A COMBINATION OF SYNCHRONOUS FLUORESCENCE SPECTROSCOPY WITH CHEMOMETRIC TREATMENT AND INTERNAL STANDARDS IN NONAQUEOUS POTENTIOMETRIC TITRATIONS OF FULVIC-ACIDS

The acid properties of a soil fulvic acid (sfua) were characterized by potentiometric titration with tetrabutylammonium hydroxide in two non-aqueous solvents with high acid-base resolution power, N,N-dimethylformamide (DMF) and acetonitrile. Synchronous fluorescence spectroscopy (SyF) was also used to monitor directly the sfua status during the potentiometric titration in DMF. The potentiometric titration curves showed no clear end-point and the analysis of the sets of spectra obtained at increasing neutralization degree, with a self-modeling curve resolution method (SIMPLISMA), revealed the existence of two components with featureless concentration profiles. Internal standards (maleic, salicylic and p-hydroxylbenzoic acids) were used to determine the amounts of acid groups with different acid strengths in the two non-aqueous solvents. It was shown that the variations observed in the SyF spectra sets of the internal standards are not correlated with those observed in the sfua data. The splitting of the sfua groups in the non-aqueous titration curves seems to be forced artificially depending on the standards used.