Fast, Easy, and Economical Quantification of Lignin Phenolic Hydroxyl Groups: Comparison with Classical Techniques

The quantification of lignin functional groups constitutes an essential step for effective assessment of lignin transformation. A new, fast, economical, and simple method without the use of harmful solvents is presented for lignin phenolic hydroxyl group quantification. The new method is based on simultaneous conductometric and acid–base titration based on pH in aqueous medium, and it provides, in a single measurement, phenolic hydroxyl and carboxyl group quantification. The new method provokes an inherent error due to the phenolic structure of lignin. This error should be 10% maximum for the low amount of condensed structures in the analyzed technical lignin samples. Four industrial lignin samples (PB-Protobind lignin, ORG-Organolsolv lignin, KF-Kraft lignin, IND-Indulin lignin) have been analyzed using the new method, and the results were compared with six classical methods (UV, 31P NMR, 1H NMR, 13C NMR, Folin Ciocalteu, and aminolysis). The results showed good statistical correlation of the new method with the UV and 31P NMR, the most significant methods, with coefficients of variation among 6 and 14%. Moreover, the carboxyl content results presented high accuracy with a traditional method as tetra-n-butylammonium hydroxide (TnBAH) potentiometric titration.