Lignin Carboxyl content

The method of choice for determining carboxyl groups in lignin is based on potentiometric titration in the presence of an internal standard, /)-hydroxybenzoic acid, using tetra- -butylammonium hydroxide as a titrant (42). The carboxyl contents of different lignins are shown in Table 6. In general, the carboxyl content of lignin increases upon oxidation. 

In the absence of any strongly acidic (for example, sulfonic) groups in the lignin sample, as indicated by the lack of an inflection near +130 mV, the equation for calculating the carboxyl content is further contracted to... 

The carboxyl contents of different lignins are shown in Table 7.5.1. In the case of lignins isolated or modified without recourse to strong oxidation procedures, the carboxyl content is generally less than 10%. However, when lignin is subjected to the action of strong oxidants, as is the case, for example, in the bleaching of chemical pulps, the carboxyl content may substantially exceed this value. 

Solid-state NMR has been used in the molecular characterization of the undegraded wheat straw and degraded samples. The NMR spectra confirmed the increase in carboxyl content but indicated that the overall lignin and methoxyl contents remained relatively constant, although some nonsyste-matic variations were observed. The spectra also showed a decrease in amorphous noncellulosic polysaccharides in relation to the crystalline cellulose upon degradation. 

H NMR data from these seven sites are presented by spectral peak-height ratios in Table III. The sites were listed in order of increasing aromatic plus olefinic carbon percentages. Fulvic acids from all the lake samples are much lower in aromatic plus oleflnic carbon content than those from river samples. These results confirm the hypothesis that autothonous inputs result in dissolved humic substances that have a low aromatic plus oleflnic carbon content. The lake samples also are lower in the ratios of peak 2 (carboxylated chains and aliphatic ketones), peak 3 (carbohydrates), and peak 4 (phenolic tannins and lignins) to peak 1 (branched methyl groups and alicyclic ali-phatics) than are the river samples. 

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