Kinetics and Possible Mechanisms of the Ce(IV) Oxidation of EDTA, CDTA, DTPA and NTA in Perchloric Acid Media

The rates of oxidation of four chelating agents with Ce(IV) in HClO4 solutions, have been studied by the stopped-flow technique. The rates first increase with increasing acidity, reach maxima which are characteristic of the chelating agent and the medium, then progressively decrease with further increasing acid concentration. At their maximum reactivities, the tendencies for oxidation decrease in the following order: trans-1,2-diaminocyclohexane tetraacetic acid (CDTA) > ethylenedinitrilotetraacetic acid (EDTA) > diethylenetriaminepentaacetic acid (DTPA) > nitrilotriacetic acid (NTA). A mechanism for the oxidation of EDTA, involving Ce4+ and Ce(OH)8+ and both unprotonated and monoprotonated chelating agent, is proposed.