The Precipitation of Sparingly-soluble Alkaline-earth Metal and Transition Metal Oxyanion Salts from Aqueous Solution at Ambient Temperatures to 100 °C: Anhydrous and Hydrated Precipitate Phases

The anhydrous and hydrated phases of the sparingly-soluble barium, strontium, calcium, magnesium, lead(II), mercury(II), cadmium, zinc, copper(II), nickel(II), cobalt(II), iron(II) and manganese(II) salts of series of inorganic and organic oxyanions, precipitated from aqueous solution at ambient temperatures to 100 °C, have been tabulated and classified. The alkaline-earth and transition metal chromates, molybdates and tungstates are mainly anhydrous. The alkaline-earth and transition metal carbonates, orthophosphates, orthoarsenates, sulphates, selenates, bromates and iodates show mixed character. The precipitated barium, some strontium, lead(II) and mercury(II) salts are anhydrous: many of the other salts are precipitated as hydrates and the hydration numbers generally increase from one-half to three with decrease in the cation radius. All the alkaline-earth and transition metal oxalates (except lead oxalate) are precipitated as hydrates (below 100 °C). The hydration numbers at ambient temperatures generally increase from two to three-four but there is no clear inverse variation with cation radius. These results are explained by Johnson's thermodynamic treatment for the free energies of solution and precipitation (and crystallisation) of ionic salts.