Dissociation ability of humic acids: Spectroscopic determination of pKa and comparison with multi-step mechanism

Humic acids are operationally defined as the fraction of humic substances which is not soluble under acidic conditions. This, does not mean, however, that their solid particles easily dissolve in water. Experimental results suggest that the dissolving of solid humic acids in an aqueous environment is more complex than the conventional solubility behaviour of sparingly soluble solids. The multi-step mechanism of their interaction with water includes partial equilibrium dissolution as well as direct equilibrium dissociation from the solid state. In this work, the pKa value of the dissolved humic acid fraction was determined on the basis of changes in the shape and intensity of their UV/VIS spectra measured in media with various values of pH. The shape of the spectra depends on the pH value, because acidic and alkaline media, in which these compounds are dissolved, shift their dissociation equilibrium towards non-dissociated molecules or to completely dissociated species. The aim of this work was to characterize the dissociation behaviour of selected humic samples by means of dissociation constants using this method. It was shown that the obtained values of pKa characterize the real dissociation behaviour of humic acids in an aqueous environment and can be used as the mean or effective value of pKa corresponding with the multi-step mechanism of dissociation of humic acids.