Aqueous solubility of arsenic compounds and thermodynamics

The solubility of a solid in a liquid is the amount of the solid that will dissolve in the liquid to form an equilibrated saturated solution (Faure, 1998), 111. In the literature, the solubility of a particular solid substance in water has been traditionally derived from laboratory measurements of the mass or moles of the solid that dissolves in a given mass or volume of distilled and deionized water. These values are often listed in milligrams per liter, grams per 100 milliliters (Lide, 2007), grams per liter, micrograms per liter Oigl-1), molal, molar, or micromolar ( iM) (Table 2.8). 

Even under ideally controlled laboratory conditions using pure chemicals, the dissolution of a solid compound in water may involve several complex reactions and the formation of numerous dissolved species. As an example, the dissolution of slightly soluble AS2S3 (orpiment) in water can be investigated in a laboratory closed system at 25°C and 1 bar pressure. Nordstrom and Archer (2003), 9 proposed the following reaction to describe the dissolution of orpiment, which initially forms H3ASO30 and H2S°  

Once formed, H2S° would partially dissociate into HS- and H+. A small amount of H2As03 and H+ would form from the dissociation of H3As03°. Some H3As03° and H2S° could also react to produce thioarsenic species, such as AsS(OH)HS (see also Section 2.7.3). Depending on the accuracy and completeness of their thermodynamic databases, geochemical computer models may be able to identify the major reactions and estimate the activities of their products. 

Arsenic compound Temperature (°C) Solubility (g/1 OOg of water, 2 SD) Reference(s)  

As an alternative to laboratory solubility measurements, solubility product constants (KSp), which are derived from thermodynamic data, can be used to calculate the solubility of solids in water (Table 2.9). Each solubility product constant describes a disassociation of a solid in water and calculates the activities or concentrations of the dissolution products in the saturated solution. The solubility product constant or another equilibrium constant of a reaction may be derived from the Gibbs free energy of the reaction (AG K) as shown in the following equation  

---