Use of the pK Spectroscopy Method in the Study of Protolytic Properties of Humic Substances and Other Soil Polyelectrolytes

A common technique for the potentiometric analysis of complex protolytic systems is mathematical data processing. This approach allows for transforming the experimental titration curve into a function of the ionogenic group distribution of the titrated object by the values of the pK (−lgKa) acid dissociation. This distribution function is called the pK spectrum, and the approach itself is called pK spectroscopy. In this case, solutions are found not in the form of discrete dissociation constants but as a distribution function of the concentrations of ionogenic groups over a continuous spectrum of dissociation constants. The purpose of this work is to test the possibility of using the pK spectroscopy method for assessing the acid–base properties of humic substances, water-soluble fraction, and soils. The results of testing the pK spectroscopy method showed that this method is expressive and more informative than the traditional method of continuous potentiometric titration. The use of the pK spectroscopy method for studying the protolytic properties of humic substances and other environmental objects is justified. The content and values of pK of ionogenic groups of fulvic acids, water-soluble fraction, and soil suspensions were determined. In the objects, the presence of three to five ionogenic groups with pK values from 3.2 to 9.6 was revealed.