Synchronous fluorescence spectra (excitation wavelength range between 280 and 510 nm and wavelength interval of 25 nm) of three samples of fulvic acids (FA) were obtained as a function of the pH, in the range from 2.0 to 10.5, and as a function of the FA concentration, in the range from 20 to 180 mg/L. FA were obtained from composted livestock materials (lsFA), composted sewage sludge (csFA), and Laurentian soil (laFA). Three-dimensional spectral matrices were obtained (wavelength, pH, and FA concentration) and multivariate curve resolution (MCR) was used to calculate spectra and fluorescence intensity profiles for the detected components. Cluster analysis of the calculated spectra showed the existence of similar and unique fluorescent properties in the three FA samples. Some of the calculated fluorescence intensity profiles have a shape compatible with acid–base species distribution diagrams, which allowed pKa values to be estimated, namely, a well-defined acid–base equilibrium with pKa 5.7 ± 0.2 (lsFA), 6.9 ± 0.4 (csFA), and 5.5 ± 0.2 (laFA); and other acid–base systems not well defined with pKa at about 3.0 and 8.6. Other spectral variations revealed the existence of inner-filter effects or self-quenching as the concentration of FA increases.

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