Effect of the Concentration of Humic Acids in Aqueous Solutions on the Structure of Their Macromolecules

The concentration dependence of the first moments M-1 of fluorescence spectra of the fractions of humic acids (HA) differing in molecular masses in aqueous solutions within the concentration range 0.5-50 mg/1 is investigated correctly for the first time. For all the HA fractions investigated, up to the concentration of -5 mg/1, a substantial increase in M-1 value is discovered. It is established that M-1 value undergoes smaller changes with an increase in the molecular mass of HA fraction. Interpretation of the concentration dependence of M-1 is carried out with the help of the hypothesis of associates. Within this hypothesis, the observed batochromic shifts of the fluorescence spectra of HA fractions with an increase in concentration are explained by the changes in the conformation of the fluorescing substance and by changes in the polarity of microsurroundings.