Solubility and solid phase of trivalent lanthanide hydroxides and oxides

An undersaturation approach was adopted to reveal the solubility and solubility-limiting solid phase of La, Eu and Tm in the sample solutions with the ionic strength (I) of 0.1 mol/L (M) and the hydrogen ion concentration range (pH c = -log [H + ]) of 6 to 12 after aging at 25 to 90C for certain periods up to 12 weeks. Solubility and solubilitylimiting solid phases were discussed with the aid of X-ray diffraction (XRD) patterns of the solid phases and temperature dependence of solubility products (log K s ) values. The XRD patterns of the solid phases indicated that the initial solid phase of La 2 O 3 (cr) transformed to La(OH) 3 (cr) after aging at 25 C and 90 C. On the other hand, Eu 2 O 3 (cr) remained stable after aging at 25 C, but after 90 C, converted to Eu(OH) 3 (cr). It was observed that Tm 2 O 3 (cr) was not transformed after aging at both 25 C or 90 C. Thermodynamic properties obtained from the temperature dependence of the solubility products agreed well with the observations in the XRD patterns.