Determination of Phenolic Hydroxyl Groups

The aliphatic hydroxyl content of a lignin expressed on a methoxyl content and G> unit basis can be obtained by subtracting the phenolic hydroxyl content reported on a methoxyl or C9 basis from the corresponding total hydroxyl value (see Chap. 7.2 for determination of phenolic hydroxyl groups). 

A Gartner, G Gellerstedt, and T Tamminen. Determination of Phenolic Hydroxyl Groups in Residual Lignin Using a Modifled UV-Method. Nord. Pulp Pap. Res. J. 14 163-170, 1999. 

E Tiainen, T Drakenberg, T Tamminen, K Kataja, A Hase. Determination of phenolic hydroxyl groups in hgnin by combined use of IH NMR and UV spectroscopy. 

Ionization of phenolic hydroxyl groups in lignin results in bathochromic and hyperchromic alterations of the absorption spectra maxima. These spectral changes are often used to determine the type and frequency of phenolic hydroxyl units in lignin samples. 

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),... 

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). 

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