The complexation behavior of PAA and lanthanide (Ln) ions was schematically investigated. As pH increased, the complexation between FAA and Ln ions will greatly strengthen because COO- groups exhibit much stronger interaction with Ln ions than COOH. When the pH values are higher than 2.5, where the ionization of PAA is about 2.0%, the PAA Ln complex would precipitate out from the solution. The precipitation is a direct indicator for the complexation between FAA and Ln ions. Besides pH values, the complex precipitation is related with the concentration, the molar ratio, and the operation procedure. At the extremely low concentration, there is no polymer metal complex precipitation but fluorescence spectroscopy reveals that the complexation between FAA and Ln ions still exists when pH value is higher than 2.5. When the molar ratio of FAA and Ln ions exceeds the certain level, it would produce soluble polymer metal complex, even at the high concentration of Ln ions. (C) 2014 Elsevier Ltd. All rights reserved.