Interaction of Organic Ligands with Metal Ions studied by Time-resolved Laser-Induced Fluorescence Spectroscopy

Migration of heavy metals in the aquatic environment is often combined with the complex formation of these metals with organic ligands, like humic substances and wood degradation products. To simulate the functionality of these polyelectrolytes simple hydroxybenzoic acids are studied.
The fluorescence properties of organic molecules were used for the determination of metal complexation. Due to short fluorescence lifetimes of the ligand molecules a dedicated spectrofluorimeter using ultra short laser pulses as excitation source was developed. The laser system consists mainly of the oscillator system for generation of femtosecond laser pulses, the amplifier system to increase the pulse energy of the generated pulses to about 2 mJ and an optical parametrical amplifier system to provide tunable laser pulses over a wide wavelength range (280 nm to about 10 µm). The emitted fluorescence was detected using a fast gating intensified CCD-camera based spectrometer. The system allows to determine time-resolved fluorescence spectra in solid and aqueous (solution) systems with fluorescence lifetimes between about 150 ps and about 10 ns and concentrations of the organic ligand down to 1 x10-6 M.
To verify the system the well known protonation constant log K3 = 3.10 ± 0.20 (I = 0.1 M, T = 298 K) [1,2] of the 2,3-dihydroxybenzoic acid was determined.
The fluorescence lifetime of C6H3(OH)2-COO- is determined to be 375 ± 32 ps and the fluorescence emission maximum was found at a wavelength of 438 nm. We obtain from fluorescence data a protonation constant of log K3 = 3.17 ± 0.05 for the first dissociation step at ionic strength of 0.1 M and 294 K exploiting the Stern-Volmer mechanism. The agreement of data and the derived error of the protonation constant demonstrate the accuracy of the developed system.
The complex formation of the 2,3-dihydroxybenzoic acid with neptunium(V) was studied in the range from pH 3.0 to 7.0. At lower pH - values a 1:1 complex was identified, showing only an interaction of the neptunium(V) with the carboxylic group of the ligand. A second 2:1 complex is formed at the higher pH-values. This indicates that the phenolic OH-group also forms complexes with the metal ion. The formation constants are found to be log K11= 0.11 ± 0.04 and log K12 = 6.97 ± 0.10. Also a third intermediate complex was detected by its fluorescence properties.

Literature:

[1] L. Lajunen, R. Portanova, J. Piispanen et al; Pure & Appl.Chem., 1997, 69, 329, [2] H. Harada; Bull.Chem.Soc.Jpn., 1971, 44, 3459