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

Evgeny Lodygin,Elena Shamrikova

doi:10.3390/agronomy11061051

Abstract

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.

Highlights

The acidity and acid–base buffering of soils are largely the products of soil formation and represent the fundamental characteristics that control the mobility of chemical elements, including many nutrients and pollutants in soils and landscapes
The study of protolytic equilibria in complex natural systems, such as soils [3,4], their particle size fractions [5], humic substances [6,7], bottom sediments [8], wine [9,10], coals [11], and organic compounds mixtures [12,13,14] is performed by the potentiometric method
Potentiometric titration of soil samples is performed in two ways, which are conventionally called continuous (CPT) and equilibrium potentiometric titration (EPT)

Summary

Introduction

The acidity and acid–base buffering of soils are largely the products of soil formation and represent the fundamental characteristics that control the mobility of chemical elements, including many nutrients and pollutants in soils and landscapes. The study of protolytic equilibria in complex natural systems, such as soils [3,4], their particle size fractions [5], humic substances [6,7], bottom sediments [8], wine [9,10], coals [11], and organic compounds mixtures [12,13,14] is performed by the potentiometric method. Potentiometric titration of soil samples is performed in two ways, which are conventionally called continuous (CPT) and equilibrium potentiometric titration (EPT). There are no unified methods for determining the acid–base buffering of both options, but when using CPT, the pH is measured a few minutes after adding the titrant, and with EPT, after a longer period of time measured in hours and days [15]

Objectives

Methods

Conclusion