Application of the Specific Ion Interaction Theory → the Solubility Product and First Hydrolysis Constant of Europium

The specific ion interaction theory (SIT) was applied to the first hydrolysis constants of Eu(III) and solubility product of Eu(OH)₃ in aqueous 2, 3 and 4 mol⋅dm⁻³ NaClO₄ at 303.0 K, under CO₂-free conditions. Diagrams of pEu_aq versus pC_H were constructed from solubilities obtained by a radiometric method, the solubility product log₁₀ K_{sp, Eu(OH)3}^I {Eu(OH)_3(s) Eu_aq³⁺+ 3OH_aq⁻ } values were calculated from these diagrams and the results obtained are log₁₀ K_sp,Eu(OH)3^I = − 22.65 ± 0.29, −23.32 ± 0.33 and −23.70 ± 0.35 for ionic strengths of 2, 3 and 4 mol⋅dm⁻³ NaClO₄, respectively. First hydrolysis constants {Eu_aq³⁺+H₂O Eu(OH)_(aq)²⁺+H⁺ } were also determined in these media by pH titration and the values found are log₁₀β_Eu,H^I = − 8.19 ± 0.15, −7.90 ± 0.7 and −7.61 ± 0.01 for ionic strengths of 2, 3, and 4 mol⋅dm⁻³ NaClO₄, respectively. Total solubilities were estimated taking into account the formation of both Eu³⁺ and Eu(OH)²⁺ (7.7 < pC_H < 9) and the values found are: 1.4 × 10⁻⁶ mol⋅dm⁻³, 1.2 × 10⁻⁶ mol⋅dm⁻³ and 1.3 × 10⁻⁶ mol⋅dm⁻³, for ionic strengths of 2, 3 and 4 mol⋅dm⁻³ NaClO₄, respectively. The limiting values at zero ionic strength were extrapolated by means of the SIT from the experimental results of the present research together with some other published values. The results obtained are log₁₀ K_{sp, Eu(OH)3}^o = − 23.94 ± 0.51 (1.96 SD) and log₁₀β_Eu,H⁰ = − 7.49 ± 0.15 (1.96 SD).