Structure of Lignosulfonates

In contrast to alkaline lignin, lignosulfonates are water-soluble and often contain

considerable amounts of carbohydrates – either dissolved in the liquor or still

attached to the lignin polymer – which must be removed prior to analysis [20].

The removal of carbohydrates from lignosulfonates is rather tedious. The linkages

between lignin and carbohydrates in bisulfite pulps have been analyzed by gel-permeation

chromatography (GPC) with multiple detection [16]. A structural model

for the high molecular-weight fraction of sulfite waste liquor was proposed by

Hachey and Bui [17].

Monomeric lignosulfonates actually identified in sulfite waste liquor are

1-(4-hydroxy-3-methoxyphenyl)-prop-2-ene-1-sulfonate [18,19]and its isomer

1-(4-hydroxy-3-methoxyphenyl)-prop-1-ene-3-sulfonate [19].

In addition to common lignin analytics 20,21], lignosulfonates can be characterized

by their degree of sulfonation (S/C9 or S/OMe), which varies for commercial

preparations from 0.4 to 0.7 sulfonate groups per phenylpropane unit [22]. Lignosulfonates

carry carboxyl groups [23], and lignosulfonates have distinct polyelectrolytic

characteristics, which often render chemical analysis more difficult.

Recent progress has been achieved in the more accurate determination of the mo-

4.3 Sulfite Chemical Pulping 415

lecular weight employing size-exclusion chromatography in combination with

light-scattering techniques [24].

4.3.4.2 Reactions of Carbohydrates: Acid Hydrolysis