. For all those chemists who wondered why the thermodynamics and phase diagrams of electrolyte systems were part of their education, these huge, highly complex, aqueous electrolyte systems provide one of the answers, as Prof. Song's overview of studies related to the Qinghai Lakes systems made clear. Concentrated electrolyte systems are found in other situations, including the winning of metals, such as aluminum, and in the processing of spent nuclear fuels; Prof. Altmaier's lecture on Np(VI) systems showed the complexities of such systems. More recently, the explosion of applications for roomtemperature ionic liquids has added a new strand to this long-established field of research. Dealing with all of these types of systems requires accurate experimental measurements, careful analysis of the thermodynamics, and, commonly, thermodynamic modeling; lectures by Profs. Gamsjäger, Hefter, Königsberger, and Wang gave insights into both the experimental and modeling challenges that these systems present. Of course, thermodynamics and solubility are not restricted to electrolyte or solid/liquid systems, as was clearly shown by Prof. De Vischer's lecture on the thermodynamics of CO2 solubility, in the context of CO 2 capture and storage. This was the 15th in the series of ISSPs, which are organized through the IUPAC Subcommittee on Solubility and Equilibrium Data (SSED). This series of symposia, which brings together some of the most eminent scientists working in the area of solubility, is traditionally one of the friendliest meetings one can attend, and the Xining meeting certainly maintained this tradition. Earle Waghorne Conference Editor

The solubility of Np(VI) was investigated in carbonate-free NaCl solutions (0.1 M ≤ I ≤ 5.0 M) at T = 22 ± 2 °C to derive thermodynamic properties of aqueous species and solid compounds formed under alkaline conditions. The experimentally derived solubility curves can be divided into four main regions: (I) ~7 ≤ pH m ≤ ~9, showing a steep decrease in Np solubility with a slope (log [Np] vs. pH m ) of –3 or –2 (depending on NaCl concentration); (II) ~9 ≤ pH m ≤ ~10.5, with a nearly pH-independent [Np]; (III) ~10.5 ≤ pH m ≤ ~13.5, showing an increase in the solubility with a well-defined slope of +1. A region (IV) with a slope ≥ +2 was only observed at I ≥ 1.0 M NaCl and pH m ≥ ~13.5. The solubility-controlling solid Np phases were characterized by X-ray diffraction (XRD), quantitative chemical analysis, thermogravimetric analysis and scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), confirming the presence of anhydrous Na 2 Np 2 O 7 (cr) in regions II and III. The same solid phase was identified in region I except for the system in 0.1 M NaCl, where a NpO 2 (OH) 2 ·H 2 O(cr) phase predominates. XRD patterns of this solid phase show a very good agreement with that of metaschoepite (UO 3 ·2H 2 O), highlighting the similarities between Np(VI) and U(VI) with respect to solid phase formation and structure. Based on the analysis of solubility data, solid phase characterization and chemical analogy with U(VI), the equilibrium reactions 0.5 Na 2 Np 2 O 7 (cr) + 1.5 H 2 O ⬄ Na + + NpO 2 (OH) 3 – and 0.5 Na 2 Np 2 O 7 (cr) + 2.5 H 2 O ⬄ Na + + NpO 2 (OH) 4 2– + H + were identified as controlling Np(VI) solubility in regions II and III, respectively. The predominance of NpO 2 + in the aqueous phase of region I (quantified by UV–vis/NIR) indicates the reductive dissolution of Np(VI) [either as Na 2 Np 2 O 7 (cr) or NpO 2 (OH) 2 ·H 2 O(cr)] to Np(V) aq . Oxidation to Np(VII) can explain the experimental observations in region IV, although it is not included in the chemical and thermodynamic models derived. The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with both the specific ion interaction theory (SIT) and Pitzer approaches. Thermodynamic data for aqueous Np(VI) species [NpO 2 (OH) 3 – and NpO 2 (OH) 4 2– ] and solid compounds [Na 2 Np 2 O 7 (cr) and NpO 2 (OH) 2 ·H 2 O(cr)] that are relevant under alkaline conditions were derived. These data are not currently included in the Nuclear Energy Agency-Thermochemical Database (NEA-TDB) compilation.

Capturing CO 2 emissions from energy production and storing it under the ground is a potential CO 2 mitigation strategy that currently receives much attention. Both CO 2 capture and CO 2 storage are solubility problems. This paper reviews some important solubility aspects of CO 2 capture and storage that are often overlooked. Until very recently, there was not a single CO 2 solubility relationship that was both applicable in a wide temperature range and thermodynamically consistent with the enthalpy of solution of CO 2 . Furthermore, very often the relationship used for the first acidity constant of carbonic acid as a function of temperature is inconsistent with the relationship used for the solubility constant (or Henry constant) of CO 2 . The removal of CO 2 from a gas stream with amine solutions is usually viewed as a chemical reaction, which leads to the practice of heating to remove the CO 2 from the solvent. However, viewing this process as a solubility phenomenon suggests the practice of using vacuum to remove the CO 2 from the solvent, a potentially more efficient approach.

Solubility equilibria between solid salts, salt hydrates, and water play an important role in fundamental and applied branches of chemistry. The continuous interest in this field has been reflected by the 15 th International Symposium on Solubility Phenomena as well as by the ongoing IUPAC-NIST Solubility Data Series (SDS), which by now comprises close to 100 volumes. Three typical examples concerning solubility phenomena of ionic solids in aqueous solutions are discussed: (1) sparingly soluble, simple molybdates; (2) sparingly soluble ionic solids with basic anions; and (3) hydrolysis of inert hexa-aqua-M(III) ions, where M is Ir, Rh, or Cr. In the first two cases, essential experimental details are discussed, an outline of thermodynamic analyses is given, and theoretical aspects are emphasized. In the third case, an educational suggestion is made.

Recent solubility measurements of a variety of solid electrolytes in water and aqueous solutions in the author’s laboratories are reviewed. The experimental challenge of performing such measurements with high accuracy is demonstrated using the solubility of solid sodium chloride in water at near-ambient temperatures as a paradigm. The special difficulties of measuring low solubilities are demonstrated using Pb(II) sulfate in various aqueous solutions and Pb(II) oxide in sodium hydroxide solutions, and the usefulness of such measurements for obtaining reliable information on homogeneous reactions in solution is briefly discussed. It is also shown, using the alkali metal triflate salts as examples, that determination of the solubilities of even highly soluble salts can be problematic. Lastly, data for the solubilities of a series of sodium carboxylate salts of industrial relevance are discussed and are used to illustrate why the theoretical prediction of solid electrolyte solubilities remains such a challenge.

The importance of solid-solution–aqueous-solution (SSAS) equilibria requires the incorporation of solid solutions into thermodynamic models for industrially and environmentally relevant applications. Insights from appropriate measurements and recent database developments have made such extensions feasible. Examples illustrating various types of stable and metastable equilibria involving solid solutions will be given.

Solubility equilibria are relevant to many aspects of chemistry and chemical engineering. Studies on solubility phenomena are of importance in understanding chemistry and in the development of chemical technology. Solubility equilibria are especially useful for geochemists investigating the formation and evolution of natural salt deposits. China possesses an enormous variety of such resources, including salt lakes, underground brines, oilfield brines, and natural soda and borate deposits, etc. With the ongoing economic development of China, the exploitation and comprehensive utilization of these kinds of mineral resources are making great progress. Although much research on aqueous salt systems has been published, solubility equilibria studies have long been an active research field in China. This paper presents a review of solubility research on salt/water systems, encompassing more than 200 papers from Chinese researchers.

A comprehensive thermodynamic model, referred to as the mixed-solvent electrolyte (MSE) model, has been applied to calculate phase equilibria, speciation, and other thermodynamic properties of selected systems that are of interest for understanding the chemistry of salt lakes and natural waters. In particular, solubilities and chemical speciation have been analyzed for various boron-containing systems, which represent an important subset of solution chemistry for such applications. The model has been shown to reproduce the speciation, solubility, and vapor–liquid equilibrium (VLE) data in the boric acid + water system over wide ranges of temperature and concentration. Specifically, solubilities have been accurately represented in the full concentration range of the B 2 O 3 + H 2 O system ( x B2O3 = 0~1), which includes H 3 BO 3 . The accuracy of the model has also been demonstrated by calculating solubilities in various aqueous borate systems, i.e., M n O + B 2 O 3 + H 2 O (where M = Li, Na, Ca, Mg), and their mixtures with a chloride salt or an acid (i.e., LiCl, NaCl, HCl). The model predicts the effects of chemical speciation, temperature, and concentrations of various acid, base, and salt components on the formation of competing solid phases.