Determination of Phenolic Hydroxyl Groups in Technical Lignins by Ionization Difference Ultraviolet Spectrophotometry (Δε-IDUS method)

The amount of hydroxyl groups, particularly phenolic, is one of the most important parameters in lignins, as it is an indicator of lignin reactivity. Ultraviolet (UV) Spectrophotometry is a simple and inexpensive method for determining phenolic hydroxyls in lignin. Ionization Difference Ultraviolet Spectrophotometry (Delta epsilon-method) relies on the analysis of solubilized lignin at neutral and alkaline conditions with a UV spectrophotometer. We added a slope analysis to the Delta epsilon-method and dubbed the resulting method Delta epsilon-IDUS (Ionization Difference UV Spectrophotometry). We assessed the reliability of Delta epsilon-IDUS by studying the well-known Indulin AT lignin. Additionally,Delta epsilon-IDUS was applied to a previously uncharacterized milox lignin. When compared to C-13-NMR, Delta epsilon-IDUS underestimated the amount of phenolic hydroxyls for Indulin AT, possibly due to neglecting second phenolic hydroxyls in some lignin units, which resist ionization because of steric hindrance. Nevertheless, the results agreed with previously reported values and confirm that Delta epsilon-IDUS is useful to screen lignins based on their phenolic hydroxyl group content.