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^–6mol/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^–6mol/L). An original, mathematical algorithm is proposed to determine a dimerization constant K and a molar coefficient of dimer absorption ε_D(λ) The value of K was (2.56 ± 0.07) ċ 10⁶L/mol. The calculated molar coefficients of HA macromolecule dimer absorption, ε_D(λ), indicate an increase in the dimer extinction coefficient ε_D(λ) compared to the double value of the molar monomer absorption 2ε_M(λ). It was established that the spectral dependence of the value β = ε_D(λ) / ε_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.