Dissociation kinetics of europium(III) cryptate complexes in aqueous buffers

Dissociation of the [EuL(1)](3+) complex (L(1) = 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane) in aqueous buffer solutions of pH 7.0-9.0 was studied by monitoring the absorbance change of its charge-transfer (c.t.) band. While the dissociation rate is linearly dependent on complex concentration, the rate constant (k(d)) is dependent on both the concentration and the type of buffer employed. In tris(hydroxymethyl)aminomethane (Tris)the measured rate constant is composed of a concentration-independent term and another term based on the square of the concentration of the basic form of Tris, i.e. k(d) = k(0) + k(2)[NH2C(CH2OH)(3)](2), which indicates that a general base mechanism is dominant at high buffer concentrations. The dissociation of the analogous complex of 4,7,13,16,21,24-hexaoxa-1, 10-diazabicyclo[8.8.8]hexacosane is more rapid. The unique c.t. band of the europium complexes was utilized to elucidate their thermodynamic behaviour in aqueous buffers. No absorbance was observed even at relatively high concentrations of the europium ion and the cryptands (0.100 mol dm(-3)). This enables an upper limit of the formation constants of 0.50 dm(3) mol(-1) to be set in aqueous buffer.