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Chemistry International
Vol. 24, No. 6
November 2002


IUPAC Projects

New Solubility Data Projects

One of the major functions of the IUPAC Subcommittee on Solubility and Equilibrium Data (formerly IUPAC Commission V.8) is to publish authoritative texts on all aspects of solubilities. The main output of this subcommittee is the ongoing series of volumes in the IUPACNIST Solubility Data Series, or simply the SDS. The priority subjects to be pursued over the next three years within the framework of this Solubility Data Project were approved by the Analytical Chemistry Division on 2 July 2002. In this context, compilations and evaluations of experimental solubility data in the following three fields are in progress:

1) Solubility Data of Compounds Relevant to Mobility of Metals in the Environment
Solubilities in aqueous media of sparingly soluble metal salts such as carbonates play an important role in chemical processes. Solubility phenomena (i.e., dissolution and precipitation reactions) frequently control procedures for preparing, separating, and purifying chemicals. Moreover, interactions of the hydrologic cycle with the cycle of rocks, as well as the naturally occurring dissolution of minerals in water and their precipitation on the ocean floor and in sediments of rivers and lakes, can often be simply described in terms of solubility equilibria.

In addition, solubility measurements have been shown to be a powerful tool for the determination of thermodynamic properties of sparingly soluble metal salts. The total concentration of alkaline earth and transition metals in carbonate-bearing natural waters is predominantly determined by the solubilities of the respective hydroxides, oxides, hydroxide carbonates, and neutral carbonates. A careful determination of thermodynamic data of sparingly-soluble metal salts is an essential prerequisite for the geochemical modeling of the release of trace elements from waste repositories. Clearly, a comprehensive compilation and evaluation of the existing solubility data is an invaluable basis for all sorts of predictive models in this field. Presently three volumes are in preparation: 1) alkaline earth metal carbonates; 2) metal carbonates (Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Hg, Pb); and 3) inorganic actinide compounds.

2) Solubility Data Related to Oceanic Salt Systems
The oceanic salt system comprises the ions Na+, K+, Mg2+, Ca2+, Cl-, and SO42-. Extended evaporitic deposits in Europe (Germany, Netherlands, Spain, France, Poland, Russia); North America (Canada); South America (Brasilia); Africa (Congo); and Asia had been formed from these ions during the evaporation-crystallization processes of seawaters in geological times. Geological surveying of these deposits, potash mining, rock salt mining, fertilizer production, and usage of rock salt mines as repository for nuclear or chemical wastes require the exact knowledge of solubility equilibria in the multicomponent oceanic salt system within a broad range of temperatures. At present, solution mining of magnesium chloride for magnesium metal production is performed or is in the planning stage in a number of places (Netherlands, Congo, Thailand, and Uzbekistan). In addition, evaporation and aerosol formation from oceans are important controlling factors for the world’s climate. In order to formulate material and energy exchange models, it is crucial to understand the coupling of evaporation and crystallization processes within the droplets transported into the upper atmosphere. Climate changes from past time periods are manifested in the complex evaporitic deposition patterns, which are interpreted on the basis of the solid-liquid equilibria of the multicomponent oceanic salt system.

Presently two volumes are under way: 1) binary systems containing sodium, potassium, and ammonium sulfate; and 2) magnesium chloride-water and calcium chloride-water and their mixtures.

3) Solubility Data Related to Industrial Processes
Gas solubility is one of the fundamental properties of various gas absorption processes in the chemical industry. The removal of carbon dioxide from gas mixtures is a necessary and expensive step in many processes. It is of particular importance, for example, in the purification of ammonia synthesis gas, in the synthesis of liquid fuels from coal, and in the upgrading of fuel gases. Absorption with suitable solvents provides a convenient method for the removal of CO2. In enhanced oil recovery, carbon dioxide is used to displace the hydrocarbons from the reservoir and the solubility of CO2 in the hydrocarbons is important to ensure that miscibility occurs, with a concomitant increase in the oil recovered. In addition, processing using supercritical fluids is an increasingly important area worldwide. Systems that employ carbon dioxide as the solvent are particularly attractive as CO2 is environmentally friendly.

The availability of accurate and reliable information on the equilibrium solubility of CO2 in absorbing solvents as a function of temperature and pressure is of utmost importance in the rational design of gas-treating units. Such data will allow more economical construction and more nearly optimum operation of gas-treating plants. Thus, compiled and evaluated data on the solubility of CO2 in various industrially important aqueous solvents and solvent mixtures are very much sought after.

Presently, three volumes are in preparation: 1) CO2 in aqueous non-electrolyte solutions; 2) CO2 and the lower alkanes at pressures above 2 bar: part 1, methane to butane; and 3) solids and liquids in supercritical CO2.

Because of the diversity of industrial processes sometimes pending problems have to be tackled when there is sufficient individual expertise and interest from contributors. Thus, volume four concerning the solubility of lead sulfate is in preparation. Lead sulfate in aqueous and non-aqueous solvents continues to present problems in the design and manufacture of the still very important lead-acid batteries.

Acetonitrile is one of the best extractive distillation solvents for separation of close boiling paraffinic and olefinic hydrocarbons. As the chemical process industry seeks more efficient and less energy-intensive separation techniques, liquid-liquid extraction based on selective solubilities is becoming more common. Volume five, which is being produced, covers this issue.

Reviewed by Heinz Gamsjäger, chairman of Subcommittee on Solubility and Equilibrium Data.


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