DETERMINATION OF HYDROXYL-GROUPS IN LIGNINS EVALUATION OF H-1-NMR, C-13-NMR, P-31-NMR, FTIR AND WET CHEMICAL METHODS

The hydroxyl group content of six lignins, analyzed in the course of an International Round Robin effort have been determined using H-1-, P-31-NMR, and FTIR spectroscopies. Two acetylated lignins were examined by C-13-NMR. In addition, the total OH group contents were determined by saponification of acetylated lignins followed by GC analysis of the acetate groups released. The phenolic OH groups were determined by difference in the methoxyl content before and after diazomethane methylation. The results obtained from the different methods were correlated by linear equations. When the relative ratios of phenolic to aliphatic OH groups were regressed, very high correlation coefficients were observed. FTIR or H-1-NMR spectroscopy is recommended for routine determination of phenolic OH/aliphatic OH ratios. P-31-NMR afforded the determination of the stereochemical configuration of beta-O-4 linkages and the phenolic OH contents belonging to G and S units. Similar results for the phenolic and secondary OH group contents were obtained when C-13- and P-31-NMR spectroscopies were employed.