Determination of Total and Aliphatic Hydroxyl Groups

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

Lignin preparation C, Unit formula Total OH (mol/C, unit) Phenolic OH Aliphatic OH 

Milled wood lignins (MWLs)a Spruce (Picea glauca) Q)H7 66O2(H2O)0 48(OCH,)o 94 1.46 0.28 1.18 

Zhong-Yang Mu (Bischofia polycarpa) CyH6 4y02(H20)() y2(OCHi)i n 1.33 0.22 1.11 

Milled bamboo lignin (MBL)J Chinese bamboo (Phyllostachys pubescens) CyH s202(H20)o 4(OCH ) 1.49 0.36 1.13 

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C -L Chen Determination of Total and Aliphatic 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). 

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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