Self-Association of the Macromolecules of Aldrich Humic Acid in Aqueous Solutions Using Ultraviolet-Visible (UV-Vis) Absorption Spectroscopy.

The ultraviolet-visible (UV-Vis) spectra of the aqueous solutions of humic acids (HA) Aldrich were obtained within the concentration range 1-20 mg/L (0.1-2ċ10-6 mol/L). A conclusion on the existence of the self-association (dimer formation) of HA macromolecules is based on the deviation of the concentration dependence of optical density (OD) from the linear one at [HA] > 10 mg/L (>10-6 mol/L). An original, mathematical algorithm is proposed to determine a dimerization constant K and a molar coefficient of dimer absorption ∊ D(λ) The value of K was (2.56 ± 0.07) ċ 106 L/mol. The calculated molar coefficients of HA macromolecule dimer absorption, ∊ D(λ) , indicate an increase in the dimer extinction coefficient ∊ D(λ) compared to the double value of the molar monomer absorption 2 ∊ M(λ) . It was established that the spectral dependence of the value β =  ∊ D(λ) / ∊ M(λ) exhibits a minimum within a wavelength range of 300-450 nm, which is due to the difference in the efficiency of the interaction of various sites upon HA macromolecule dimerization. Thus, an approach of studying the processes of self-association is proposed using the method of UV-Vis absorption spectroscopy. This method is implemented for molecules that do not have characteristic absorption bands. The proposed method can also be successfully applied to molecules with characteristic absorption bands.