A new method for the titration of free phenolic groups in pulps

Abstract

Abstract A fast and semi-quantitative wet chemical method was developed for the titration of free phenolic groups in residual lignins of pulps. The method is based on selective chlorine dioxide reaction with phenolic units of lignin at 0°C. A known amount of ClO2 is reacted for up to 30 min with various substrates and ClO2 consumption is measured. Experiments with monomeric lignin-like model compounds, a dioxane/HCl lignin (isolated from kraft pulp), kraft pulp, and one thermomechanical pulp (TMP) were performed. The ideal stoichiometric consumption of 1 mol of ClO2 per Ar-OH was approximated in a few cases. More model compound experiments are necessary to establish the possibilities and limitations of the method, but the results of these preliminary experiments are promising. First studies on pulps and comparison of the data with those obtained by 13C NMR analysis on dioxane/HCl lignins showed some deviations in the first instance, which were attributed to reaction of the intermediates HClO and Cl2 involved in the overall mechanism. However, using DMSO as HClO trapping agent improved the ClO2/Ar-OH stoichiometric ratio close to the theoretical value of 2, which essentially yielded better results. This improved stoichiometry was verified for three model compounds, one kraft pulp and one TMP, yielding results were similar to those obtained by 13C NMR analysis.