Kinetic analysis of the complexation of aqueous lanthanide(III) ions by arsenazo III†

Abstract

The complexation behavior of Ln³⁺ (aq) (Ln = Eu, Gd or Tb) ions by arsenazo III [3,6-bis(o-arsonophenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid] has been investigated by stopped-flow kinetic techniques (ambient and high-pressure). The reaction takes place in three stages. The first kinetic stage differs in pressure dependence for the different metal ions, probably related to a co-ordination number changeover. The second and third kinetic stages were also studied under a variety of conditions. A step by step ring-closure mechanism is suggested to account for the three kinetic stages. The results are discussed with reference to related literature data.