Thermodynamic data bases for multivalent elements: An example for ruthenium

A careful consideration and understanding of fundamental chemistry, thermodynamics, and kinetics is absolutely essential when modeling predominance regions and solubility behavior of elements that exhibit a wide range of valence states. Examples of this are given using the ruthenium-water system at 298.15 K, for which a critically assessed thermochemical data base is available. Ruthenium exhibits the widest range of known aqueous solution valence states. Known solid anhydrous binary oxides of ruthenium are crystalline RuO/sub 2/, RuO/sub 4/, and possibly RuO/sub 3/ (thin film), and known hydroxides/hydrated oxides (all amorphous) are Ru(OH)/sub 3/ . H/sub 2/O, RuO/sub 2/ . 2H/sub 2/O, RuO/sub 2/ . H/sub 2/O, and a poorly characterized Ru(V) hydrous oxide. Although the other oxides, hydroxides, and hydrous oxides are generally obtained as precipitates from aqueous solutions, they are thermodynamically unstable with regard to RuO/sub 2/(cr) formation. Characterized aqueous species of ruthenium include RuO/sub 4/ (which slowly oxidizes water and which dissociates as a weak acid), RuO/sub 4//sup -/ and RuO/sub 4//sup 2 -/ (which probably contain lesser amounts of RuO/sub 3/(OH)/sub 2//sup -/ and RuO/sub 3/(OH)/sub 2//sup 2 -/, respectively, and other species), Ru(OH)/sub 2//sup 2 +/, Ru/sub 4/(OH)/sub 12//sup 4 +/, Ru(OH)/sub 4/, Ru/sup 3 +/, Ru(OH)/supmore » 2 +/, Ru(OH)/sub 2//sup +/, Ru/sup 2 +/, and some hydroxytetramers with formal ruthenium valences of 3.75 greater than or equal to Z greater than or equal to 2.0. Potential pH diagrams of the predominance regions change significantly with concentration due to polymerization/depolymerization reactions. Failure to consider the known chemistry of ruthenium can yield large differences in predicted solubilities.« less