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

Potentiometric titration in a fast and simultaneously high-resolution modality was proposed for the identification and quantification of protolytic groups of variable strength at the surface of primary soil particles. The method is implemented by titrimetric data processing as multicomponent spectra (pKa spectrotitrimetry). Due to the high resolution, the error of acidity-constant assessment (3–5%) is lower, compared to existing approaches; due to the fast titration, the effect of soil hydrolysis is minimized. The soil profiles for acidic Retisols (podzolic soils)—under a spruce crown and in the intercrown space—were studied. These soils, which have similar bulk properties and genesis but developed under different plant covers, were distinguished by pKa spectral features at 4–5; 5.5–6.5; 6.5–8.5; 7.5–8.5; and 9–10, as well as total group concentrations. Differences in acidic and basic-group distribution (carboxyl groups, amorphous aluminosilicates, carbonate species, amino groups, soluble (poly)phenolic compounds, phospholipids) and Al and Fe complex compounds within the same soil profiles and between two Retisols were found and quantified. The acidity constants and group concentrations found by pKa spectrotitrimetry were compared with conventional soil-composition indicators (total organic carbon, oxalate-soluble Fe and Al, and phosphorus), using principal component analysis. The main correlations are between the concentrations of oxalate-soluble Al and groups with pKa values of 5.0–6.5 and 8.5; oxalate-soluble Fe and pKa values of 9.0–10.0; and P2O5 and pKa values of 4.0–6.0 and 6.5–8.5. The method provides a set of major acidity values without a priori information on a soil sample and can be used for screening and identifying similar soils.