Abstract
Aqueous molybdate-EDTA solutions were studied by temperature jump in the pH range 7.25-8.25 at 25° and ionic strength 0.1 M (NaN03). The upper time limit of the apparatus was extended to allow accurate measurement of relaxation times in the 1-3 sec range by interfacing the signal to an on-line averager. The rate con- stant for the reaction HMoOr + AH22- — Mo03AH3- + H20 is k¡ = (2.26 ± 0.23) X 10s M-1 sec-1, where AH22- is the diprotonated form of EDTA. The composite rate constant for the reactions Mo03A4- + HMoOr — 03MoAMo034- + OH- (k6) and M0O3AH3- + Mo042- — 03MoAMo034- + OH- (k7) is A:67 = (3.26 ± 0.88) X 10s M-2 sec-1. Upper limits for the individual rate constants are ke S 3.26 X 104 and k- S 1.03 101 M-1 sec-1. Comparing k5 with rate constants for chelation of HMoOr by 8-hydroxyquinoline and catechol shows the EDTA reaction to be slower. Bimolecular steps in the polymerization of molybdenum(VI) are also more rapid. The nature of the rate-limiting step appears to depend on the ligand; for EDTA, the process of forming molybdenum- ligand bonds may be rate limiting.