Differentiation of the acidic groups of fulvic acids from lignite by potentiometric titration in acetone, acetonitrile and isopropanol

Potentiometric titrations in acetone, acetonitrile and isopropanol, with tetrabutylammonium hydroxide as titrant, were carried out to differentiate between the acidic groups of fulvic acids. Titrations in pure solvents were able to differentiate at least three acidic groups of different acidic strength. The amount of acidic hydrogen titrated increased with the solvent's basicity: isopropanol < acetone < acetonitrile. The addition of small amounts of water to the solvents had a strong effect on the titrations, leading to very similar values for the three inflection points found in each solvent. This was attributed to preferential solvation of fulvic acids by water.