Total Aliphatic Hydroxyl Content

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). 

Aliph OH/OCH3 (moles/mole, based on lignin) 

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Table 7.1.1. Total, phenolic and aliphatic hydroxyl contents of some representative milled wood and bamboo lignins (MWL and MBL) and technical lignins based on C9 unit... 

In general, the aliphatic hydroxyl content of a lignin preparation is determined by subtraction of the phenolic hydroxyl content (see Chap. 7.2) from the total hydroxyl content of the lignin. The total hydroxyl content of lignin (partial structure H) can be determined by acetylation with acetic anhydride-pyridine reagent (10) followed by saponification of the acetate (partial structure 12) and titration of the resulting acetic acid (14) with a standard 0.05 N sodium hydroxide solution (Roth 1953). 

Structure. Otherwise, the total and aliphatic hydroxyl contents of lignin should be expressed in terms of methoxyl content (see Chap. 7.6.4 for the significance of the methoxyl analysis). 

The total hydroxyl content of lignin is determined by acetylation with an acetic anhydride-pyridine reagent, followed by saponification of the acetate, and followed by titration of the resulting acetic acid with a standard 0.05 N NaOH solution. Either the Kuhn-Roth (43) or the modified Bethge-Lindstrom (44) procedure may be used to determine the total hydroxyl content. The aliphatic hydroxyl content is determined by the difference between the total and phenolic hydroxyl contents. Total hydroxyl content has been determined, after derivitization, by... 

Solomon (16,has uset a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the 0-H stretching band near 3450 cm were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In principle, this approach appears to be sound, but there are a number of problems. One difficulty, discussed above, is general to all infrared methods that have been employed so far what errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area Other problems involve the correct use of curve resolving techniques and the measurement of hydroxyl groups, which we will now consider in more detail. 

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