Charge characteristics of humic and fulvic acids: Comparative analysis by colloid titration and potentiometric titration with continuous pK-distribution function model

Charge characteristics of humic and fulvic acids of a different origin (inshore soils, peat, marine sediments, and soil (lysimetric) waters) were evaluated by means of two alternative methods – colloid titration and potentiometric titration. In order to elucidate possible limitations of the colloid titration as an express method of analysis of low content of humic substances we monitored changes in acid–base properties and charge densities of humic substances with soil depth, fractionation, and origin. We have shown that both factors - strength of acidic groups and molecular weight distribution in humic and fulvic acids – can affect the reliability of colloid titration. Due to deviations from 1:1 stoichiometry in interactions of humic substances with polymeric cationic titrant, the colloid titration can underestimate total acidity (charge density) of humic substances with domination of weak acidic functional groups (pK > 6) and high content of the fractions with molecular weight below 1 kDa.