Survey of Analytical Approaches

Both physical and chemical methods or a combination of both have been used to estimate the phenolic hydroxyl content of lignin (Adler 1977). Common physical methods include potentiometric and conductometric titration (Chap. 7.5) ionization UV spectroscopy (Chap. 5.1), and NMR spectroscopy (Chap. 5.3). 

The titration method is based on the acidity of phenolic hydroxyl groups and, under proper conditions, can be used to determine this group together with carboxyl or sulfonate groups that may be present coincidentally in lignin. An improved nonaqueous titration method in DMF developed by Pobiner (1983), 

Springer Series in Wood Science Methods in Lignin Chemistry (Edited by S.Y. Lin and C.W. Dence) 

Chemical methods for analysis of phenolic hydroxyl groups include determination of the increase in methoxyl content resulting from diazomethane methylation (Bjorkman and Person 1957), the increase in phenolic acetyl group content after acetylation (Lenz 1968, Mansson 1983), and low-molecular weight compounds derived from the degradation of phenolic structures (Chap. 5.2). The phenolic acetyl group of acetylated lignin may also be determined by an NMR spectroscopic technique (Lenz 1968, Robert et al. 1986) or by a selective deacetylation in pyrrolidine (aminolysis) (Mansson 1983). 

A method developed by Adler et al. (1958) is based on the oxidation of simple phenolic guaiacyl (Adler and Hernestam 1955) or syringyl (Chang et al. 1975) compounds with aqueous sodium periodate solution to ortho-quinone structures in which process nearly lmol of methanol per mol of phenolic hydroxyl group is released [Eq. (1)]. 

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