Complex formation of uranium with 2,5-dihydroxybenzoic Acid

To study of the complex formation of dihydroxybenzoic acids with uranium(VI), we measured the spectra of 2,5-dihydroxybenzoic acid (2,5-DHBA) as a function of pH (2.0 - 4.0) and concentration of uranium (0 - 5x10-3 M). The intensity of the spectra changes with pH and uranium concentration. As we found only spectra with one emission maximum and mono-exponential fluorescence decay, we concluded that no excited state reactions could be observed in the experimental conditions. On the other hand, a change in the fluorescence lifetime could be obtained. The fluorescence lifetimes were found to be in the range from 1.8 ns to 5.7 ns. This means that a dynamic quench process occurred. In order to estimate the formation constant of the 2,5-DHBA with uranium, firstly, we evaluated the protonation constant of the carboxylic group of the 2,5-DHBA. It was found to be log K3 = 2.77 ± 0.05, by taking into account the self-quenching effect. This is in good agreement with literature data (2.82) /1/. The 2,5-DHBA itself and uranium quench the fluorescence of the 2,5-DHBA. We evaluated the dynamic quench constants to be KSV = 216 ± 4 for the ligand itself and KSV = 45 ± 1 for uranium. Both have to be taken into consideration for estimating the complex formation constant. Furthermore, in the pH range 2.0 to 4.0, slopes between 0.91 and 1.39 were found for the validation of the stoichiometry. We conclude, therefore, that a one to one complex forms between uranium and 2,5-DHBA. The dependence of the formation constants on pH values was investigated in order to determine the number of protons involved in the complex formation. Since the slope of the pH dependence was -0.79 ± 0.1, we conclude that one proton at most is involved in this complex formation. This suggests that only the carboxylic group is involved in the complex formation. Based on these considerations, the complex formation constant was assigned to be log K = -0.57 ± 0.26. As uranium shows fluorescence properties, the complex formation can be also confirmed using the changes in the fluorescence behavior of uranium. The formed complex itself does not show fluorescence properties, and only the emission of the non-complexed uranium is observed. As example at pH 4.0, formation constants of log K = 3.29 ± 0.16 were assigned for the ligand study and of log K = 3.17 ± 0.32 for the uranium study, respectively./1/ L. Lajunen, R. Portanova, J. Piispanen et al; Pure & Appl.Chem.,69,329 (1997)