EFFECTS OF CROWN-ETHERS AND ANIONS ON REDOX BEHAVIORS OF EUROPIUM (III/II) IN AQUEOUS-SOLUTION

The effects of crown ethers (12-crown-4, 15-crown-5, and 18-crown-6) and anions (ClO4-, NO3-, Cl-, Br-, and I-) on the redox behaviors of europium(III/II) were investigated by cyclic voltammetry in an aqueous solution. The complexations of europium(II) with crown ethers were discussed by means of the shifts of redox potential. Addition of 18-crown-6 led to the appearance of a new oxidation wave at -0.28V and the decrease of the peak height of the oxidation wave at -0.55 V. For 15-crown-5 and 12-crown-4, similar behaviors were observed, but the magnitude of the changes for oxidation peak heights was very small. These differences depend upon the relative sizes of europium(II) ion to the internal cavities of crown ethers. As europium(II) forms a stable 1 : 1 complex with 18-crown-6, the stabilization of europium(II) is remarkable. The small anion such as Cl- strongly interacts with europium (II) complex with 18-crown-6 to weaken the stabilization effect of europium(II) by crown ether.