Complex formation of Eu(III) with polyacrylic acid

For the quantitative description of the interaction of metal ions with humic substances, it is necessary to clarify the effects of both polyelectrolyte and heterogeneous nature of humic substances. To estimate the polyelectrolyte effect separately, polyacrylic acid (MW = 90 000) has been selected as a representative of well-defined, homogeneous polymeric weak acids, and its interaction with Eu(III) has been investigated by a solvent extraction method using ¹⁵²Eu (∼ 10^-8 M) with TTA and TBP in xylene. By defining the apparent complex formation constants as β_α = [ML]/([M][R]), where [M] = [Eu³⁺], [ML] is the concentration of Eu(III) associated with polyacrylic acid and [R] = C_Rα (C_R is a total concentration of proton exchanging sites and α is a degree of dissociation determined by potentiometric titration), the apparent constants have been obtained at several pcH and ionic strength (0.1 M and 1.0 M NaClO₄). The constants increased with pcH and decreased with an increase of ionic strength, that is, the values of log β_α varied from 6.0 (at pcH = 4.7) to 7.6 (pcH = 5.5) at 0.1 M NaClO₄ and from 4.8 (pcH = 4.4) to 6.5 (pcH = 5.4) at 1.0 M. The plots of log β_α versus log α revealed almost linear relationship both at 0.1 and 1.0 M NaClO₄.