that many organic dicarboxylic acids do not behave as biden-The 7 F 0 r 5 D 0 excitation spectra of Eu( III) complexed with tate ligands in aqueous solution: Phthalate, maleate, and sucpolyfunctional dicarboxylic acid containing neutral O, N, and S cinate form seven membered chelation (h 6 ) rings that are donors were investigated using Eu(III ) luminescence spectrosunstable in aqueous solution (3). Malonate may be one excopy. Three distinct peaks are seen in the excitation spectra in ample capable of forming a stable six-membered (h 5 ) ring the Eu( III) -diglycolic acid ( ODA) and Eu (III ) -iminodiacetic chelation ligand unit (8). A malonate-like component was acid (IMDA) systems showing the formation of three species, in one of the ''average'' structures proposed for the FA from where each tridentate ODA 20 and IMDA 20 displace approxi-Suwannee River (7). mately three coordinated water molecules; however, the central S FA also has neutral O donors and minor amounts of N donor of Eu(III) -thiodiglycolate and 2,5-thiophenedicarboxylate are shown to be poor chelators for free Eu 3/ ions. The neutral O and S donors, and they together with carboxylates may form and N donors in combination with neighboring carboxylates may another class of strong binding sites of FA. A Eu(III) lumiform strong metal chelation sites on fulvic acid. α§ 1997 Academic Press nescence study showed that dipicolinic acid (DPA) behaves Key Words: europium; luminescence probe; diglycolic acid; imias a tridentate ligand for Eu(III) ion ( 9). In this work, the nodiacetic acid; thiodiglycolic acid; 2,5-thiophenedicarboxylic use of Eu(III) excitation spectroscopy involving the 7 F 0 r acid; hydration number; fulvic acid. 5 D 0 transition as a luminescence probe to study the complexation behaviors of polyfunctional carboxylic acids containing neutral O, N, and S donors in aqueous solutions is