Spectrofluorimetric study of the complexes between calcein and lanthanide(III) ions

The equilibrium of calcein, an H6L type fluorescent ligand, with lanthanide (III) ions, Ln(III), was studied spectrofluorimetrically in aqueous solutio n at constant ionic strength mu=0.1 (KCl), pH 8.0 and 25.0+/-0.1 degrees C. Application of the mole ratio and continuous variation methods reveals the formation of 1:1 complexes. The conditional stability constants (beta') we re calculated from the analysis of the observed fluorescence vs. [Ln(III)]/ [calcein] mole ratio data by using an iterative non-linear least-squares co mputer program. The values obtained for beta' are in the range 5.24x10(6)-5 .77x10(7). The thermodynamic stability constants (beta) were estimated by c alculating the side-reaction coefficients (alpha) for lanthanides and calce in. The beta values obtained were from 3.2x10(12) to 3.6x10(13).