Abstract: A polarographic catalytic prewave of Eu(Ⅲ) with a-naphthaleneacetic acid(NAA) was obtained in a solution of 0. 2 mol/L NH₄Ac-0. 05 mol/L K₂SO₄, pH 7. 0. The concentration of Eu(Ⅲ) from 3. 0 × 10^-6to 1. 0 ×10^-4mol/L, and that of NAA from 5. 0×10^-6to 3. 0× 10^-5mol/Lare directly proportional to the peak heights. The mechanism was investigated by several methods, such as linear sweep, cyclic voltammetry, ACpolarography, differential pulse polarography,etc.Wesuggest that NAAbe adsorbed on the surface of mercury electrode, and it complexes with Eu(Ⅲ) diffusing to the surface. The formation of complex on the mercury surface facilitates the reduction of Eu(Ⅲ). In the similar way, we have also obtained the catalytic prewaves of Eu(Ⅲ) with other auxins, such as indole-3-acetic acid (LAA), indole-3-propanoic acid(IPA) and indole-3-butanoic acid (IBA). The formation of polarographic prewaves could be useful to get some information about the additive effect of rare earths with auxins for promoting the growth of plant roots.

Key words: Eu(Ⅲ)-α-naphthaleneacetic acid, Adsorption on mercury electrode, Catalytic prewave