Hydration and ion association of La3+ and Eu3+ salts in aqueous solution

Aq. solns. of five lanthanide salts: LaCl3, La(NO3)3, La2(SO4)3, Eu(NO3)3 and Eu2(SO4)3 were studied at 25° by dielec. relaxation spectroscopy over the frequency range 0.05 ≤ ν/GHz ≤ 89. Detailed anal. of the solvent-related modes located at higher frequencies showed that both La3+ and Eu3+ are strongly hydrated, even including partial formation of a third hydration shell similar to that of Al3+(aq). Up to two solute-related modes could be detected at lower frequencies, due to the formation of various types of 1 : 1 ion pairs (IPs). All five salts showed modest levels of assocn. in the order Cl- < NO3- « SO42-, mostly in the form of double-solvent-sepd. IPs with small amts. of solvent-shared IPs. Overall assocn. consts., K0A, calcd. from the stepwise IP formation consts. were consistent with literature values