Analytical methods for determining functional groups in various technical lignins

In this paper we compare various analytical methods for determining functional groups in technical lignins of five different origins: kraft, sulfite, soda/anthraquinone, organosolv and ethanol process lignins. These lignins were characterized in terms of methoxyl, phenolic and aliphatic hydroxyl, carbonyl, carboxyl and sulfonate groups. The analytical methods used were: gas chromatography, aminolysis, UV-spectroscopy, H-1 and(13) CNMR spectroscopy, the oximating method, IFTIR spectroscopy, acid number determination, and non-aqueous and aqueous potentiometry. The statistical comparison of the various analytical methods for hydroxyl groups (phenolic and aliphatic) shows that the results obtained are not fully comparable. Aminolysis and non-aqueous potentiometry are assumed to be the most reliable for phenolic hydroxyl. We observed the same trend for the methods for carbonyl groups and selected the oximating method as reliable for determining total carbonyl. The results for the methods used for carboxylic groups showed correspondence at a significance level of 0.05. We selected aqueous and non-aqueous titration as being reliable for the lignins studied. We also compare all the commercial lignins in terms of functional groups. Finally, by completely characterizing the functional groups of various technical lignins, we have established the most complete representative expanded formula C-9 for each lignin under study. (C) 2007 Elsevier B.V. All rights reserved.