Laser-induced breakdown spectroscopy (LIBS) was applied to selectively analyze the aqueous suspension of Eu<sub>2</sub>O<sub>3</sub>(s) particles in the presence of the Eu<sup>3+</sup> aquo ion. A plasma was generated by focusing a pulsed Nd:YAG laser beam (λ = 532 nm) into the sample. The light emission from the plasma was detected by a spectrograph equipped with a gated intensified charge-coupled device (ICCD) in the wavelength range of 275-525 nm. The atomic emission intensity of the Eu<sub>2</sub>O<sub>3</sub>(s) suspension was about two orders of magnitude higher than that of the Eu<sup>3+</sup> aquo ion. The detection limits for Eu<sup>3+</sup>(aq) and Eu<sub>2</sub>O<sub>3</sub>(s) were found to be 3.3 × 10<sup>-5</sup> mol/L and 2.0 × 10<sup>-7</sup> mol/L, respectively. Such a difference allows the selective determination of colloidal europium particles. This capability of LIBS was used to study the formation of Eu(OH)<sub>3</sub>(s) colloids in the aqueous Eu<sup>3+</sup> solution by varying pH until the solubility limit was exceeded. The appraisal of the threshold pH for the solubility limit led to the determination of the solubility product of colloidal Eu(OH)<sub>3</sub>(s), which was then calculated to be log K<sup>0</sup><sub>sp</sub> = -25.5 ± 0.4.
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