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

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.

The Role of Organic and Mineral Fertilization in Maintaining Fertility and Productivity of Cryolithozone Soils

“…Enrichment of the arable layer of Eutric Albic Retisols normally includes inorganic phosphorus compounds. The content of organic phosphates increases insignificantly [49]. Among the mineral phosphorus compounds, soil is dominated by aluminum, and not by iron and calcium phosphates.…”

“…Being not fertilized and agriculturally used for a long period of time, Eutric Albic Retisols, the organic matter of which is highly mobile, normally develop undesirable humus features accompanied with a decrease in the C HAs /C FAs ratio mainly due to an increase in FAs (including aggressive fractions) and to a lesser degree due to a decrease in HAs. The most serious changes in the humic composition were recorded during long-term cultivation of row crops [46][47][48][49].…”

“…Particularly, noticeable changes occur under the influence of long-term application of fertilizers and liming, which affect not only the arable layer but also the underlying soil horizons. During a stationary field experiment, data on the degree of impact of fertilizers and liming on the physicalchemical properties of silty-loam Eutric Albic Retisol were obtained [12,49]. Mineral fertilizers (superphosphate, ammonium nitrate, and potassium chloride) were annually applied in the crop rotation at the rate of 60 kg/ha of the active substance of each element.…”

Fast High-Resolution pKa Spectrotitrimetry for Quantification of Surface Functional Groups of Retisols

“…We used a pK a range of 2-13 for the model systems and 4-10 for real soil samples. The step was 1 or 0.5 [41][42][43][44]. Contents of groups with pK a equal to 5.0; 6.5; 8.5; 9.0; and 10.0; or ranges 4.0-5.0; 4.0-6.0; 5.5-6.5; 6.5-8.5; 7.5-8.5; and 9.0-10.0 were determined by potentiometric titration of all the horizons.…”

“…This technique is implemented in the form of pK a spectrometry (spectrotitrimetry), i.e., using the processing of the resulting continuous titration curves as arbitrary multicomponent acidity spectra. The technique was previously used for the concentrations and pK a values of ionogenic groups of mono-and polyfunctional polyelectrolytes: polyacrylic and polymethacrylic acids, gelatin, and humic substances [41][42][43][44]. However, so far, it has not been implemented to whole soil samples due to the lack of instruments capable of acquiring high-resolution titration curves at the required speed.…”