2020 年 20 巻 p. 32-42
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 (pHc = –log [H+]) of 6 to 12 after aging at 25 to 90 °C for certain periods up to 12 weeks. Solubility and solubility-limiting solid phases were discussed with the aid of X-ray diffraction (XRD) patterns of the solid phases and temperature dependence of solubility products (log Ks°) values. The XRD patterns of the solid phases indicated that the initial solid phase of La2O3(cr) transformed to La(OH)3(cr) after aging at 25 °C and 90 °C. On the other hand, Eu2O3(cr) remained stable after aging at 25 °C, but after 90 °C, converted to Eu(OH)3(cr). It was observed that Tm2O3(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.